Arxiu d'etiquetes: dinosaur

From lab to big screen (II)

As I told you in the previous article on genetics and cinema, there is a wide variety of films that talk about genetics. In the next article we will talk about science fiction, with two well-known films. Beware: spoilers!

GATTACA (1997)

Director: Andrew Niccol

Cast: Ethan Hawke, Uma Thurman, Jude Law

Genre: Science fiction

Story line: Vincent is one of the last “natural” babies born into a sterile, genetically-enhanced world, where life expectancy and disease likelihood are ascertained at birth. Myopic and due to die at 30, he has no chance of a career in a society that now discriminates against your genes, instead of your gender, race or religion. Going underground, he assumes the identity of Jerome, crippled in an accident, and achieves prominence in the Gattaca Corporation, where he is selected for his lifelong desire: a manned mission to Saturn’s 14th moon (titan). Constantly passing gene tests by diligently using samples of Jerome’s hair, skin, blood and urine, his now-perfect world is thrown into increasing desperation, his dream within reach, when the mission director is killed – and he carelessly loses an eyelash at the scene! Certain that they know the murderer’s ID, but unable to track down the former Vincent, the police start to close in, with extra searches, and new gene tests. With the once-in-a-lifetime launch only days away, Vincent must avoid arousing suspicion, while passing the tests, evading the police, and not knowing whom he can trust.

Relation with genetics: GATTACA is the “genetic” film par excellence. Starting with the title, this is formed by the initials of the four nitrogenous bases that make up DNA (guanine, adenine, thymine and cytosine). In addition, the helical shape of the DNA is repeated in several moments of the film, as in the stairs of Vincent’s house.

The main issue is about genetic selection, all children born have been genetically selected, closely linked to bioethics. The idea of ​​this selection is to reach eugenics, that is, to improve the population by selecting the “best” humans. This concept can be related to the Hitler’s Germany, who believed that Germans belonged to a superior group of races called “Aryan”. Hitler said that German Aryan race had been better endowed than the others and that this biological superiority destined Germans to oversee an empire in Eastern Europe.

Although nowadays genetic selection is valid and is used to avoid diseases, it is not applied with the same purposes as those of the film. At present, it is decided to carry out genetic selection after having studied the family and carried out the appropriate genetic counselling. It aims to help patients and their families avoid the pain and suffering caused by a genetic disease and should not be confused with the eugenic objective of reducing the incidence of genetic diseases or the frequency of alleles considered to be deleterious in the population.

This is very related to the genetic discrimination, case also exposed in the film. Gattaca is situated in a possible future in which genetics, trying to improve the quality of life of society, causes a movement of discrimination.

When we talk about discrimination, we tend to think about racial discrimination. This is defined as the different or exclusive treatment of a person for reasons of racial or ethnic origin, which constitutes a violation of the fundamental rights of individuals, as well as an attack on their dignity. Racism has been present throughout the history of mankind, especially in the twentieth century with racial discrimination in the United States and apartheid in South Africa.

For some time now, genetic discrimination has been gaining weight. It happens when people are treated differently by their company or insurance company because they have a genetic mutation that causes or increases the risk of a hereditary disorder. Fear of discrimination is a common concern among people who undergo genetic testing, and is a current problem that concerns the population because your own genome does not have to be a curriculum vitae that opens or closes doors as happens in the film. Vincent goes to work in Gattaca after performing a urine test and a blood test, since in Gattaca they do not choose workers for their ability or ability but for their DNA.

However, the film ends with the sentence “There is no gene for the human spirit”. This means that, although the society in which Gattaca is located is based on genetic modification, it does not affect the morality and final character of people because there is no way to genetically relate to the spirit, only the body has the genetic information.

Video 1. Trailer Gattaca (Source: YouTube)

JURASSIC PARK (1993)

Director: Steven Spielberg

Cast: Sam Neill, Laura Dern, Jeff Goldblum

Genre: Science fiction

Story line: Huge advancements in scientific technology have enabled a mogul to create an island full of living dinosaurs. John Hammond has invited four individuals, along with his two grandchildren, to join him at Jurassic Park. But will everything go according to plan? A park employee attempts to steal dinosaur embryos, critical security systems are shut down and it now becomes a race for survival with dinosaurs roaming freely over the island.

Relation with genetics: In the first film of this saga, from dinosaur’s fossils scientists extract DNA to be able to clone dinosaurs. The cloned dinosaurs will be part of the Jurassic park on which the film is based.

It is true that DNA can be extracted from bones, widely used in forensic genetics. Same as the issue of cloning, which was known by the Dolly sheep, the first large animal cloned from an adult cell in July 1996. But the film goes further and raises the possibility of reintroducing, in today’s world, extinct species and challenge natural selection.

Video 2. Trailer Jurassic Park (Source: YouTube)

REFERENCES

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Cutting up dinosaur’s evolutionary tree

For more than 130 years dinosaurs have been classified into two distinct orders, the saurischians and the ornithischians. But as it is common in biological sciences, every theory is true until the opposite is proved. A new study has called into question classical dinosaur classification, destroying and redistributing some of the different dinosaur groups. Even if this new hypothesis isn’t 100% sure yet, in this entry we’ll explain what this dinosaur reordering consists in.

TRADITIONAL DINOSAUR CLASSIFICATION

Since the XIX century, dinosaurs have been divided into two large orders based on their hip anatomy. The order Saurischia (lizard-hipped) includes theropods (carnivorous dinosaurs and current birds) and sauropodomorphs (large, long-necked herbivores); the order Ornithischia (bird-hipped) includes ornithopods (herbivorous and duck-billed dinosaurs), marginocephalians (dinosaurs with horns and hardened skulls) and thyreophorans (armored dinosaurs).

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Traditional dinosaur evolutionary tree by Zureks, with the two different hip morphologies at the bottom.

Yet, this classification doesn’t have the last word. Palaeontology is an extremely volatile science, as with each new discovery, you can dismantle everything you knew at that moment, even if it’s a centenary-old hypothesis. This is what has recently happened with dinosaurs.

THE RISE OF A NEW HYPOTHESIS

A new study published in March 2017, has caused the reconsideration of traditional dinosaur classification. Many previous studies assumed the Saurichia/Ornithischia classification as true and so, the used characters and taxons were all focussed on this classification. However, this new study has pioneered in many aspects:

  • It includes a larger number of species and taxons (many more than in previous investigations).
  • Previous studies gave more importance to basal theropod and sauropodomorph dinosaurs (traditional saurischians), as they were the first dinosaurs to diversify, including few basal ornithischians.
  • It has also included many dinosauromorph archosaurs (non-dinosaur taxons).
  • Older studies had assumed many ornithischian characters to be symplesiomorphies (ancestral characters of all dinosaurs) and they only focused on a few synapomorphies (characters found in a monophyletic group).

This study has detached from many of the previous assumptions on dinosaur phylogeny and has analysed a large number of species and many characters not included in previous investigations. This has made the resulting evolutionary tree pretty different from the ones obtained before.

RESHAPING THE TREE

Then, how does the dinosaur’s evolutionary tree stand according to this hypothesis? Well, the matter is somewhat complex, even if the different groups are still divided in two orders:

  • Order Saurischia which, according to this study, only includes sauropodomorphs and herrerasaurids (a group of carnivorous, non-theropod saurischians).
  • The new order Ornithoscelida (bird-limbed) that includes the traditional ornithischians and theropods, which are no longer saurischians.

Keeping this in mind, let’s now see the characteristics that define these two orders.

Saurischians

The order Saurischia is almost the same, except that theropods are no longer part of this group. This order presents the original saurischian hip structure, which the dinosaurs’ ancestors also had. According to this new hypothesis,  herrerasaurids and sauropodomorphs are all included as saurischians.

Herrerasaurids (Herrerasauridae family) were a small group of basal saurischians that evolved towards meat-eating. That’s why for a long time it was thought that they were the sister-taxon of theropods, but it was later seen that they were found among the first saurischians. Even if they were pretty specialized, they were probably displaced by competition with other predators, appearing during the middle Triassic and becoming extinct at the end of it.

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Photo by Brian Smith of a Herrerasaurus skeleton and model, from the Field Museum of Natural History of Chicago.

Herrerasaurids occupied a similar ecological niche as theropods. The new hypothesis implies that hypercarnivory (feeding exclusively on meat) evolved independently twice in dinosaurs, which makes some palaeontologist question it. Yet the herrerasaurid and theropod anatomy differed in some aspects, such as the anatomy of their hands (more generalistic in herrerasaurids) and the jaw structure.

The first sauropodomorphs were biped animals just like herrerasaurids, even if they were omnivorous. Yet, sauropodomorphs would end up becoming huge herbivorous quadrupeds with characteristic long necks.

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Thecodontosaurus skeleton (by Qilong), a basal sauropodomorph and a reconstruction of Plateosaurus (from Walters, Senter & Robins) a more advanced one. Even if it cannot be appreciated in this image, sauropodomorphs would increase very their size very much during their evolution (Thecodontosaurus 2 metres, Plateosaurus up to 10 metres).

Ornithoscelidans

The new dinosaur order is Ornithoscelida, which groups theropods with ornithischians. This taxon is supported by more than twenty skeletal synapomorphies (derived characters shared by a clade), present both in basal theropods and ornithischians. Some of these characteristics include the presence of a gap between premaxillar and maxillar teeth (diastema) and the fusion of the ends of the tibia and the fibula into a tibiotarsus (even if these characteristics are only found on the most basal species).

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Scheme from Baron et al. (2017) of the skulls of two basal ornithoscelidans, Eoraptor (a theropod, top) and Heterodontosaurus (an ornithischian, bottom).

Both theropods and the first ornithischians were bipedal animals. Also, the presence of heterodont teeth in the ancestral members of both groups leads us to think that the first ornithoscelidans were omnivorous, which would later specialise in feeding on meat and on plants (theropods and ornithopods respectively).

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Reconstruction of the face of Daemonosaurus, one of the first theropods, by DeadMonkey8984.

A curiosity about the new classification is that accepting Ornithoscelida as a valid taxon, all feathered dinosaurs are put together into one group. Everyone knows that many theropods presented feathers (as they were the ancestors of birds) but, what most people don’t know is that feathers have also been found in some basal ornithischians and in more advanced ones too.

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Reconstruction by Tom Parker of Kulindadromeus, a ornithischian which feathers have been proved to be present on most of its body.

KEEP INVESTIGATING

Then, is this hypothesis irrevocable? Well, no of course. Even if it’s pretty tempting to assume that the dinosaur’s natural history has been changed, we cannot say that from now on dinosaurs will be classified this way.

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Dinosaur evolutionary tree according to Baron et al. (2017), in which we can see the different clades; Dinosauria (A), Saurischia (B) and Ornithoscelida (C).

Even if this study shows really interesting results about the origin of dinosaurs, we cannot dismiss hundreds of previous studies about this group of animals. We’ll have to remain alert to new articles that step by step will keep unveiling more information about the relationships between these Mesozoic reptiles. And that’s what’s so stimulating about biology, that there’s nothing sure! And that with new investigation techniques and new discoveries, little by little we learn more about the world around us.

Keep your mind open and keep investigating!

REFERENCES

The following sources have been consulted during the elaboration of this entry:

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Dinosaurs from the North Pole: Live at Prince Creek

When we think about dinosaurs, we probably imagine them walking through a dense, tropical jungle or wandering in a warm, foggy swamp. But as a matter of fact, some dinosaur species lived in very high latitudes, as the ones found in the Prince Creek formation. This Alaskan geologic formation is one of the most important sources of arctic dinosaurs, as many fossils have been found in it. In this entry, we’ll describe some of these dinosaurs from the North Pole, and we’ll explain some of the difficulties they had to endure in order to survive in the northernmost point of the planet.

ALASKA 75 MILLION YEARS AGO

The Prince Creek formation is situated in the north of Alaska and dates from around 80-60 million years ago, at the end of the Cretaceous, the last period of the Mesozoic. At that time, North America was divided by the Western Interior Seaway; the eastern continent or Appalachia, and the western continent or Laramidia, north of which the Prince Creek formation was deposited.

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Map of North America at the end of the Cretaceous period, with the Prince Creek formation marked in red, from the article New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism.

At the end of the Cretaceous period, the Prince Creek formation was further north than it is today. Yet, at that time the Earth was going through a greenhouse effect phase, so the climate was a little warmer than it is today. It is thought that the mean annual temperature at Prince Creek was about 5°C, with summer maximums at about 18-20°C. Still, the difference in temperature between summer and winter would have been quite remarkable (currently, at the same latitude, it’s about 56°C).

Even if temperatures were not as low as the ones of present-day Alaska, the dinosaurs of Prince Creek had to endure long, dark winter months. Yet, the slightly higher temperatures and the proximity to the sea, produced a higher diversity of plant species. Observing the fossilized flora, we know that the landscape was that of a polar woodland, with angiosperm-dominated forests and a large number of fern, moss and fungus species, with some areas of seasonally-flooded grasslands.

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Drawing by Julio Lacerda of Prince Creek’s landscape and wildlife.

As for the fauna, palaeontologists were surprised at the great number of big animals found. The fact that dinosaurs were found in such high latitudes is what makes us think that these were endotherm animals that generated their own body heat. Also in Prince Creek, there aren’t any fossils of other ectotherm reptiles like turtles, crocodiles or snakes, which are usually found in other United States deposits of the same period. Currently, dinosaurs are thought to be neither endotherm nor ectotherm, but mesotherm animals, which generated body heat metabolically, but were unable to control its temperature or keep it stable.

TOUGH HERBIVORES

The relatively abundant vegetation, allowed the presence of a great diversity of plant-eating dinosaurs in such high latitudes. While the smaller herbivores had little trouble because of their low energetic requirements, the larger herbivores probably had more difficulties in order to find enough food, especially during the harsh winter months. The dinosaur fossil found at the highest latitude is Ugrunaaluk (literally “ancient grazer” in Inupiaq language from northern Alaska) a hadrosaurid or “duck-billed dinosaur”. This ornithopod measured up to 10 metres long and weighed around 3 tonnes, making it one of the largest animals in Prince Creek.

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Reconstruction by James Havens of a herd of Ugrunaaluk kuukpikensis, moving under the polar lights.

Ugrunaaluk were herbivorous animals that lived in groups. Even if many author think that these animals performed long migrations like today’s birds and mammals in order to avoid the lack of food during the winter, some others argue that young Ugrunaaluk (which had a less active metabolism than current endotherms) would had been unable to endure such long journeys. Ugrunaaluk probably moved to areas were the vegetation better tolerated the severity of winter, even if it’s thought that these great herbivores survived during the dark winters feeding on bark, ferns and probably aquatic vegetation during the coldest months.

The other great Prince Creek plant-eater was Pachyrhinosaurus (literally “thick-nosed lizard”) a ceratopsid widely-distributed through the United States, with a large protuberance on its nose which may have been used as a weapon during intraspecific combats, and a pair of laterally-facing horns on the top of its frill. Pachyrhinosaurus was the largest animal of Prince Creek, measuring up to 8 metres long and weighing up to 4 tonnes. It is possible that it used its nasal protuberance to shovel through the snow to reach the plants buried under it, similar to today’s bisons.

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Reconstruction of a pair of Pachyrhinosaurus perotorum by James Havens.

All the animals of Prince Creek had arduous lives. Almost all the fossils of both Ugrunaaluk and Pachyrhinosaurus, indicate that these species matured quickly and died young. Observing the growth of the different bones that have been found, it is thought that these dinosaurs rarely lived for over 20 years of age, probably due to the harsh conditions of their habitat but also to the presence of predators.

PREDATORS LARGE AND SMALL

The largest predator of the region was Nanuqsaurus (“polar bear lizard”, from Inupiaq language), a tyrannosaurid. This animal had a highly developed sense of smell which allowed it to detect their prey or animal carcasses in low light during the polar winter. Also, although there is no evidence, it was probably covered in feathers which would have protected it from the cold, as many closely-related theropods presented feathers to some extent.

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Reconstruction of Nanuqsaurus hoglundi by Tom Parker.

What’s more surprising about Nanuqsaurus is its size, much smaller than that of its relatives. While other tyrannosaurids from the same time measured between 10 or 12 metres long and weighed up to 9 tonnes, Nanuqsaurus was more of a pygmy tyrannosaur, with an estimated length of 6 metres and a weight of 800 kg. This diminutive size was probably caused by the fact that it lived in an environment where food availability varied through the seasons. Apart from the fact that their prey’s population densities probably weren’t very high, during winter months many herbivores would migrate to other areas.

By contrast, there was another theropod that presented the opposite adaptation. Troodon (“wounding tooth”) was a relatively small dinosaur, about 2.9 metres long and 50 kg of weight. This is a pretty abundant dinosaur in many North American deposits. Troodon was a highly active carnivorous animal, with a good binocular vision and it’s also believed to be one of the most intelligent Mesozoic dinosaurs.

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Reconstruction of two Troodon inequalis playing in the snow by Midiaou.

While Nanuqsaurus was smaller by the lack of abundant prey, Troodon specimens found at Prince Creek were characterized by their bigger size, compared with the ones from other deposits. This is what is called the Bergmann’s Rule, according to which the populations of a species that live in colder climates tend to be larger than the populations living in warmer climates, as this way they lose less body heat. Also, the larger eyes of Prince Creek’s Troodon, would give them advantage hunting during the long winter nights.

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Image from the article A Diminutive New Tyrannosaur from the Top of the World, in which we can see the size of Nanuqsaurus (A) compared with some other tyrannosaurids (B, C, D and E) and two Troodon specimens (F and G) from different latitudes.

As you can see, dinosaurs not only thrived in warm and tropical environments. Even if their populations weren’t very large and their living conditions were harsher, these dinosaurs were able to adapt and survive in the polar forests of Prince Creek, and many of them surely gazed at the spectacular northern lights of 75 million years ago.

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Assembly of the different dinosaur species from the Prince Creek formation by James Kuether.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

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Feathered dinosaurs: the origin of birds

The presence of feathers is one the main characteristics of modern birds. Currently many dinosaur fossils show us that feathers appeared long before birds. Yet the feathers that those Mesozoic animals had weren’t exactly the same as the ones current birds have. The evolution of feathers was a long and gradual process, and in this entry we’ll review the most important evolutionary stages that brought those dinosaurs to develop anatomically modern feathers.

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Today’s feathers

Feathers are fundamental structures for the life of modern birds. Feathers help them insulate from cold and hot weather, make them waterproof, camouflage, allow them to fly and in many species, feathers are very important in the mating rituals. In many birds, plumage allows us to differentiate between different species, telling a male and a female apart, and even allows us to know the age of an individual.

Chrysolophus_pictus_walkingMale golden pheasant (Chrysolophus pictus) photographed at Kuala Lumpur’s Bird Park, showing us different types of feathers. Photo by Bjørn Christian Tørrissen.

Feathers are the most complex integumentary structures found in vertebrates. These are formed in the epidermis, in little follicles which produce keratin. The β-keratin of the bird’s feathers, claws and beak is much more folded than the α-keratin found in mammalian’s hair, hooves or horns, making the first a much stronger structure. Feathers are resistant and light structures, but in many birds they correspond to a third of their body weight.

Modern feathers have a central shaft divided into two parts: the proximal part which inserts to the body called the calamus, and the rachis, the distal part from which the laminar part of the feather appears. This is called the vane and is disposed on both sides of the rachis. The laminar part is made of parallel ramifications called barbs, which have ramifications called barbules which also have ramifications in the shape of small hooks called barbicels that make barbules cross-attach to each other. The superior end of the vane (pennaceous part) barbules are perfectly arranged by the barbicels, while in the inferior end (plumulaceus part) barbules lack barbicels and so they float free from each other.

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Parts of a feather:

  1. Vane
  2. Rachis
  3. Pennaceous barbs
  4. Plumulaceous barbs or afterfeather
  5. Calamus

According to its structure, in current birds we can find two main types of feathers:

Contour feathers: These are the feathers that make up the shape of the bird. These are long, flat feathers with a well-developed rachis and well-arranged barbs. These can be further classified into generic contour feathers, which cover the head, neck, trunk and limbs of the animal, and the flight feathers, called rectrices the ones in the tail (symmetric) and remiges the ones in the wings (asymmetric).

Parrot-featherFeathers of a macaw. Photo by Jörg Gorβ.

Down feathers: These are found forming a second layer under the contour feathers. These are feathers with a short rachis and with disordered barbs floating freely. Its main function is to thermally insulate the bird. Natal down feathers covering most bird hatchlings in some time of their lives are called “neossoptilus”.

Young_barn_owl_(Tyto_alba_pratincola)Barn owl hatchling (Tyto alba) covered in down feathers. Photo by Maxgreen.

Apart from these two types, there are other kinds of feathers in birds, such as the semiplumes (with an intermediate structure between contour and down feathers) and the bristles and filoplumes (with few barbs and mainly with a sensory function).

Tipos_de_plumasDifferent types of feathers we can find on modern birds, drawings by Osado. From left to right: Rectrix (tail), remex (wing), generic contour feather, semiplume, down feather, bristle and filoplume.

Origin and evolution of feathers

Probably dinosaurs develped the first feathers as a system to avoid the loss of body heat. Having a covering feathers, a layer of warm air becomes trapped around the animal, making its body temperature more stable. That’s why some scientists think that many dinosaur species had an almost endothermic metabolism (mesothermy), with high and constant body temperature. Nevertheless, primitive feathers or “protofeathers” were very different from modern feathers.

Deinonychus_im_NHM_WienReconstruction of Deinonynchus by Stephen Czerkas, at the Natural History Museum of Vienna. Photo by Domser.

As we will now see, protofeathers went through different evolutionary stages before becoming modern feathers. Even if here we present you these stages linearly, it doesn’t mean that when a new kind of protofeather appeared the previous one disappeared. Just like modern birds sport different kinds of feathers, many dinosaurs presented different combinations of protofeathers, which only represented different levels of specialization.

Stage 1: A single filament

Feather_evolution_StageI_v2Drawing about the origin and formation of the first protofeathers. Extracted from Prum & Brush (2002).

The first known protofeathers were nothing more than a cylindrical hollow spine-like filament, which formed on a follicle’s collar. Even though feathers and protofeathers are typically exclusive characteristics of theropods, this first protofeathers have also been found in two groups of non-theropod dinosaurs. These are the Heterodontosauridae and Psittacosauridae families, many species of which had spines homologous to stage 1 protofeathers which probably also served to retain body heat.

FruitadensReconstruction of a heterodontosaurid named Fruitadens. Drawing by Smokeybjb.

In theropods, feathers appeared in a group named Coelurosauria, which includes animals like the tyrannosaur, the velociraptor and modern birds. The oldest feathered coelurosaur known is Sciurumimus, which literally means “squirrel mimic”. This fossil got its name for its fully feathered tail, covered in filamentous protofeathers similar to a squirrel’s.

sciurumimus_skeleton_by_franz_josef73-d5osy3yReconstruction of a juvenile Sciurumimus based on the skeleton found in Bavaria. Drawing by Franz Joseph.

Stage 2: A plumulaceous protofeather

Feather_evolution_Stage2_v2Second stage in the evolution of feathers, in which a division in the follicle produces various barbs with a single origin. Extracted from Prum & Brush (2002).

The next step on the evolution of feathers was the division of the cellular collar of the follicle, which brought the branching of the filament. The result is a plumulaceous protofeather with unbranched barbs originating in a calamus. Stage 2 protofeather are similar to down feathers of current birds and have been found in a wide variety of theropod fossils.

These protofeathers provided a better insulation, helping the animal to keep its body heat. It is also believed that it’s likely that the smallest dinosaurs were more fully covered in protofeathers, since smaller animals loose heat faster than bigger animals and so, they need more mechanisms to retain body heat. Bigger coelurosaur species like Tyrannosaurus may have lost their protofeathers much like modern elephants have lost almost all their body hair. Yet, it is possible that some species presented protofeathers after birth and during the first stages of life, and after growing up they would either loose them or only present them on some body parts.

Juravenator_by_Tom_ParkerReconstruction of a juvenile Juravenator in which we can appreciate how it was covered both with protofeathers and scales. Drawing by Tom Parker.

Yet in a Chinese paleontological site, the two biggest feathered dinosaurs known were discovered. The first to be discovered was Beipiaosaurus, a strange looking coelurosaur of about 3 metres long with long claws, which presented protofeathers both filamentous (stage 1) and plumulaceous (stage 2). This species shared its habitat with Yutyrannus, a 9 metre-long animal up to 1400 kilos of weight, which had almost all its body covered in plumulaceous protofeathers. These two animals probably lived in a humid and cold environment, and their coat of protofeathers helped them to keep their warmth when temperatures would fall.

dino-herdReconstruction of four Yutyrannus and a pair of Beipiaosaurus on their habitat. Drawing by Brian Choo.

Stage 3: Fusion and branching

Feather_evolution_Stages1to3bDrawing of the evolution of feathers from stage 1 to 3. Extracted from Sues (2001).

The third stage in the evolution of feathers gave rise to a protofeather with a central rachis made from the fusion of some barbs (3a) and a protofeather with barbules branching from the main barbs (3b). The combination of these two characters produced a pennaceous, vaned protofeather similar to the ones found in modern birds but less firm, as it lacked the hooked barbicels of modern feathers.

Feather_evolution_StageII_IIIa_v2Fossils of stage 3a protofeathers where we can see a central rachis from which various barbs extend. Extracted from Perrichot (2008).

Stage 4: Hooks to maintain order

Feather_evolution_3-5_v2Modified drawing from Prum & Brush (2002) of the apparition of hooks on the barbules of the stage 4.

It is in this stage where we can start talking about present day feathers. The stage 3 structure with a rachis, barbs and barbules, developed small hooks on the barbules which made them cross-attach and keep the vane together. These feathers were like the ones found in modern birds, the contour feathers, which present a central shaft and a symmetric vane.

Anchiornis_martyniukReconstruction of the troodontid called Anchiornis, where the wide cover of feathers it presented can be seen. Drawing by Matt Martyniuk.

These feathers are found in many different dinosaurs, many of which had begun to develop adaptations for flight, or at least gliding. Despite this, we can also find them in typically running dinosaurs like Velociraptor, a terrestrial predator about the size of a turkey, with a long mouth and a sickle-shaped claw on its hind legs. This claw is thought to be used mainly to kill their prey, but some scientists think that they used their claw to climb trees and ambush their prey from above. Maybe their feathers served them to make more controlled leaps when they fell on their victims.

Velociraptor_restraining_an_oviraptorosaur_by_durbedDrawing of a velociraptor attacking an oviraptotosaur. Drawing by Durbed.

These feathers are also found in the oviraptorosaurs, a coelurosaurian group with beak and few or no teeth. Even if they couldn’t fly, they probably used their arm feathers to incubate their eggs (like the Avimimus genre) and the ones on the tail for display and communication with other members of their species (like the Caudipteryx, and Nomingia genera).

Nomingia_gobiensisReconstruction of the oviraptorosaurian Nomingia, in which we can see the fan of feathers on its tail. Drawing by Smokeybjb.

Other dinosaurs like Scansoriopteryx had an arboreal lifestyle, and the feathers on its arms allowed it to glide from one tree to the other both to hunt and to escape predators. A relative of this animal called Epidexipteryx, even though not having feathers on its arms (as far as is known) presented long vaned feathers on the tail, probably to send visual signals to other members of its species.

Epidexipteryx_NTReconstruction of Epidexipteryx in which the long vaned tail feathers can be appreciated. Reconstruction by Nobu Tamura.

Stage 5: Asymmetry brings flight

Amber_feathersDrawings and fossils of all the different stages of the evolution of feathers. Extracted from McKellar et al (2011).

Finally the last stage in the evolution of feathers is the appearance of asymmetric feathers, with a displaced rachis making one half of the vane wider than the other. These feathers are the remiges found on the wings of birds, which not only increase drag during gliding, but also allow the animal to leave the ground and fly.

poecile-montanus-kittila-85459_1920Photo of a Willow tit (Poecile montanus) taking flight, where we can perfectly see the asymmetric remiges on its wings. Photo by David Mark.

Even if it’s generally assumed that apart from birds no other dinosaur group accomplished powered flight, there’s one group which got really close. The microraptorians were a group of small feathered dinosaurs characterized by presenting flight feathers, not only on their front limbs but also on their hind limbs. The most famous of them, Microraptor, had asymmetrical flight feathers on its arms, legs and, unlike modern birds, on its tail.

Microraptor4Drawing of the silhouette of Microraptor gliding. Extracted from Xu et al.

Even if it’s usually considered a glider, some authors argue that possibly Microraptor was capacitated to fly. Some skeletal characteristics indicate that some microraptorians may have been better suited for flight than Archaeopteryx, the ancestor of modern birds. For example, Microraptor presented a fused more developed sternum than Archaeopteryx, which would have made for a major anchor point for the flight muscles.

Video from the American Natural History Museum of the reconstruction of the appearance of Microraptor.

Nevertheless, Archaeopteryx is still considered the nearest to the ancestor of modern birds that, even if it wasn’t a great flier, it already had the different kinds of feathers found on current birds. Probably many more dinosaurs were covered with feathers or protofeathers, but in this entry we have only seen the species which show irrefutable evidence of having them. As we have seen, the road to reach modern feathers was long and gave rise to a wide diversity of dinosaur species, but after a meteorite practically extinguished life on Earth 65 million years ago, only one group of feathered dinosaurs survived and thrived.

This is labeled USNM# 4178. The original fossil is at Humboldt Museum, East Be rlin. The rock was found in Solnhofn, West Germany, and it is from the Jurassi c period.Fossil of Archaeopteryx lithographica from the late Jurassic found at southern Germany. Photo by James L. Amos.

Difusió-anglès

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References

The next sources have been consulted during the elaboration of this entry:

Danger, poisonous mammals!

We usually associate snakes, spiders, jellyfish, etc. as venomous animals par excellence, but did you know that there are poisonous mammals? In this article we will discover who are they and the nature and use of their poisons.

THE PLATYPUS

The platypus (Ornithorhynchus anatinus) is the most famous among the poisonous mammals, and not just for this feature. With a peak like a duck and oviparous (laying eggs), when it was discovered some scientists thought it was a fraud.

platypus ornitorrinco ornitorinc
Platypus (Ornithorhynchus anatinus). Photo by Jonathan Munro

They belong to the order monotremes, which means “one hole” in reference to the cloaca, the end of the digestive and reproductive systems. Some evolutionary biologists refer to them as the “missing linkbetween reptiles and mammals, as they have characteristics of both groups. Monotremes are the only mammals that lay eggs, but his body is covered with hair and the young are fed with breast milk. They are distributed by Australia, Tasmania and New Guinea.

Platypuses have a spur on the hind legs, which only in the case of males, release poison produced by femoral glands (located in the leg). The male uses it mainly to defend their territory and establish their dominance during the mating season, although if it is bothered also uses it as a defense. This poison can kill small animals, including dogs, and cause severe pain and swelling in humans. This pain can last days or months.

Platypus spur, espolón ornitorrinco
Spur on the hind leg of a platypus. Photo by E. Lonnon

Toxins are four proteins, three of which are unique to the platypus. They are like the defensins (DLP, defensin-like proteins). These are globular proteins, small and compacted, involved in the activation of pain receptors. Understanding how these toxins act it has special interest because they cause a lasting and severe pain; it may open new chances in the synthesis of analgesic drugs.

short-beaked echidna, equidna de nariz corta, equidna de nas curt
Short-beake echidna (Tachyglossus aculeatus). Photo de Tony Britt-Lewis

Echidnas (family Tachyglossidae) complete the order of monotremes with the platypus; consequently they are also oviparous. The family consists of four species, with the common characteristic of having the body covered with dense hair and spines. They are mainly insectivores specializing in ants and termites.

Like the platypus, they also have spurs behind the knees, but their secretions are not poisonous. The substances are used to mark their territory, according to the recent studies.

SLOW LORIS

As we saw in a previous post, lorises are primates in the prosimians suborder. They are nocturnal, arboreal and feed primarily on insects, vegetables and fruits. The slow lorises (Nycticebus) living in Southeast Asia, are the only poisonous primate. They possess poison glands on the elbows (brachial gland), and poison their body with arms and tongue, which can also join saliva and be transmitted by bitting.

lori pigmeo, nycticebus pigmaeus,
Pygmy slow loris (Nycticebus pigmaeus). Photo by Ch’ien C. Lee

In this case the poison is used as a defense against predators, causing them pain, inflammation, necrosis (cell death) in the area of the bite, hematuria (blood in urine) or in some cases anaphylactic shock (allergic reaction) which can lead to death, even in humans (some are threatened by the illegal pet trade and traditional Chinese medicine). The poison also serves as protection for the young, they are licked by their parents and the poisonous secretion is distributed throughout the coat. Being poisonous, unusual among primates, can help counteract the disadvantages of its slow movements. Exudate from glands, as in echidnas, can also give olfactory information of range and territory between individuals of loris (Hagey et al., 2007).

Loris de Kayan (Nycticebus kayan). foto de Ch'ien C. Lee
Kayan loris (Nycticebus kayan). Photo by Ch’ien C. Lee

Toxins are polypeptides (generated when glandular secretion is mixed with saliva) and an unidentified steroid. Secretion is similar to the allergen Fel d 1 which is in the domestic cat and cause allergies in humans (Hagey et al., 2006; Krane et al., 2003).

It is believed that slow lorises even have converged evolutionarily with cobras, for his defensive behavior when threatened, whistling and raising his arms around his head. (Nekaris et. al, 2003).

Loris, cobras, evolucion, convergencia
Mimicry between loris and cobras. 1. Javan slow loris, 2 y 3. Spectacled cobra, 4. Bengal slow loris. Photo by Nekaris et. al.

In the following video a lazy lori is disturbed and hisses like a snake while trying to bite:

SOLENODON OR ALMIQUI

They are small and nocturnal mammals, basically insectivores, that live in the West Indies. The Hispaniolan solenodon (Solenodon paradoxus), also known as the Dominican solenodon, Haitian solenodon or agouta, lives on the island de La Española (Dominican Republic and Haiti) while The Cuban solenodon or almiqui (Solenodon cubanus) is distributed throughout Cuba. They are considered living fossils because they have similar characteristics to primitive mammals of the end of the Mesozoic Era (kingdom of the dinosaurs).

solenodonte de La Española (Solenodon paradoxus
Hispaniolan solenodon (Solenodon paradoxus). Photo by Eladio M. Fernández.

Unlike other poisonous mammals, toxic saliva is produced under the jaw (submandibular glands), which is transported by pipes to the front of the mouth. The second incisor teeth have a groove where toxic saliva accumulates to promote their entry into the wounds. They are the only mammals that inject venom through its teeth, similar to the way snakes do.

diente, solenodon, teeth, surco
Paradoxus Solenodon lower jaw incisor showing the groove. Photo by Phil Myers

The main function of this venom is to immobilize prey, as well as insects they can hunt small vertebrates such as reptiles, amphibians and birds.

Almiquí, Cuba, Solenodon, cubanus, Cuban giant shrew
Cuban solenodon (Solenodon cubanus). Photo by Julio Genaro.

This poison may have been developed to keep alive but immobilized prey during times of shortage, to aid in digestion, minimize energy expenditure in the struggle for hunting and face prey even twice as big as them. This venom is not deadly to humans.

SHREWS

The northern short-tailed shrew (Blarina brevicauda), the Eurasian water shrew (Neomys fodiens) and the Mediterranean water shrew (Neomys anomalus) also have submandibular glands similar to solenodons. They are distributed by North America (northern short-tailed shrew) and Europe and Asia (water shrews), including the Iberian Peninsula.

Musaraña colicorta americana (Blarina brevicauda). Foto de Gilles Gonthier.
The northern short-tailed shrew (Blarina brevicauda). Photo by Gilles Gonthier.

The short-tailed shrew can consume up to three times its weight in food per day. Their saliva is the most poisonous and uses it to paralyze their prey, to eat them or keep them alive in times of shortage. The water shrews also store its immobilized prey under rocks.

Musgaño (Neomys anomalus). Foto de rollin Verlinde.
Mediterranean water shrew (Neomys anomalus). Photo by Rollin Verlinde.

These animals attack from behind and bite the neck of its prey so that the poison acts more quickly, affecting the central nervous system (neurotoxins). The respiratory and vascular system is also affected and causes seizures, incoordination, paralysis and even death of small vertebrates.

Musgaño patiblanco-Neomys_fodiens, Wasserspitzmaus
Eurasian water shrew (Neomys fodiens). Photo by R. Altenkamp.

Its teeth don’t have grooves as the solenodons do, but a concave surface to store the toxic saliva.

neomys, anomalus, mandibula, dientes, veneno
Lower jaw of Neomys anomalus. Photo by António Pena.

It is suspected that other mammals also produce toxic saliva similarly, as the European mole (Talpa europaea) and other species of shrew, but there are no conclusive studies.

MANED RAT

The maned rat or crested rat (Lophiomys imhausi), lives in Africa and  uses his poisoned hair to protect themself from predators.

Rata crestada Lophiomys_imhausi, rata de crin, maned rat
Maned rat (Lophiomys imhausi). Photo by Kevin Deacon

Unlike other mammals that produce their own poison, the crested rat gets toxin (called ouabain) from the bark and roots of a tree (Acokanthera schimperi). Chews the bark and the mixture of saliva and toxins are distributed on the body. Their hairs are cylindrical whith a perforated microscopic structure, which favors the absorption of venom. In case of danger, it bristles and shows his brown coat with white stripes, warning of its potential danger. This strategy of persuasion based on brightly colored warning is known as aposematism present in many animals, such as bees.

In this BBC video you can see a crested rat and a hair under the microscope absorbing ink, showing its porous structure:

It is unknown how it is immune to the toxin, since it is the same substance used by some African tribes for hunting such large animals like elephants.

Ouabain is a glycoside which controls the heartbeat, causing infarcts if absorbed in large quantities. The study of the mechanisms that protect the crested rat of a substance that regulates the heartbeat, can help develop treatments for heart problems.

European hedgehogs (Erinaceus europaeus) have similar behavior (smearing the body with foreign poison), but it is not established whether the objective is defensive because it does not scare away predators.

In conclusion, strategies, practices and nature of the poison in mammals are varied and their study may have important medical implications for drug development and increase awareness of the evolutionary relationships between different groups of living animals (reptiles-mammals) and their ancestors.

REFERENCIAS

MIREIA QUEROL ALL YOU NEED IS BIOLOGY

Spinosaurus: the first aquatic dinosaur?

Recently, the BBC documentary series “Planet Dinosaur” has premiered on TVE2. In this series the latest paleontological discoveries concerning the biology of dinosaurs are explained. On my last entry we talked about the theropod dinosaurs, one of which is the spinosaur, one of the largest predators that have ever existed. On this entry I’m going to explain some of the facts that paleontology has revealed about the lifestyle of this creature.

TAXONOMY

The spinosaur (scientific name Spinosaurus aegyptiacus) belonged to the Spinosauridae family, a group of specialized theropods which appeared during the late Jurassic and became extinct about 93 million years ago during the late Cretaceous. This group was characterized by being relatively large theropods, with conic teeth and long snouts similar to crocodiles, and elongated neural spines through its back forming a sail-like structure (that’s where the name Spinosauridae comes from, meaning spine reptiles).

Spinosauridscale
Comparition of the different sizes of various spinosaurids by Scott Hartman. From right to left: Irritator challengeri, Baryonyx walkeri, Suchomimus tenerensis and Spinosaurus aegyptiacus.

Some of the more famous members on this family are, the Baryonyx from Europe, which had long curved claws on its hands to capture the fish it fed on, similar to its close relative the Suchomimus from northern Africa. Furthermore, there was the smaller Irritator of about 3 metres tall found in Brasil and finally, the Spinosaurus from northern Africa, which measuring between 12 and 18 metres long and wheighing between 7 and 20 tons, was one of the biggest predators to ever walk on land.

HABITAT AND DISTRIBUTION

The genus Spinosaurus was distributed in the zone of what is now the north of Africa. This genus lived during the Cretaceous, appearing about 112 million years ago and disappearing about 97 million years ago.

94_mya_Texas_Geology
Map of the World 94 million years ago by Joshua Doubek, during the middle Cretaceous period.

During that period, the northern part of Africa was a very humid zone with high temperatures and lots of wetlands. Spinosaurs probably lived in areas with large rivers and mangrove forests next to the sea, where tidal movements flooded its habitat during certain seasons of the year. This is in accordance with the vision that spinosaurids preferred wet semiaquatic habitats with plenty of great fish to prey upon.

Spinosaurus1DBa
Reconstruction from 2010 of Spinosaurus aegyptiacus by Dmitry Bogdanov.

Currently there are two possible spinosaur species. The most famous is Spinosaurus aegyptiacus from Egypt, the species of which we have more information. A possible second species is Spinosaurus maroccanus from Morocco, which some authors consider simply as a subpopulation of Spinosaurus aegyptiacus.

FUNCTION OF NEURAL SPINES

Spinosaurs were discovered in 1912 from a fossil which included its characteristic dorsal spines. These spines grow up to a length ten times that of the vertebra from which they emerged.

The scarcity of spinosaur fossils means that the function of the spines is still a mystery for science, although there are some hypothesis. One of these is that the spines formed a “sail” along the back of the animal which was highly irrigated and helped the animal’s thermoregulation, as such a big animal probably would have had problems losing heat. Therefore its sail would have helped the spinosaur to evade overheating, orienting it towards fresh winds to cool down.

Subadult_Spinosaurus
Reconstruction of the skeleton of a subadult spinosaur (Japan Museum, photo by Kabacchi).

Another hypothesis tells us that the spines held a hump-like structure similar to that of camels, which the animal would have used as a fat reserve system to store fat to withstand periods with little available feeding resources.

Lots of paleontologists think that both hypothesis could be correct and that the spinosaur used the sail both to regulate its body temperature and also to store fat to resist periods of low prey abundance. It is also possible that the sail made the spinosaur appear bigger than it actually was and that they used it during mating rituals similar to those of the modern peacock.

FEEDING STRATEGIES

The Spinosaurus‘s skull shows adaptions to a piscivorous diet. The snout is longer and slender than on other theropods. Aside from this, observing the snout of Spinosaurus it has been seen that it presents a series of little holes similar to those found on crocodiles. It is thought that these structures indicate the presence of pressure receptors which helped them detect the movement of their preys underwater.

Spinosaurus
Upper jaw of Spinosaurus from the Museo di Storio Naturale di Milano, where the holes which possibly contained the pressure receptors can be seen.

While the teeth of most carnivorous theropods where curved and serrated on their posterior part to tear flesh, spinosaur teeth were conic in shape and had no serration, more similar to those of crocodiles. These teeth were more useful for catching and holding fast and slippery prey and to prevent them from escaping (for example, a fish). Also, various Spinosaurus fossils have been found to have between their theeth scales and bones of large prehistoric fish which probably populated many rivers during the Cretaceous period.

new_mawsonia_by_hyrotrioskjan-d5qjb39
Reconstruction by Joschua Knüppe of two Mawsonia species, the rests of which have been found between the teeth of Spinosaurus.

Nevertheless, it is generally believed that the spinosaur was probably an opportunistic predator, feeding mainly on fish, also hunting small dinosaurs when it had the opportunity and stealing prey from smaller predators using its great size to intimidate them.

POSTURE AND LIFESTYLE

Spinosaurs have traditionally been represented as bipedal animals, as most similarly-sized theropods have. Eventhough most fossils are actually pretty incomplete, it is known that its forelegs were more developed than in most theropods, having long curved claws.

Traditionally it was thought that Spinosaurus hunted in a manner similar to a grey heron, roaming through zones of shallow water, sinking its long snout underwater to detect prey using the pressure receptors, and catching fish with its jaws. It then, probably used its front legs as hands to tear its prey to small pieces easy to swallow.

spinosaurus_by_hyrotrioskjan-d5ate1h
Reconstruction by Joschua Knüppe of Spinosaurus aegyptiacus in hunting posture.

At the end of 2014 a new Spinosaurus fossil was discovered which has changed the view we had on this animal. For the first time, scientists found a fossil which shows the structure of the hind legs of this dinosaur and they have observed a number of characteristics not found in any other theropod not even in other spinosaurids. This fossil shows that the hind legs of Spinosaurus were much more massive than those of other theropod dinosaurs, in which the bones are usually hollow to make them more agile (like present day birds). Also, in this fossil the hind legs are actually much shorter in relation to the size of the animal than in any other theropod, leading some scientists to think that Spinosaurus was actually a quadrupedal animal. This has made some paleontologist think that maybe the lifestyle of the spinosaurs was much more similar to that of a crocodile and that they spent much more time living in water than on land, making the Spinosaurus the first known aquatic dinosaur.

spinosaurs_aegyptiacus_2014_by_rodrigo_vega-d7zj8yn
Reconstruction by Rodrigo Vega of Spinosaurus based on the skeleton found in 2014.

Anyway, many paleontologists argue that the biology of a species cannot be based on a single fossil and advise caution when generalizing to the whole species (the fossil could belong to an adult and a juvenile that died together or could even come from an individual which had suffered some kind of embryonic malformation that kept its legs from developing normally). Paleontology is a science in which with every new discovery we can unravel the tree of life and the evolution of the different groups of living beings. With a little of luck, future discoveries will enable us to clarify the anatomy of Spinosaurus aegyptiacus and define the lifestyle of such a unique and extraordinary reptile.

REFERENCES

The following sources have been consulted in the elaboration of this entry:

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The kingdom of the reptiles: what is a dinosaur?

Dinosaurs (superorder Dinosauria, “terrible reptiles”) are a group of reptiles which dominated all terrestrial ecosystems during the Mesozoic (Secondary Era or “the Age of Reptiles”). Even today, to most people there’s still some confusion over what a dinosaur is and what is not, and the term “dinosaur” is often used to refer to all the reptiles that evolved during the Secondary Era. In this entry I’ll try to give account of some of the different groups of reptiles that appeared during the Mesozoic and I’ll explain the classification of the different dinosaurian groups and some of their adaptations.

MESOZOIC REPTILES: DINOSAURS AND LOTS MORE

The rise of the dinosaurs was possible thanks to a mass extinction phenomenon which occurred 251 million years ago (Permian-Triassic extinction event). That phenomenon annihilated up to 96% of marine species and up to 70% of terrestrial species in that time, leaving lots of empty ecological niches to be inherited by new animal species.

Sin título
Modified graphic from Rohde & Muller (2005) showing the great massive extinction. The darker zone corresponds to the Mesozoic period.

During the Triassic period (in the early Mesozoic) many different groups of reptiles evolved. One of these groups was the Dinosauria, which at that moment was far from being the dominant group of terrestrial animals. Some other reptilian groups of that time were the terrestrial rauisuchians (clade Rauisuchia) and fully aquatic groups like the sauropterygians (superorder Sauropterygia) and the ichthyopterygians (superorder Ichthyopterygia).

Reconstructions by Dmitry Bogdanov of Prestosuchus (a rauisuchian, top), Nichollsia (a suropterygian, left bottom) and Platypterigius (an ichthyopterygian, right bottom).
Reconstructions by Dmitry Bogdanov of Prestosuchus (a rauisuchian, top), Nichollsia (a suropterygian, left bottom) and Platypterigius (an ichthyopterygian, right bottom).

A second mass extinction in the late Triassic and the early Jurassic put an end to most of the dominant reptile groups, allowing the yet small dinosaurs to expand and evolve, along with some new groups like the crocodilomorphs (superorder Crocodylomorpha, ancestors of crocodilians), the flying pterosaurs (order Pterosauria).

Reconstructions by Dmitry Bogdanov of Dakosaurus (a crocodilomorph, top) and Scaphognathus (a pterosauria, bottom).
Reconstructions by Dmitry Bogdanov of Dakosaurus (a crocodilomorph, top) and Scaphognathus (a pterosauria, bottom).

As we can see, dinosaurs are only one of many reptile groups that evolved during the Mesozoic. During the Jurassic period, dinosaurs diversified into many different groups, but they were mostly restricted to terrestrial ecosystems, which they would rule until their practical extinction 65 million years ago at the end of the Cretacic period.

DINOSAUR CLASSIFICATION

The first dinosaurs evolved around 231 million years ago during the mid-Triassic period. They were small in size and were characterized by their limb’s posture, which contrary to most reptiles, grew vertically elevating their body from the ground. That gave them more agility and a more active lifestyle.

eorpohd
Top: Skeleton of Eoraptor, one of the oldest known dinosaurs (Museum of Japan, photo by Kentaro Ohno). Bottom: Representation of the posture common among most reptiles (left) and the posture characteristic of dinosaurs (right).

Then dinosaurs diverged into two different orders: the Saurischia and the Ornithischia. These two groups were distinguished by the structure of their pelvis; saurischians conserved a pelvis more closely similar to that of the other reptiles, while the ornithischians evolved a pelvis superficially similar to that of modern birds.

pjjhsfjsfdorni
Representation showing the structure of saurischian hips (left) and ornitischian hips (right). The animals represented are facing left.

ORNITISCHIA: BIRD HIPPED

ORNITHISCHIA
Evolutionary tree of Ornithischia, modified by Zureks.

Ornithopoda (“bird feet”): Ornithopods were the most diverse group of Ornithischia, characterized by their three-toed feet similar to that of birds. They were herbivores that could combine bipedal and quadruped walking. Among them we can find the Iguanodon, one of the first dinosaurs to be discovered by science.

IGUANODOOOONT
Iguanodon feet (right) and reconstruction by O. C. Marsh (1896).

Ornithopods acquired many different adaptations; some groups had duck-like bills to feed on aquatic vegetation, others developed specialized hands with a sharp thumb and an opposable little finger to grasp the plants they fed on. Many groups developed bony crests which are thought to be used both for species identification and for communication between members of the same species.

parasauro
Reconstruction of Parasaurolophus (missing author), an ornithopod which presented a big hollow crest to amplify the sounds it made.

Marginocephalia (“fringed heads”): The so-called marginocephalians were a group of herbivorous dinosaurs related to the ornithopods characterized by a great cranial ossification. These can be divided into two separated groups:

Pachycephalosaurians (suborder Pachycephalosauria, “thick-headed reptiles”) were bipeds which had an extremely thick skull and a series of lateral osteoderms (keratin-covered ossifications) flanking it. It is believed that pachycephalosaurians resolved territorial fights and disputed reproductive rights via head-ramming, similar to goats.

Pachycephalosauria_jmallon
Reconstruction of Pachycephalosaurus by Jordan Mallon.

The other members of the group are the ceratopsians (suborder Ceratopsia, “horned faces”), quadrupeds which presented; neck frills making their skulls look bigger and the “rostral bone”, which formed a beak-shaped structure on the mouth. Lots of species also developed facial horns which could protrude from the cheek-bone, the eyebrow or the neck frill.

CERAAA
Reconstruction of Rubeosaurus by Lukas Panzarin (left) and skull of Triceratops (right), photo by Zachi Evenor.

Thyreophora (“shield bearers”): This basal group of ornithischians was exclusively composed of quadruped herbivores characterized by the presence of heavy osteoderms that constituted their main defence. This group can be divided into:

Stegosaurians (suborder Stegosauria, “roofed reptiles”) were big herbivorous dinosaurs characterized by having two rows of dorsal osteoderms from the neck to the tail, which served as protection and helped them in their thermoregulation. Some species also developed caudal spines called “thagomizers” used as weapons to defend themselves from predators.

STEGo
Mounted “thagomizer” at Denver Museum of Nature and Science (left) and reconstruction of Stegosaurus by Nobu Tamura (right).

Anchylosaurians (suborder Ankylosauria, “fused reptiles”) developed heavy bony armours that covered most of the body. Some of them, like the Ankylosaurus, developed big bony clubs at the end of the tail to fend off predators.

Euoplocephalus_BW
Reconstruction of Euoplocephalus by Nobu Tamura.

SAURISCHIA: REPTILE HIPPED

SAURISCHIA
Evolutionary tree of Saurischia, modified by Zureks.

Sauropodomorpha (“reptile-shaped feet”): The sauropodomorphs are better known as the “long-necked dinosaurs”. That’s because they adapted to feed on the highest strata of vegetation.

Macronaria_scrubbed_enh
Reconstruction of different sauropodomorphs (left to right): Camarasaurus, Brachiosaurus, Giraffatitan and Euhelopus.

Most species became large quadrupeds, with pillar-like legs similar to those of elephants and long necks to reach the leaves of the highest trees. Later species reached tremendous sizes, like the Amphicoelias which could grow up to 60 metres long.

Theropoda (“beast feet”): This last group is mostly known for two reasons. First of all is that this group includes some taxons of great predators like the Tyrannosauridae and Dromeosauridae families. The second reason is that theropods are the only dinosaurian group that includes living species, because modern birds are included in the suborder Theropoda.

AllosaurusAMNH5753
Skeleton of Allosaurus from american museum collections (1915).

All theropods are bipedal and most of the Mesozoic species were carnivorous, with sharp replaceable teeth adapted to predation. Theropods present a saurischian pelvis but later on, birds evolved a hip structure more similar to that found in ornitischian dinosaurs.

Davide-Bonnadonna-theropod-size-evolution-600-px-tiny-July-August-Darren-Naish-Tetrapod-Zoology
Reconstruction by Davide Bonnadonna of the different clades that led to the aparition of birds (left to right): Neotheropoda, Tetanurae, Coelurosauria, Paraves and finally the Archaeopteryx, believed to be the first bird species that ever existed.

Some species had feathers to help thermoregulation. Birds from these groups evolved at the end of the Jurassic period.

REFERENCES

The following sources have been consulted in the elaboration of this entry:

If you enjoyed this article, please share it on social networks tospread it. The aim of the blog, after all, is to spread science and reachas many people as possible.

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