Arxiu d'etiquetes: bird

Reptiles and mammals: same origin, different stories

Did mammals evolve from reptiles? The truth is they didn’t. Reptiles and mammals both have independent evolutionary histories that separated soon after the apparition of the so-called amniotic egg, which allowed the babies of these animals to be born outside of water. Previously, we talked about the origin of vertebrates and about how they managed to get out of the sea to start walking on land for the first time. In this entry we’ll explain how the ancestors of reptiles and mammals, the AMNIOTES, became independent of the aquatic medium and became the dominant land animals.

THE AMNIOTIC EGG

The characteristic that unites reptiles and mammals in the same group is the amniotic egg. While amphibian eggs are relatively small and only have one inner membrane, the eggs of amniotes are much bigger and present various membranes protecting the embryo and keeping it in an aqueous medium. The outer layer is the eggshell which, apart from offering physical protection to the embryo, prevents water loss and its porosity allows gas interchange. Beneath the eggshell we can find the next membranes:

512px-Crocodile_Egg_Diagram.svgDiagram of a crocodile egg: 1. eggshell 2. yolk sac 3. yolk (nutrients) 4. vessels 5. amnion 6. chorion 7. air 8. alantois 9. albumin (white of the egg) 10. amniotic sac 11. embryo 12. amniotic fluid. Image by Amelia P.
  • Chorion: The first inner membrane, which offers protection and, together with the amnion, forms the amniotic sac. Also, being in contact with the eggshell, it participates in gas interchange, bringing oxygen from the outside to the embryo and carbon dioxide from the embryo to the outside.
  • Amnion: Membrane that surrounds the embryo and constitutes a part of the amniotic sac. It offers an aqueous medium for the embryo and connects it with the yolk sac (a structure that brings food and that is also found in fish and amphibians).
  • Allantois: The third layer, it is used as a storage for nitrogen waste products, and together with the chorion, helps in gas interchange.
512px-Amphibian_Egg_Diagram.svgDiagram of an amphibian egg: 1. jelly capsule 2. vitelline membrane 3. perivitelline fluid 4. yolk 5. embryo. Image by Separe3g.

All these different kinds of membranes eliminate the need amphibians had of laying their eggs in water. Also, unlike amphibians, amniotes don’t go through a gilled larval stage, but are instead born as miniature adults, with lungs and legs (at least those that have them). All these made the first amniotes completely independent of the aquatic medium.

AMNIOTE ORIGINS

The first amniotes evolved around 312 million years ago from reptiliomorph tetrapods. At the end of the Carboniferous period lots of tropical forests where the great primitive amphibians lived disappeared, leaving a colder and drier climate. This ended with many of the big amphibians of that time, allowing the amniotes to occupy new habitats.

Solenodonsaurus1DBReconstruction of Solenodonsaurus janenschi, one of the candidates in being the first amniote, which lived around 320-305 million years ago in what is now the Czech Republic. Reconstruction by Dmitry Bogdanov.

CHARACTERISTICS

These early amniotes had a series of characteristics that set them apart from their semiaquatic ancestors:

  • Horny claws (amphibians don’t have claws) and keratinized skin that prevents water loss.
  • Bigger large intestine and higher density of renal tubules to increase water reabsorption.
  • Specialized lacrimal glands and a third membrane in the eye (nictitating membrane) which keep the eye wet.
  • Larger lungs.
  • Loss of the lateral line (sensory organ present in fish and amphibians).

The skeleton and musculature also evolved offering better mobility and agility on a terrestrial medium. The first amniotes presented ribs that encircled their body converging at the sternum, making their inner organs more secure, and a series of muscular receptors offered them better agility and coordination during locomotion.

AMNIOTE SKULLS

Traditionally, the different amniotes were classified based on the structure of their cranium. The characteristic used to classify them was the presence of temporal openings (fenestrae), by which we have three groups:

  • Anapsids (“no arches”): No temporal openings (turtles).
Skull_anapsida_1Diagram of an anapsid skull, by Preto(m).
  • Synapsids (“fused arches”): With only one temporal opening (mammals).
Skull_synapsida_1Diagram of a synapsid skull, by Preto(m).
  • Diapsids (“two arches”): With two temporal openings (reptiles, including birds).
Skull_diapsida_1Diagram of a diapsid skull, by Preto(m).

Previously it was believed that the first amniotes presented an anapsid skull (without openings, like turtles) and that subsequently they separated into synapsids and diapsids (the temporal openings formed “arches” that offered new anchor points for the jaw’s musculature). Yet, it has been discovered that this three-group classification is not valid.

Even though we still believe that the first amniotes were anapsid, it is currently known that these, soon after their apparition, separated into two different lineages: the synapsids (clade Synapsida) and the sauropsids (clade Sauropsida).

SYNAPSIDA

This lineage includes mammals and their amniote ancestors. Even though the first synapsids like Archaeothyris looked externally like lizards, they were more closely related to mammals, as they shared one temporal fenestrae where the jaw muscles passed through.

Archaeothyris.svgDrawing of the skull of Archaeothyris, which is thougth to be one of the first synapsids that lived around 306 million years ago in Nova Scotia. Drawing by Gretarsson.

The ancestors of mammals were previously known as “mammal-like reptiles”, as it was thought that mammals had evolved from primitive reptiles. Currently it’s accepted that synapsids form a different lineage independent of reptiles, and that they share a series of evolutionary trends that makes them closer to modern mammals: the apparition of different kinds of teeth, a mandible made of one single bone, the vertical posture of their limbs, etc…

Dimetrodon_grandisReconstruction of Dimetrodon grandis, one of the better known synapsids, from about 280 million years ago. Reconstruction by Dmitry Bogdanov.

Even though most modern mammals don’t lay eggs and give birth to live offspring, all groups maintain the amniote’s three characteristic membranes (amnion, chorion and allantois) during embryonic development.

SAUROPSIDA

Sauropsids include current reptiles and their amniote ancestors. Currently, in many scientific papers the word “sauropsid” is used instead of “reptile” when discussing phylogenies, as the sauropsids also includes birds. The first sauropsids were probably anapsids, and soon after their appearance they separated into two groups: the Parareptilia which conserved anapsid skull, and the Eureptilia which include the diapsids (current reptiles and birds).

Traditional_ReptiliaEvolutionary tree of current vertebrates, in which green color marks the groups previously included inside reptiles. As you can see, the traditional conception of "reptile" includes the ancestors of mammals and excludes birds. Image by Petter Bøckman.

Diapsids are currently the most diversified group of land vertebrates. They diversified greatly in the late Permian period (about 254 million years ago), just before the Mesozoic (the Age of Reptiles). These can be divided into two main groups: the Lepidsaurs and the Archosaurs, both with representatives in our days.

LEPIDOSAURIA: SMALL AND PLENTIFUL

Lepidosaurs (literally “reptiles with scales”) appeared in the early Triassic (around 247 million years ago) and, even if most of them didn’t grow to big sizes, they are currently the largest group of non-avian reptiles. These are characterized by presenting a transversal cloacal slit, by having overlapping scales and shedding their skin whole or in patches and by other skeletal characters.

Rat_Snake_moulted_skinShed skin of a rat snake. Photo by Mylittlefinger.

The current lepidosaurs belong to one of two different orders:

  • Order Rhynchocephalia: That includes the two species of tuatara. Currently endangered, they are considered living fossils because they present skulls and characteristics similar to the Mesozoic diapsids.
Sphenodon_punctatus_(5)Photo of a tuatara (Sphenodon punctatus), by Tim Vickers.
  • Order Squamata: Current squamates include iguanas, chameleons, geckoes, skinks, snakes and other legless lizards. With more than 9000 living species, squamates are a large group with a wide array of adaptations and survival strategies.
Sin títuloPhotos of some squamates, from left to right and from top to bottom: Green iguana (Iguana iguana, by Cary Bass), king cobra (Ophiophaga Hannah, by Michael Allen Smith), Mexican mole lizard (Bipes biporus, by Marlin Harms) and Indian chameleon (Chamaeleo zeylanicus, by Shantanu Kuveskar).

ARCHOSAURIA: ANCIENT KINGS

Archosaurs (literally “ruling reptiles”) were the dominant group of land animals during the Mesozoic. These conquered all possible habitats until the extinction of most groups at the end of the Cretaceous period. Some of the extinct groups were the pseudosuchians (relatives of modern crocodiles, order Crocodylia), the pterosaurs (large flying reptiles) and the dinosaurs (excepting birds, clade Aves).

Massospondylus_Skull_Steveoc_86Drawing of the skull of the dinosaur Massospondylus in which we can see the different characteristic openings of diapsid archosaurs. Image by Steveoc 86.

As you see, both groups of modern archosaurs couldn’t be more different. Yet, crocodiles and birds share a common ancestor, and they are both more closely related with each other than with the rest of reptiles.

Yellow-billed_stork_kazingaPhoto of two species of modern arcosaurs: a Nile crocodile (Crocodylus niloticus) and a yellow-billed stork (Mycteria ibis). Photo by Tom Tarrant.

AND WHAT ABOUT TURTLES?

Turtles (order Testudines) have always been a group difficult to classify. Turtles are the only living amniotes with an anapsid skull, without any post-ocular opening. That’s why previously they had been classified as descendants of primitive amniotes (clade Anapsida, currently disused) or as primitive anapsid sauropsids (inside the Parareptilia clade)

KONICA MINOLTA DIGITAL CAMERASkeleton of the extinct tortoise Meiolania platyceps which lived in New Caledonia until 3000 years ago. In this photo it can be seen the compact cranium without openings. Photo by Fanny Schertzer.

Recent molecular studies have revealed that turtles are actually diapsids that lost their temporal openings secondarily. What still divides the scientific community is if testudines are more closely related to Lepidosauromorphs (lepidosaurs and their ancestors) or to Archosauromorphs (archosaurs and their ancestors).

Leopard_tortoiseIndividual leopard tortoise (Stigmochelys pardalis) from Tanzania. Photo by Charles J. Sharp.

As you have seen, the evolution of amniotes is an extremely complex matter. We hope that with this entry some concepts have been clarified:

  1. Mammals (synapsids) come from an evolutionary lineage different from that of reptiles (sauropsids).
  2. Sauropsids include traditional reptiles (lepidosaurs, archosaurs and turtes) and birds (inside archosaurs).
  3. There’s still so much to investigate about the placement of turtles (testudines) in the evolutionary tree of sauropsids.
Figure_29_04_03Modified diagram about the evolutionary relationships of the different amniote groups.

REFERENCES

During the elaboration of this entry the following sources have been consulted:

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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.

Parts_of_feather_modified

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.

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References

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