Arxiu de la categoria: AMPHIBIANS

Herpetological parachuting: gliding amphibians and reptiles

Currently, the only flying reptiles are birds, direct descendants of theropod dinosaurs. Although the age of the great flying reptiles has passed, nowadays, various species of reptiles and amphibians have acquired the ability of gliding to escape their predators. Gliding is defined as falling at an angle less than 45o from the horizontal with the help of membranes that create resistance to the air. In this entry I’ll show you some gliding herp species which currently exist.

Gliding frogs

Gliding frogs (also called “flying frogs”) include species from the Polypedates, Rhacophorus (Rhacophoridae family) and Ecnomiohyla (Hylidae family) genera. These have gained similar characteristics through a process of convergent evolution.

Ecnomiohyla_rabborumPhoto of Ecnomiohyla rabborum by Brian Gratwicke.

Both hylids and rhacophorids are popularly known as tree frogs. Their limbs are specialized for an arboreal lifestyle, with long legs and fingers with sucker-like structures for a better grip.

 Male and female false Malabar's gliding frogs (Rhacophorus pseudomalabaricus) mating. Video by Sandesh Kadur.

The gliding genera have also acquired big membranes on its limbs and between their toes to help them glide and therefore, being able to escape predators more efficiently.

frog_m_1804347aWallace's flying frog (Rhacophorus nigropalmatus) gliding.

Gliding geckoes

Between the members of the Gekkonidae family there are two Southeast Asian genera which have acquired adaptations for gliding: the Ptychozoon genus and the Luperosaurus genus.

P1100785Photo of a Kuhl's flying gecko (Ptychozoon kuhli) by Bernard Dupont.

Geckoes are a group of lizards which have evolved for an arboreal lifestyle which allows them to adhere practically on any surface. Their feet present tiny filaments which allow them to move even upside down.

Ptychozoon_kuhli_mâleDetail of the underside of a Kuhl's flying gecko (Ptychozoon kuhli) in which the skin flaps can be appreciated. Photo by Fenchurch.

The Ptychozoon and Luperosaurus genera also present membranes on their neck, body, limbs and tail that help them to blend in the surface of trees and also to glide at some extent from tree to tree to escape possible predators.

Flying snakes

Speaking of predators, the snakes from the Chrysopelea genus also have developed an efficient method to move through the rainforest’s canopy. The snakes from this genus are diurnal, feed on lizards, frogs, birds and bats, and are found throughout Southeast Asia.

Chrysopelea_paradisi_(6032067972)Couple of Paradise flying snakes (Chrysopelea paradisi) in the Singapore Zoo, by Alan Couch.

Unlike the former gliding herps, flying snakes have no membranes to slow down their descent, instead they have a more complex method. When arriving at the tip of a tree branch, these snakes drop themselves. After a brief fall, they retract their inner organs, compressing them against their thoracic cavity and flaring out their ribs laterally, taking a semi-concave shape, similar to that of a plane’s wing.

biomechanics_1Explicative image of the gliding mechanism of the flying snakes. Image from Biomechanics.

With this method and with the help of serpentine movements, the snakes of the Chrysopelea genus can control with great precision the direction of their descent. These snakes have a more controlled glide than many gliding mammals such as the gliding squirrel and are able to glide through a horizontal length of up to 100 metres.

 Group of scientists testing a Paradise flying snake’s (Chrysopelea paradisi) ability to glide. Video by All of These Videos.

Flying dragons

And we finally get to the most spectacular of all flying herps, the reptiles known as flying dragons. These agamids (Agamidae family) of the Draco genus are found in the tropical forests of Asia, where they survive hunting insects on the forest canopy.

Sans nom-399Photo of a five-lined flying dragon (Draco quinquefasciatus) from Sarawak, Malaysia. Image by Bernard Dupont.

The main characteristic of flying dragons is their ribs, some of which are extremely elongated and present dermal membranes between them acquiring the function of wings. These “wings” are usually retracted against the body and can be erected for both gliding and sending visual signals to other members of their species (wings are usually brightly coloured).

Flying_Dragon_MivartDrawing from the book On The Genesis of Species of the skeleton of a Draco volans.

Flying dragons use their wings to move from tree to tree, to hunt, to escape predators, to chase their own kind during both territorial disputes and courtship. Aside from their brightly coloured wings, many species also present colourful dewlaps (especially males) to indicate their reproductive state to other members of their species.

Draco_spilonotusPhoto of a Sulawesi's lined flying dragon (Draco spilonotus) by A. S. Kono.

The flight record of these agamids is of 60 metres of distance with a vertical descent of only 10 metres. Flying dragons are small, fast and active animals, so few predators are able to hunt them. In addition, they are totally arboreal with only females descending to the ground to lay their eggs underground.

 Flying snake chasing a flying dragon. Video found in Venomous Animals.

As we have seen, most species of gliding amphibians and reptiles live in tropical climates. This is due to the fact that these are habitats with a dense vegetation cover and trees grow very close to one another, allowing these animals to glide from one tree to the other easily. The main threats to these creatures are deforestation and habitat loss since, without an optimal vegetation cover, these animals may be preyed easily by terrestrial predators.

References

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

Difusió-anglès

Anuncis

Frogs, toads and newts: the last amphibians

With about 7000 living species, amphibians currently occupy almost all the habitats on Earth. While in the last entry we explained the origin of the first tetrapods and how those gave rise to the different groups of primitive amphibians, in this entry we will explain in more detail the characteristics of current amphibians, the so-called lissamphibians.

AMPHIBIANS AND LISSAMPHIBIANS

The term “Lissamphibia” (“smooth amphibian”) is used to name current amphibians and it’s useful to tell them apart from the rest of fossil amphibians, while the term Amphibia (“double life” referring to the aquatic larval stage of most species), is used to name all tetrapods except the amniotes (reptiles, birds and mammals).

Most authors consider lissamphibians a monophyletic group (a group which includes all the descendants of a common ancestor) which includes the different groups of modern amphibians. The main characteristics of this group are:

Dermal characteristics

  • Smooth, scaleless, permeable skin that allows gas exchange (both pulmonary and cutaneous respiration) and the absorption of water (most amphibians usually do not need to drink water). This makes them susceptible to skin infections like the one from the Batrachocytrium dendrobatidis fungus.
FrogSkinSection through frog skin by Jon Houseman. A: Mucous gland, B: Chromophore, C: Granular poison gland, D: Connective tissue, E: Stratum corneum, F: Transition zone, G: Epidermis, and H: Dermis.
  • Two types of skin glands: mucous (the majority, to maintain humidity) and granular (less numerous, secrete toxins of different intensity).

Skeletal characteristics

  • Pedicellate and bicuspid teeth.
teethPhoto of pedicellate teeth, in which the crown and base are made of dentine and are separated by a narrow layer of uncalcified dentine.
  • A pair of occipital condyles.
  • Short, stiff ribs not encircling the body.
  • Four digits on the front limbs and five digits on the hind limbs.
10050622254_8cffbfb0e4_oSkeleton of giant salamander in which we can see some of the characteristics of lissamphibians. Photo by Graham Smith.

Auditory characteristics

  • Papilla amphibiorum, a group of specialized cells in the inner ear which allow them to hear low frequency sounds.
  • Stapes-operculum complex which are in contact with the auditory capsule, improve reception of aerial and seismic waves.

Other characteristics

  • Fat bodies associated with gonads.
  • Presence of green rods in the visual cells (these allow the perception of more colours).
  • Presence of a muscle elevator of the eye (called levator bulbi).
  • Forced-pump ventilation system (their short ribs do not allow pulmonary ventilation, so they pump the air through their mouth).
Two_stroke_buccal_pumpingExplicative diagram about buccal ventilation in lissamphibians, by Mokele.

TAXONOMY AND EVOLUTIONARY THEORIES

Nowadays only three living amphibian orders persist: the order Salientia or Anura (which includes frogs and toads), the order Caudata or Urodela (salamanders and newts) and the order Gymnophiona or Apoda (caecilians). The second name of each order refers to the current species and their recent ancestors, while the first name refers to the whole order since the separation of each order.

There are two hypotheses regarding the relationships between the three orders. The most accepted both by anatomic and molecular analyses is that Salientia and Caudata are grouped together into the clade Batrachia, while the other one is that Caudata and Gymnophiona together form the clade Procera.

Batrachia proceraTwo hypothetical evolutionary trees by Marcello Ruta & Michael I. Coates (2007), showing the Batrachia and Procera hypotheses on the relationships between Salientia (S), Caudata (C) and Gymnophiona (G).

Currently there are three groups of hypotheses of the origin of lissamphibians: the temnospondyl hypotheses, the lepospondyl hypotheses and the polyphyletic hypotheses.

Temnospondyls are the main candidates to be the ancestors of lissamphibians, as they share many characteristics, such as the presence of pedicellated, bicuspid teeth, and short, stiff ribs. Authors defending these theories say that lissamphibians suffered during their evolution a process known as paedomorphosis (retention during the development of juvenile characteristics), this way explaining why temnospondyls reached such large sizes while lissamphibians are much smaller and usually have lighter and less ossified cranial structures.

temnospondyliDrawings from Marcello Ruta & Michael I. Coates (2007) of skeletons belonging to Celteden ibericus (left, a lissamphibian) and Apateon pedestris (right, a temnospondyl) to show similitudes in skeletal structure.

Hypotheses regarding a lepospondyl origin for lissamphibians do not have such a strong support as the temnospondyl hypotheses. However, recently some statistical studies combining anatomic and molecular data have given some support to these hypotheses.

Nevertheless, there is a third group of hypotheses we must consider, the ones that say that lissamphibians are a polyphyletic group (with different origins for the different orders). According to one of these theories, frogs and salamanders (clade Batrachia) would have a temnospondyl origin, while caecilians (order Gymnophiona or Apoda) would have originated from lepospondyl ancestors, many of which had already suffered a limb reduction process.

 Lissamphibian_phylogenyModified outline of the three different hypotheses regarding the origins of the lissamphibians; 1. Lepospondyl origin, 2. Temnospondyl origin, 3. Polyphyletic origin.

Still, most authors support a monophyletic and temnospondyl origin for lissamphibians, but alternative hypotheses shouldn’t be discarded.

SALIENTIA OR ANURA

With up to 4750 species, frogs and toads form the most diverse lissamphibian order. The first known Salientia is Triadobatrachus, which, despite having a tail, already presented some typical characteristics of modern frogs, such as a short spine with few vertebras and the hind limbs longer than the front limbs.

TriadobatrachusInterpretation by Pavel Riha, of the ancient Salientia, Triadobatrachus massinoti.

The anatomy of modern anurans is unique among the animal kingdom. Their skeleton seems totally dedicated to allow these animals to jump (even though many species move simply by walking). Some of their characteristics are:

  • A short and stiff trunk (less than 12 vertebras), an especially long pelvic girdle and the vertebras of their posterior end (that in other amphibians form the tail) are reduced and fused forming the urostyle.
  • Long hind limbs, with the tibia and fibula fused together (to aid in impulse during jumping) and short and strong front limbs (to resist the impact on the landing).
3888291918_f779053a0a_oPhoto of a pig frog (Rana grylio), a typical american anuran.

Also, of all current amphibians frogs are the ones with the most developed hearing apparatus and vocal organ. Males, usually present specialized structures to amplify sound during the mating season.

Litoria_chloris_callingRed eyed tree frog (Litoria chloris) showing the vocal sac, used to amplify the sound of its calls.

Size in anurans varies from 3 kg in weight and 35 centimetres in length of the goliath frog (Conraua goliath) to the 7, 7 millimeters long recently discovered Paedophryne amanuensis, currently the smallest known vertebrate.

Paratype_of_Paedophryne_amauensis_(LSUMZ_95004)Photo from Rittmeyer EN, Allison A, Gründler MC, Thompson DK, Austin CC (2012)  of Paedophryne amanuensis, the smallest known vertebrate in the world on a US dime.

With such a diversity, vital strategies of anurans vary greatly and it’s difficult to generalize on their reproductive biology, even though most show indirect development (born as tadpoles and passing through a metamorphosis process) and they mate and lay their eggs in an aquatic medium.

BufoBufoTadpolesTadpoles of common toad (Bufo bufo) from northern Germany by Christian Fischer.

URODELA OR CAUDATA

The urodeles or caudates are the order of lissamphibians which externally most resemble primitive amphibians. This group includes salamanders and newts, most of which have a long body, a well-developed tail and four relatively short legs. Most urodeles are terrestrial and are distributed mainly in the northern hemisphere, with a few species inhabiting the tropics.

Salamandra_TigrePhoto of an eastern tiger salamander (Ambystoma tigrinum) from the House of Sciences, Corunna - Spain. Taken by Carla Isabel Ribeiro.

Most species present internal fertilization and are oviparous. Most also present indirect development (larvae, metamorphosis, adult), and the larvae usually resemble miniaturized adults with external, ramified gills. Various groups of salamanders suffer neoteny phenomenon, in which individuals, even though sexually developing into adults, externally keep larval characteristics.

Joung_and_very_large_larva_of_Salamandra_infraimmaculata,_Ein_Kamon,_IsraelYoung and very large larvae of near eastern fire salamander (Salamandra infraimmaculata), Ein Kamon, Israel. Photo by Ab-Schetui.

Currently, urodeles are classified into three suborders: the Sirenoidea, the Cryptobranchoidea and the Salamandroidea. Sirenoideans are urodeles with both specialized and primitive characteristics, such as the loss of hind limbs and the presence of external gills. Cryptobranchoideans are large primitive salamanders (up to 160 centimetres) which present external fertilization, while salamandroideans are the most numerous group of urodeles (with more than 500 species) and the most diverse, with most species being terrestrial and having internal fertilization using packs of sperm called spermatophores.

20090924201238!P_striatus_USGSPhoto of a northern dwarf siren (Pseudobranchus striatus) a sirenoidean from the United States.

GYMNOPHIONA OR APODA

The most ancient known member of the order Gymnophiona is Eocaecilia micropodia, an amphibian about 15 centimetres long with a considerably long body, a short tail and really small limbs.

Eocaecilia_BWRestoration by Nobu Tamura of Eocaecilia micropodia an ancient Gymnophiona from the early Jurassic.

Current caecilians (order Apoda) have completely lost any trace of limbs, girdles or tail, due to their adaptation to a subterranean lifestyle. That’s why they also suffered a process of cranial hardening and their eyes are extremely reduced. They also present a series of segmentary rings all along their bodies, which make them look somewhat like earthworms.

Ichthyophis kohtaoensis, ca 12Yellow-striped caecilian (Ichthyophis kohtaoensis) from Thailand, by Kerry Matz.

There are currently about 200 species of caecilians divided into 10 families. Their size varies from about 7 centimetres in the species Idiocranium russelli from Cameroon, to up to 1,5 meters of Caecilia thompsoni from Colombia. They present a pantropical distribution, internal fertilization and a great variation in their development (there are viviparous and oviparous species and some which endure metamorphosis while some have direct development).

KONICA MINOLTA DIGITAL CAMERAPhoto of Gymnopis multiplicata an american caecilian. Photo by Teague O'Mara.

REFERENCES

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

 Difusió-anglès

The evolution of amphibians: the conquest of the land

Amphibians were the first group of vertebrates to develop limbs and to be able to leave the water to conquer the land. Even if they are seen as simple and primitive animals by most people, amphibians show a wide diversity of survival strategies which have allowed them to occupy most terrestrial and fresh-water habitats. On this entry we’ll explain some of the aspects related to their evolution, explaining how our ancestors managed to get out of the water.

ORIGIN OF THE AMPHIBIANS

Current amphibians, together with reptiles, birds and mammals are found within the superclass Tetrapoda (“four limbs”), the vertebrate group that abandoned the sea to conquer the land. These first tetrapods were amphibians and they evolved around 395 million years ago during the Devonian period from lobe-finned fish named sarcopterygians (class Sarcopterygii, “flesh fins”) within which we find the coelacanth and the current lungfish.

6227540478_88c4b03cd2_o
Specimen of coelacanth (Latimeria chalumnae) a sarcopterygian fish, photo by smerikal.

This group of fish is characterized by its fins which, instead of being formed by rays like in most bony fish, they have a bony base that allowed the subsequent evolution of the limbs of the first amphibians. Within the sarcopterygians, the nearest relatives of the tetrapods are the osteolepiformes (order Osteolepiformes) a group of tetrapodomorph fish that got extinct about 299 million years ago.

Eusthenopteron_BWRestoration of Eusthenopteron, an extinct osteolepiform, by Nobu Tamura.

ADAPTATIONS TO LIVE ON LAND

The conquest of land was not done from one day to the other; it was possible with the combination of multiple adaptations. Some of the most important characteristics that allowed the first amphibians to leave the water were:

  • Evolution of lungs, which are homologous to the gas bladder that allows fish to control its buoyancy. Lungs appeared as an additional way to get oxygen from the air. In fact, there is actually a sarcopterygian family the members of which have lungs to get oxygen from the air, for they live in waters poor on oxygen.
  • Lungs_of_Protopterus_dolloiDissection of Protopterus dolloi a sarcopteryigian fish with lungs.
  • Development of the choanaes, or internal nostrils. While fish present a pair of external nostrils at each side of its snout through which water passes on while swimming, the ancestors of the tetrapods only had one external nostril at each side connected to the internal nostrils, the choanae, which communicated with the mouth. This allowed them to get air through their noses using lung ventilation and this way to smell outside of water.
  • Apparition of the quiridium-like limb. The quiridium is the tetrapod’s most basic characteristic. This limb is known for having the differentiated parts: the stylopodium (one bone, the humerus or the femur), the zeugopodium (two bones, the radius or tibia and ulna or fibula) and the autopodium (fingers, hands, toes and feet). While the stylopodium and zeugopodium derived from the sarcopterygian’s fins, the autopodium is a newly-evolved structure exclusive from tetrapods.
Quiridio
Simplified drawing of the structure of the quiridium, by Francisco Collantes.

In short, the relatives of the osteolepiformes developed the tetrapod’s typical characteristics before ever leaving water, because they probably lived in brackish, shallow waters, poor in oxygen and that dried out quickly and often.

THE FIRST AMPHIBIANS

Probably the creature known as Tiktaalik is the closest animal to the mid-point between the osteolepiformes and the amphibians. The first recorded amphibians were labyrinthodonts meaning that their teeth had layers of dentin and enamel forming a structure similar to a maze.

Labyrinthodon_MivartCross-section of a labyrinthodont tooth, form "On the Genesis of Species", by St. George Mivart.

There were four main groups of primitive amphibians, each characterized by: a group that includes the first animals that were able to get out of water, a second group which contains the ancestors of the amniotes (reptiles, birds and mammals) and two more groups, both candidates to be the ancestors of modern amphibians.

Order Ichthyostegalia

Ichthyostegalians were the first tetrapods to be able to leave the water. They appeared at the late Devonian period and they were big animals with large wide heads, short legs and an aquatic or semi aquatic lifestyle (they probably were pretty clumsy on land). They moved around using mainly their muscular tail with rays similar to that of fish.

5212816060_da1a11e94e_oFossil and restoration of Tiktaalik. Photo by Linden Tea.

Similarly to current amphibians, they presented a lateral line (sensory organ that allows fish to detect vibrations and movement underwater) and were able to breathe through their skin (they lost the cosmoid scales of their ancestors). Also, the eggs were laid in the water, from which the tadpoles emerged and later on, they suffered a metamorphosis process to become adults just like current amphibians. Subsequently ichthyostegalians gave rise to the rest of amphibian groups.

ichthyostega(1)Skeletons of Ichthyostega and Acanthostega, two typical ichthyostegalians.

Clade Reptiliomorpha

Reptiliomorphs were the ancestors of amniotes and appeared about 340 million years ago. Most of them were usually large and heavy animals, which presented more advanced adaptations to live on land (laterally-placed eyes instead of dorsally-placed ones and a knobby more impervious skin). Even though, reptiliomorphs still laid their eggs in the water and had larval-stages with gills. It wouldn’t be until the late Carboniferous period when the first amniotes (animals that could lay their eggs on dry land) would emancipate completely from water.

Diadectes_phaseolinusMounted skeleton of Diadectes a large herbivorous reptiliomorph from the American Museum of Natural History, photo by Ghedoghedo.

Order Temnospondyli

This group is one of the possible candidates to being the ancestors of modern amphibians. This is the most diverse group of primitive amphibians and it survived until the early Cretaceous period, about 120 million years ago. The temnospondyls varied greatly in shape, size and lifestyle.

Eryops1DBRestoration of Eryops megacephalus a large temnospondylian predator, by Dmitry Bogdanov.

Most of them were meat-eaters, but some were terrestrial predators, some were semi aquatic and some had returned completely to water. Even though, all species had to return to water to breed for the fertilization was external; while the female was laying clutches of eggs in the water, the male released the sperm over them.

Buettneria
Mounted skeleton of Koskinonodon a 3 metres long temnospondyl, from the American Museum of Natural History, photo by Lawrence.

Within the temnospondyls we can find some of the biggest amphibians that ever lived, such as Prionosuchus, with an estimated length of 4,5 meters and about 300 kilograms of weight. Also, even though their skin was not covered with scales, it wasn’t completely smooth like in modern amphibians.

Prionosuchus_DBRestoration of Prionosuchus by Dmitry Bogdanov.

It is believed that this group could be the sister-taxon of modern amphibians, even though there’s one last group which could be a candidate to that post.

Order Lepospondyli

Lepospondyls were a small group of primitive animals which appeared at the early Carboniferous and disappeared at the late Permian period. Even though lepospondyls were not as numerous and smaller than the temnospondyls, they presented a wide range of body shapes and adaptations.

Diplocaulus_BWRestoration of Diplocaulus magnicornis, of about 1 metre long was the biggest of all lepospondyls, by Nobu Tamura.

The first lepospondyls looked superficially like small lizards, but subsequently lots of groups suffered processes of limb reduction or loss.

Pelodosotis1DBRestoration of Pelodosotis, an advanced lepospondyl, by Dmitry Bogdanov.

The relationship of the lepospondyls with the rest of tetrapods isn’t very clear. Different hypothesis go from some authors arguing that they are a group separated from the labyrinthodonts, some thinking that they are the ancestor of current amphibians and reptiles, and some even saying that they are the ancestors of only a portion of modern amphibians.

LysorophusRestoration of Lysorophus, a Permian lepospondyl, by Smokeybjb.

As we can see, the classification of primitive amphibians can be an extremely complex thing. On this entry I tried to make a summary of the most important groups of ancient amphibians and, on the next one, we’ll center on the evolution of modern amphibians, the so-called “lissamphibians”, and we’ll look in more detail all the controversies surrounding these curious animals.

REFERENCES

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

Difusió-anglès