Amniota
From Palaeos
| REPTILIOMORPHA | |
| Taxonomy | Phylogeny |
Subphylum: Vertebrata
|
o Tetrapoda crown group `--o REPTILIOMORPHA |--Embolomeri (=Anthracosauria) |?--Chroniosuchia `--o?--Gephyrostegidae `--+--Seymouriamorpha `--+--Westlothiana lizziae `--o--Diadectomorpha `--+--Casineria kiddi `--o Amniota |--Sauropsida `--Synapsida |
Contents |
[edit] Introduction
The amniotes are the most successful clade of tetrapods. They are defined by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis ref. The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments, or the massive size and yolk content of eggs designed for direct development to a larger size.
However this does not mean that early amniotes themselves had necessarily moved to more terrestrial environments, as some or even many early reptile ecosystems remained close to water at least until the later Middle Permian.
Following the evolutionary emergance of the amniotes, three main lines developed. These were the Eureptiles, which soon gave rise to the diapsids, the Anapsida or Parareptilia, which included an essemblage of diverse small forms (one of which, the pareiasaurs, grew to large size) and the Pelycosauria, constituting the first evolutionary radiation of the Synapsida (mammal-like reptiles and mammals). Thus within a short time the earliest members of all of the major groups had evolved.
| AMNIOTA | |
| Taxonomy | Phylogeny |
Subphylum: Vertebrata |
o Cotylosauria |--Diadectomorpha `--+?--Casineria `--o AMNIOTA |--o Sauropsida | |--Anapsida/Parareptilia | `--Eureptilia `--Synapsida |
[edit] Taxonomy
The following taxonomy includes all living and some fossil groups. Indenting roughly follows the phylogenetic arrangement, but with the addition of Linnaean ranks.
- Series Amniota (true amniotes)
- Class Sauropsida - Reptiles
- Subclass Anapsida/Parareptilia
- Order Mesosauridae (uncertain placement)
- Order Procolophonia
- Order Testudines (turtles) (traditional placement)
- (unranked) Eureptilia
- Family Captorhinidae
- Family Protorothyrididae (paraphyletic as traditionally defined)
- Subclass Diapsida
- Order Araeoscelida
- Infraclass Lepidosauria
- Order Sphenodontida (tuatara)
- Order Squamata (lizards & snakes)
- Infraclass Archosauria
- Order Crocodilia (crocodiles)
- Superorder Dinosauria (Dinosaurs)
- Class Aves - Birds
- Subclass Anapsida/Parareptilia
- Class Synapsida - Mammal-like reptiles/Theropsids(beast faces)
- Order Pelycosauria (paraphyletic)
- Order Therapsida (paraphyletic)
- Class Mammalia - Mammals
- Order Therapsida (paraphyletic)
- Order Pelycosauria (paraphyletic)
- Class Sauropsida - Reptiles
[edit] Characteristics
[edit] General characteristics
Lungs: complex and in-folded, joined pharynx by trachea with cartilaginous support. Lungs used for CO2 dumping as well as O2 intake due to keratinized skin.
Neck: Often lengthened and more flexible.
Head: Buccal pumping eliminated, so head can be smaller and more domed.
Skull: (Captorhinid) Like "Anthracosaurs" but no otic notch or intertemporal. Postparietal, tabular and supratemporal reduced and on occipital surface only. Supraoccipital supports posterior of braincase, large stapes supports it laterally. Transverse flange on pterygoid. Elimination of large fangs. Basicranial articulation with palate moveable. Basioccipital and exoccipital form occipital condyle. Fits in ring formed intercentra and arches of atlas: ball and socket joint. Palatoquadrate reduced to quadrate and epipterygoid. Lower jaw has 1-2 coronoids and splenial.
Vertebrae: Spool-shaped centra. Small, crescentic intercentra.
Ventral axial muscles: Development of intercostals used to move ribs in respiration.
Ribs: lighter and may be joined ventrally by sternum as specialization for intercostal ventilation. Postural role assumed by epaxial muscles which are no longer primary locomotor muscles.
Limb bones: lighter – possibly reflecting proprioreceptor system. Feet used as levers for propulsion, rather than holdfasts. Ankle forms distinct hinge joint (mesotarsal). Tibiales, other bones of pes fuse to form astragalus. 23453 manus, 23454 pes.
Pelvis: Sacrum expanded from one vertebra to 2-3.
Hearing: convergent development of stapes (hyomandibula) as principal sound conduction mechanism of middle ear (i.e. connects tympanum with inner ear).
Excretion: Duct linking kidney & cloaca. Bladder not used as much for water recovery. This function tends to be performed by kidney.
Amniotic Egg: additional membranes (amnion, allantois chorion) in egg act to permit gas exchange but avoid water loss, permit large amounts of yolk storage, isolate waste products during development.
Features of amniotes designed for survival on land include a sturdy but porous leathery or hard eggshell, and an allantois designed to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta.
The skeletal remains of amniotes can be identified by their having at least two pairs of sacral ribs and an astragalus bone in the ankle.
[edit] Characters
Technical characters include: premaxilla with palatal, maxillary & nasal processes [MR05]; frontal contacts orbit; various patterns of fenestration related to additional musculature for jaw from dermal skull and development of musculature to supply static pressure at jaw; squamosal contributes to margin of posttemporal fenestra; hemispherical & ossified occipital condyle; pterygoid with distinct palatal surface, transverse flange and quadrate ramus [MR05]; pterygoid quadrate ramus with separate dorsal flange extending from basicranial articulation to dorsal process of quadrate, supporting elongate epipterygoid [MR05]; loss of labyrinthodont teeth, caniniform tooth present on maxilla; 2 centers of ossification in scapulocoracoid; astragalus present. Numerous additional characters listed above.
References: Müller & Reisz (2005) [MR05].
[edit] Amniota ecology
A lot is said about Amniotes being able to exploit terrestrial environments. This was certainly true at least to those species with the necessary adaptations. But most early reptiles seem to have remained tied to water - Sphenacodontids such as Dimetrodon (above) and other early Permian reptiles frequented pond and river margins, Edaphosaurs may have been semi-aquatic, Ophiacodon and perhaps also Secodontosaurus were semi-aquatic piscivores, and Mesosaurs were already fully freshwater aquatic. Even well into the Middle Permian, Parieasaurs and some dinocephalian ecosystems remained semi-aquatic (Olson, 1962).
So just as the evolution of legs may have had nothing to do with life on land, and terrestriality only came later, so the amniote egg may have been an adaptation of largely semi-aquatic animals. The real advantage of the amniote egg would be that the eggs, hidden on land, are safe from preditors.
More fully terrestrial animal ecosystems only began to appear in the Roadian and Wordian ages, where a complex caseid-captorhinid - basal therapsid community emerged, and only became important from the following Capitanian age onwards, when both small and large herbivores for the first time exploited fully terrestrial ecological niches, thrus providing a food source for carnivores, and breaking the previous aquatic-detrivore dependence on fresh-water ecosystem invertebrates.
[edit] Evolutionary History
[edit] The Emergence of Amniotes
Amniotes evolved some time during the Early Caboniferous (Late Visean or Serpukhovian). Their closest known relatives are the diadectomorphs. Casineria kiddi could be even closer, but is poorly known and has not been described in detail. (Westlothiana lizziae, originally heralded as the first amniote, is probably the basalmost lepospondyl instead: Vallin & Laurin 2004, Ruta & Coates 2007.)
By Bashkirian times, true amniotes had already appeared, represented by the earliest known sauropsid, Hylonomus lyelli, and the earliest known synapsid (although this is disputed because of the incomplete specimen), Protoclepsydrops haplous.
Thus we are looking at an evolutionary succession or continuum, at some point of time along which there was an evolutionary breakthrough represented by the amniote egg.
How this happened is still a matter of some speculation. The following account from Wikipedia, which suggests first a small soft shell, and then a larger hard shell, is as good as any:
[edit] Soft to Hard Shells
The first amniotes looked like small lizards, and their eggs were small and covered with a membrane, not a hard shell like often seen today. Some modern amphibians lay eggs on land but without any protection to speak of, while others like some lungless salamanders and Amphiuma also lay small eggs on land, but these are covered by a rubber-like membrane even if they lack advanced traits like an amnion. This kind of egg became possible with internal fertilisation. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land. It was probably because the embryos were safer on land than in water that some species got the habit of laying them out of the water. One can assume the ancestors of the amniotes laid their eggs in moist places, as such modest-sized animals wouldn't have too many difficulties in finding depressions under fallen logs or other suitable places in the ancient forests, and dry conditions were probably not the main reason why the soft shell emerged.
In fish and amphibians there is only one inner membrane, also called an embryonic membrane. In amniotes the inner anatomy of the egg has evolved further, new structures have developed to take care of the gas exchanges between the embryo and the atmosphere, as well as dealing with the waste problems.
To grow a thicker and tougher shell there were no other alternatives than finding new ways to supply the embryo with oxygen, as diffusion alone wouldn't be enough any more. After the egg had gotten these structures, further sophistication of them allowed the amniotes to lay much bigger eggs in much drier habitats.
It has been assumed that bigger eggs meant bigger offspring, and bigger adults meant bigger eggs, which meant the amniotes had gotten the opportunity to grow bigger than their ancestors. However, no such size increase seems to have happened (Laurin, 2004).
[edit] The Amniote Evolutionary Radiation
Freedom from being tied to water to lay eggs meant that the first amniotes could explore and radiate out into many new terrestrial ecological niches. This was the first great reptilian radiation, and was represented by three distinct groups, the Paraeptiles or Anapsids, the Eureptiles, and the Pelycosaurs. The last-named quickly became the dominant animal type of the latest Carboniferous and Early Permian period, and the first dynasty of fully terrestrial animals. Ironically, many of these early amniotes remained close to water, one even became semi- or fully-aquatic (e.g. Mesosaurus). The only real advantage of the amniote egg may be that the eggs are safe from predators. Fully terrestrial animal ecosystems did not emerge until the Middle Permian.
For further evolutionary developments of the amniotes, see entries of each the groups in the phylogeny menu (top of this page) and of those groups that evolved from them.
[edit] Phylogeny
<==Amniota [Haemothermia] |--? Casineria kiddi |--Theropsida/Synapsida [Pelycosauria, Theromorpha] | | i. s.: Protoclepsydrops | `--+--Caseasauria | | |--Caseidae | | `--Eothyrididae | | |--Eothyris | | `--Oedalops | `--+--Ophiacodontidae | | |--Archaeothyris | | `--+--Ophiacodon | | `--Varanosaurus | `--+--Varanopidae | | `--+--Archaeovenator | | `--+--Aerosaurus wellesi | | `--+--Varanops brevirostris | | `--Varanodon | `--+--Edaphosaurus [Edaphosauridae] | | `--E. pogonias | `--+--Therapsida | `--Sphenacodontidae | |--Dimetrodon | | |--D. grandis | | |--D. limbatus | | |--D. milleri | | |--D. natalis | | |--D. occidentalis | | `--D. teutonis Berman et al. 2001 | |--Haptodus | `--Secodontosaurus `--Sauropsida
* Type species of genus indicated
[edit] References
Bailon, S., J. Garcia-Porta & J. Quintana-Cardona. 2002. Première découverte de Viperidae (Reptilia, Serpentes) dans les îles Baléares (Espagne): Des vipères du Néogène de Minorque. Description d’une nouvelle espèce du Pliocène. Comptes Rendus Palevol 1: 227-234.
Baird, D., & R. L. Carroll. 1967. Romeriscus, the oldest known reptile. Science 7 July 1967: 56-59.
Berman, D. S., A. C. Henrici, S. S. Sumida & T. Martens. 2004. New remains of Dimetrodon teutonis (Synapsida: Sphenacodontidae) from the Lower Permian of Germany. Annals of Carnegie Museum 73 (2): 48-56.
Cheng Z. & Li J. 1997. A new genus of primitive dinocephalian - the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43.
Krzeminski, W., & C. Lombardo. 2001. New fossil Ephemeroptera and Coleoptera from the Ladinian (Middle Triassic) of Canton Ticino (Switzerland). Rivista Italiana di Paleontologia e Stratigrafia 107 (1): 69-78.
Laurin, M., & R. R. Reisz. 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113: 165-223.
Laurin, M. 2004. The evolution of body size, Cope’s rule and the origin of amniotes. Systematic Biology 53: 594–622.
Maisch, M. W., & A. T. Matzke. 2000. The Ichthyosauria. Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 298: 1-159.
Maisch, M. W., & A. T. Matzke. 2003. Observations on Triassic ichthyosaurs. Part XII. A new Early Triassic ichthyosaur genus from Spitzbergen. Neues Jahrbuch für Geologie und Paläontologie 229 (3): 317-338.
Modesto, S. P., R. J. Damiani & H.-D. Sues. 2002. A reappraisal of Coletta seca, a basal procolophonid reptile from the Lower Triassic of South Africa. Palaeontology 45 (5): 883-895.
Norman, D. 1985 (reprinted 2000). The Illustrated Encyclopedia of Dinosaurs. Salamander Books: London.
Okamura, C. 1987. New facts: Homo and all Vertebrata were born simultaneously in the former Paleozoic in Japan. Original Report of the Okamura Fossil Laboratory 15: 347-573.
Olson, E. C. 1962. Late Permian terrestrial vertebrates, USA and USSR. Transactions of the American Philosophical Society 52 (2): 1–196. See diagram p.162
Prothero, D. R. 1998. Bringing Fossils to Life: An introduction to paleobiology. WCB McGraw-Hill: Boston.
Ruta, M., & M. I. Coates. 2007. Dates, nodes and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Paleontology 5: 69-122.
Ruta, M., M. I. Coates & D. L. J. Quicke. 2003. Early tetrapod relationships revisited. Biological Reviews 78: 251-345.
Sidor, C. A. 2003. The naris and palate of Lycaenodon longiceps (Therapsida: Biarmosuchia), with comments on their early evolution in the Therapsida. Journal of Paleontology 77 (5): 977-984.
Smithson, T. R. 1985. The morphology and relationships of the Carboniferous amphibian Eoherpeton watsoni Panchen. Zoological Journal of the Linnean Society 85: 317-410.
Spencer, P. S., & G. W. Storrs. 2002. A re-evaluation of small tetrapods from the Middle Triassic Otter Sandstone Formation of Devon, England. Palaeontology 45 (3): 447-467.
Tverdokhlebov, V. P., G. I. Tverdokhlebova, A. V. Minikh, M. V. Surkov & M. J. Benton. 2005. Upper Permian vertebrates and their sedimentological context in the South Urals, Russia. Earth-Science Reviews 69: 27-77.
Vallin, G., & M. Laurin. 2004. Cranial morphology and affinities of Microbrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians. Journal of Vertebrate Paleontology 24: 56-72.
Wellnhofer, P. 1991. The Illustrated Encyclopedia of Pterosaurs. Salamander Books: London (reprinted 2000, in The Illustrated Encyclopedia of Dinosaurs (D. Norman & P. Wellnhofer). Salamander Books).
[edit] Credits
MAK061016 & 17, includes material from Wikipedia; ATW051015 (Public domain) - Palaeos - Amniotes (Characters); Phylogeny CKT071203
