Reptiliomorpha
From Palaeos
| TETRAPODA - Four legged animals | |
| Taxonomy | Phylogeny |
o==Elpistostegalia `--o TETRAPODA `==Elginerpetonidae `--+--Acanthostega `--+--Ichthyostega `--+--Crassigyrinus `--+--Colosteidae `--+--Baphetidae `--o--Temnospondyli |--Lepospondyli `--Reptiliomorpha |
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Contents |
[edit] Gaining Ground
Our subtitle here is the same as the title of Dr. Jenny Clack's recent book about the origin and early evolution of the Tetrapoda. The irony of Clack's title is that she has shown, rather convincingly, that none of the early tetrapods were likely to have been terrestrial to any marked degree. She argues, convincingly, that many of the critical "terrestrial" adaptations of the early tetrapods can just as easily be interpreted as adaptations to aquatic lifestyles. Elsewhwere, we have discussed that this point may have been overstated. The particular suite of adaptations we see in early tetrapods may have succeeded, not because they were aquatic adaptations, and not because they were terrestrial adaptations, but precisely because they were both. Thus, Clack's thesis is certainly valid to the extent it states that tetrapods were primitively not evolving toward terrestriality. Rather, they were becoming better and better amphibians.
The real turn to terrestriality begins here, with the Reptilomorpha. We tentatively define this clade as one of the supporting stems of crown group Tetrapoda*. As a reminder, Tetrapoda* is defined (at various places in Palæos, as dogs + frogs, princes + frogs, or Prof. Michael Coates + frogs. These are all equivalent definitions. Reptilomorpha is then dogs (or princes, or Prof. Coates) > frogs.
In the not unlikely event that living amphibians are descended from temnospondyls, then the reptilomorphs will include a few more taxa but will otherwise change little. In terms of evolutionary grade, this section covers things from the anthracosaurs through the first steps of the amniote radiation. Temporally, this spans a very limited period. This whole process was probably compressed into about 30 million years, between the middle Mississippian and the middle of the Pennsylvanian. Yet, in that brief period, the tetrapods diversified from a rather homogeneous, if peculiar, bunch of air-breathing freshwater fishes into truly terrestrial amniotes, including anapsids (the turtle lineage), synapsids (the mammal lineage), and the true reptiles.
The critical points in this process involved the acquisition of a large group of amniote adaptations, most of which are listed here.
| REPTILIOMORPHA | |
| Taxonomy | Phylogeny |
Subphylum: Vertebrata
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o Tetrapoda crown group `--o REPTILIOMORPHA |--Embolomeri (=Anthracosauria) |?--Chroniosuchia `--o?--Gephyrostegidae `--+--Seymouriamorpha `--+--Westlothiana lizziae `--o--Diadectomorpha `--+--Casineria kiddi `--o Amniota |--Sauropsida `--Synapsida |
[edit] Information
Stratigraphic Range: (as Linnaean taxon) Carboniferous to Triassic ; (as Clade) Carboniderous to Recent
Original Author: Säve-Söderbergh, 1934
[edit] Evolutionary History
During the Carboniferous and Permian periods, tetrapods evolved along a number of parallel lines towards a reptilian condition. Some of these tetrapods (e.g. Archeria, Eogyrinus) were elongate, eel-like aquatic forms with diminutive limbs, while others (e.g. Seymouria, Solenodonsaurus, Diadectes, Limnosceles) were so reptile-like that until quite recently they actually have been considered true sauropsids, and it is likely that to an observer they would have appeared like small or large reptiles.
Although the first amniote reptile probably appeared as early as the latest Mississippian period (Middle Carboniferous), reptilomorph tetrapods continued to flourish alongside their fully reptilian descendents and relatives for many millions of years.
By the middle Permian the terrestrial forms had died out, but several aquatic groups continued to the end of the Permain, and in the case of the Chroniosuchids survived the end Permian mass extinction, only to die out at the end of the Early Triassic. Meanwhile, the single most successful daughter-clade of the Reptiliomorphs, the Amniotes, continued to flourish and to inherit the Earth.
[edit] Characters
Characters: Domed skull, retaining kinesis (skull roof loosely attached to cheek); skull usually with fine radiating grooves; tabular large & attached to parietal; deep otic notch in quadrate; some with slight, impedance-matching stapes; pleurocentrum dominant element of vertebrae; well-developed limbs; 5 digits; usually terrestrial. ATW021214
Dr Michael Benton (2000, 2004) gives the following characteristics for the Reptiliomorpha:
- narrow premaxillae (less than half the skull width)
- vomers taper forward
- phalangeal formulae (number of joints in each toe) of foot 2.3.4.5.4-5
MAK050701
[edit] Note
A reminder that we are referring to the crown group Tetrapoda*, i.e. frogs + dogs. Reptilomorpha is one stem from the tetrapod node by definition. The other stem really ought to be called something other than Lepospondyli, since we really don't know if the traditional lepospondyls included the ancestor of modern amphibians. ATW021214.
[edit] Reference
- Benton, M. J. (2000), Vertebrate Palaeontology, 2nd Ed. Blackwell Science Ltd 3rd ed. 2004
[edit] Credits
- ATW040821 (Palaeos com - Reptilomorpha), copied to Palaeos org MAK060930 (Gaining Ground)
- MAK050701 (Wikipedia (GNU Free Documentation License), MAK060930 (Evolutionary History)
- ATW021214 Palaeos com (public domain), MAK050701 (Wikipedia) MAK060930 (Characters)
