Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as Pan-Amniota
The informal variant of the name, "reptiliomorphs", is also occasionally used to refer to stem-amniotes, i.e. a grade of reptile-like tetrapods that are more closely related to amniotes than they are to lissamphibians, but are not amniotes themselves; the name is used in this meaning e.g. by Ruta, Coates and Quicke (2003).[Benton, M. J. (2000), Vertebrate Paleontology, 2nd Ed. Blackwell Science Ltd 3rd ed. 2004 – see also taxonomic hierarchy of the vertebrates, according to Benton 2004] While both anthracosaurs and/or embolomeres are suggested to be closer to , some recent studies either retain them as amphibians or argue that their relationships are still ambiguous and are more likely to be stem-tetrapods.
As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word. The two major hypotheses for lissamphibian origins are that they are either descendants of Dissorophoidea Temnospondyli or "Lepospondyli". If the former (the "temnospondyl hypothesis") is true, then Reptiliomorpha includes all tetrapod groups that are closer to amniotes than to temnospondyls. These include the Diadectomorpha, Seymouriamorpha, most or all "lepospondyls", Gephyrostegidae, and possibly the Embolomeri and .
Changing definitions
The name Reptiliomorpha was coined by Professor Gunnar Säve-Söderbergh in 1934 to designate amniotes and various types of late Paleozoic tetrapods that were more closely related to amniotes than to living amphibians. In his view, the amphibians had evolved from fish twice, with one group composed of the ancestors of modern
salamanders and the other, which Säve-Söderbergh referred to as Eutetrapoda, consisting of anurans (
), amniotes, and their ancestors, with the origin of
being uncertain. Säve-Söderbergh's Eutetrapoda consisted of two sister-groups: Batrachomorpha, containing anurans and their ancestors, and Reptiliomorpha, containing anthracosaurs and amniotes.
Säve-Söderbergh subsequently added Seymouriamorpha to his Reptiliomorpha as well.
Alfred Romer rejected Säve-Söderbergh's theory of a Polyphyly amphibia and used the name Anthracosauria to describe the "labyrinthodont" lineage from which amniotes evolved. In 1970, the German paleontologist Alec Panchen took up Säve-Söderbergh's name for this group as having priority,[Panchen, A. L. (1970). Handbuch der Paläoherpetologie - Encyclopedia of Paleoherpetology Part 5a - Batrachosauria (Anthracosauria), Gustav Fischer Verlag - Stuttgart & Portland, 83 pp., .] but Romer's terminology is still in use, e.g. by Carroll (1988 and 2002) and by Hildebrand & Goslow (2001).[Carroll, R. L., (1988): Vertebrate paleontology and evolution. W. H. Freeman and company, New York.][Gauthier, J., Kluge, A.G., & Rowe, T. (1988): "The early evolution of the Amniota". In The Phylogeny and Classification of the Tetrapods: Volume 1: Amphibians, Reptiles, Birds. Edited by M.J. Benton. Clarendon Press, Oxford, pp. 103–155.]
In 1956, Friedrich von Huene included both amphibians and Anapsida in the Reptiliomorpha. This included the following orders: Anthracosauria, Seymouriamorpha, Microsauria, Diadectomorpha, Procolophonia, , Captorhinomorpha, Testudines.[Von Huene, F., (1956), Paläontologie und Phylogenie der niederen Tetrapoden, G. Fischer, Jena.]
Michael Benton (2000, 2004) made it the sister-clade to Lepospondyli, containing "anthracosaurs" (in the strict sense, i.e. Embolomeri), seymouriamorphs, diadectomorphs and amniotes.[ Systema Naturae (2000) / Classification Superorder Reptiliomorpha ]
Several phylogenetic studies indicate that amniotes and diadectomorphs share a more recent common ancestor with Lepospondyli than with seymouriamorphs, Gephyrostegus and Embolomeri (e.g. Laurin and Reisz, 1997,
Characteristics
Gephyrostegids, seymouriamorphs and diadectomorphs were land-based, reptile-like amphibians, while embolomeres were aquatic amphibians with long bodies and short limbs. Their anatomy falls between the mainly aquatic
Devonian labyrinthodonts and the
Captorhinidae. University of Bristol paleontologist Professor Michael J. Benton gives the following characteristics for the Reptiliomorpha (in which he includes embolomeres, seymouriamorphs and diadectomorphs):
-
narrow (less than half the skull width)
-
Vomer bone taper forward
-
phalanges formulae (number of joints in each toe) of foot 2.3.4.5.4–5
Cranial morphology
The groups traditionally assigned to Reptiliomorpha, i.e. embolomeres, seymouriamorphs and diadectomorphs, differed from their contemporaries, the non-reptiliomorph temnospondyls, in having a deeper and taller skull, but retained the primitive kinesis (loose attachment) between the
skull roof and the cheek (with exception of some specialized taxa, such as
Seymouria, in which the cheek was solidly attached to the skull roof
[Carroll, R. L., (1988): Vertebrate paleontology and evolution. W. H. Freeman and company, New York, p. 167 and 169.]). The deeper skull allowed for laterally placed eyes, contrary to the dorsally placed eyes commonly found in amphibians. The skulls of the group are usually found with fine radiating grooves. The
quadrate bone in the back of the skull held a deep
otic notch, likely holding a spiracle rather than a tympanum.
[Palaeos Reptilomorpha ]
Postcranial skeleton
The
vertebrae showed the typical multi-element construction seen in labyrinthodonts. According to Benton, in the vertebrae of "anthracosaurs" (i.e.
Embolomeri) the
intercentrum and
pleurocentrum may be of equal size, while in the vertebrae of
Seymouriamorpha the pleurocentrum is the dominant element and the intercentrum is reduced to a small wedge. The intercentrum gets further reduced in the vertebrae of amniotes, where it becomes a thin plate or disappears altogether.
[Chapter 4: "The early tetrapods and amphibians." In: Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd.] Unlike most labyrinthodonts, the body was moderately deep rather than flat, and the limbs were well-developed and ossified, indicating a predominantly terrestrial lifestyle except in secondarily aquatic groups. Each foot held five digits, the pattern seen in their
amniote descendants.
[Alfred Romer. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)] They did, however, lack the reptilian type of ankle bone that would have allowed the use of the feet as levers for propulsion rather than as holdfasts.
[Palaeos Reptilomorpha: Cotylosauria ]
Physiology
The general build was heavy in all forms, though otherwise very similar to that of early reptiles.
The skin, at least in the more advanced forms probably had a water-tight epidermal horny overlay, similar to the one seen in today's reptiles, though they lacked horny claws.
In
and some
seymouriamorpha, like
Discosauriscus, dermal scales are found in post-metamorphic specimens, indicating they may have had a "knobbly", if not scaly, appearance.
With reptiliomorph anthracosaurs having evolved small near-circular keratinous scales, their amniote descendants further covered almost their entire body with them, and also formed
claws of keratin, with both scales and claws making cutaneous respiration and water absorption impossible, making them breathe through their mouths and nostrils, and drink water through mouth.
Seymouriamorphs reproduced in amphibian fashion with aquatic eggs that hatched into (tadpoles) with external gills;
Evolutionary history
Early reptiliomorphs
During the
Carboniferous and
Permian Geologic period, some
started to evolve towards a
reptile 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,
Limnoscelis) were so reptile-like that until quite recently they actually had been considered to be true reptiles, and it is likely that to a modern observer they would have appeared as large to medium-sized, heavy-set
. Several groups however remained aquatic or semiaquatic. Some of the
show the build and presumably habits of modern crocodiles and were probably also similar to crocodylians in that they were river-side predators. While some other
possessed elongated
newt- or
eel-like bodies. The two most terrestrially adapted groups were the medium-sized insectivorous or carnivorous
Seymouriamorpha and the mainly herbivorous
Diadectomorpha, with many large forms. The latter group has, in most analysis, the closest relatives of the
.
[Laurin, M. (1996): Phylogeny of Stegocephalians, from the Tree of Life Web Project]
The earliest known fossil evidence of reptiliomorphs are amniote tracks from the early Mississippian (Tournaisian) of Australia. These discoveries suggest that contrary to prior assumptions, reptiliomorphs must have diverged from amphibians almost immediately after the start of the Carboniferous, and potentially even before it (during the Devonian).
From aquatic to terrestrial eggs
Their terrestrial life style combined with the need to return to the water to lay eggs hatching to
(tadpoles) led to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians.
[Duellman, W.E. & Trueb, L. (1994): Biology of amphibians. The Johns Hopkins University Press] Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family
Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitate gas exchange between the egg and the atmosphere. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Origin of amniotes
Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved
poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including
Solenodonsaurus,
Casineria and
Westlothiana. Such small animals laid small eggs, 1 cm in diameter or less. Small eggs would have a small enough volume to surface ratio to be able to develop on land without the amnion and chorion actively affecting gas exchange, setting the stage for the evolution of true amniotic eggs.
Although the first true amniotes probably appeared as early as the Middle Mississippian sub-epoch, non-amniote (or amphibian) reptiliomorph lineages coexisted alongside their amniote descendants for many millions of years. By the
Guadalupian the non-amniote terrestrial forms had died out, but several
Aquatic animal non-amniote groups continued to the end of the Permian, and in the case of the
Chroniosuchidae survived the end Permian mass extinction, only to die out prior to the end of the
Triassic. Meanwhile, the single most successful daughter-clade of the reptiliomorphs, the amniotes, continued to flourish and evolve into a staggering diversity of tetrapods including
,
, and
.
==Gallery==
, an aquatic embolomere]]
, an indeterminate Carboniferous reptiliomorph]]
, an indeterminate Carboniferous reptiliomorph]]
, a Triassic chroniosuchian]]
, an "advanced" reptiliomorph]]
, an amniote-like reptiliomorph]]
, a diadectomorph]]
'', an amniote sauropsid]]
See also
