Palaeos Vertebrates Amniota : Stem Amniotes

A some point in evolutionary history, at a time when the first great coal swamps were only just emerging, and before even insects had taken wing, a tiny reptile-like amphibian aquired the ability to lay small waterproof eggs on land. This great grand mother of all reptiles, birds, and mammals (including us) probably wasn't more than 10 cm (or 4 inches in old imperial measurement), in length, and for all the world resembled a small salamander or lizard. It would have lived amongst leaf litter, rotten logs, and other hideaways, where it was safe from predators, and could scrurry after and munch happily on the numerous arthropods that even then inhabited the Earth. We don't know the transitiobnal stages that enabled it to liberate its reproductive cycle from water, culminating in the ability to lay waterproof eggs that hatched into miniature adult-like young, bypassing the amphibian tadpole stage altogether. We can be pretty sure that this creature was prior by some millions of years to the most recent common ancestor of all living reptiles birds and mammals, but we don't know exactly by how much, or how physiologically primitive or advanced it was. It may have resembled one of the two creatures featured on this page, or primitive(Solenodonsaurus featured on a previous page. Or it may have different to all of them. We do know that that that tiny creatuere was the beginning of one of the most successful animal clades to inhabit the Earth, the amniotes. MAK111109

Descriptions

Comment: This taxon is erected here as a hypothetical apomorphy based node, defined by the presence of the first amniote egg. This stem-based taxon is prior to, and hence more inclusive than, the crown-based taxon (crown amniotes) referred to later. We don't know when this appeared, so the position of this clade is purely sp[eculative, as are the relations to it of the other taxa on this and related pages, which may or may not have been actual amniotes. MAK111121.

Casineria: C. kiddi Paton, Smithson & Clack 1999

Early Carboniferous (Asbian - Middle Visean) of Scotland. (Cheese Bay Shrimp Bed of the East Lothian Lower Oil Shale)

Stem Amniota : Westlothiana + Crown Amniota + *

Comments: Poorly known form, known only from a headless partial skeleton, but with several tantalising early aamniote features, such as unfused ankles and clawed toes, five fingers, and gracile build with light leg-bones. Of course these may simply be convergences due to common adaption of a shared environment, or it may be that Casineria is indeed the earliest known reptile. In any case, this is the earliest known fully terrestrial tetrapod. The single tiny specimen shows bones that are strongly ossified (not cartiligeinous) indicating that the animal was almost or completely grown when it died. This ties in with the hypothesis proposed by Carroll that the first amniotes would have been very small, mo more than 10 cm, due to the initial lack of extra-embryonic membranes that are unique to amniotes (amnion, chorion, and allantois) and hence problems with build up of carbon dioxide (Carroll 1970, 1991, Laurin 2004 p.594). Initial cladistic analysis (Paton et al 1999) suggested a polytomy with Westlothiana and Eureptilia (Captorhinus, Paleothyris, Petralocosaurus), but the consensus tree was highly instable. Supertree analysis by Ruta et al 2003b place it close to Westlothiana, another enigmatic form, which may or may not be an amniote, and place both species outside a surprising clade in which diadectomorphs are nested in a paraphyletic amniota. For now we have tentatively and provisionally placed Casineria as a very primitive amniote. The generic name is is a latinization of Cheese Bay, the site near Edinburgh, where the specimen was found (not a native Australasian evergreen).

Links: Out of the Swamps - How early vertebrates established a foothold with all 10 toes on land - Richard Monastersky .; Wikipedia; Jenny Clack - Other Early Tetrapod Projects (Casineria gets a brief mention)

Image: Casineria kiddi holotype from Wikipedia; skeleton reconstruction and silhouette David Peters - Reptile evolution

References: Paton et al 1999 MAK111106

Westlothiana: W. lizzaie Smithson & Rolfe 1990

Early Carboniferous (Brigantian - Late Visean) of Scotland. (lower East Kirkton Limestone, West Lothian Upper Oil Shale Formation, Scotland)

Stem Amniota : Casineria + Crown Amniota + *

Comments: named after the West Lothian district where it was discovered, and known from two nearly complete skeletons. A transitional form that combines amphibian and reptilian features, it was originally cosidered a a eureptile based on similarities of the skull, humerus, and vertebrae. Features that asociate it with amniotes rather than amphibians include unfused ankle bones, lack of labyrinthodont infolding of the dentin, a lack of an otic notch and a generally small skull. Later placed outside the amniote crown because of the absence of diagnosable characters, and now considered to belong on the amniote stem, close to either diadectomorphs or lepospondyls [1]. However its precise affinities remain uncertain, and the absense of crown group traits need not mean it is not an amniote, albeit a stem group amniote. As with Casineria , and in keeping with Carroll's hypothesi of amniote origins, these were small, terrestrial animals (total length 20 cm). The elongate body and small legs could be an adaption to burrowing, as with skinks.

Image: Skeleton reconstruction and silhouette David Peters - Reptile evolution; life reconstruction Nobu Tamura Wikipedia

References: Smithson & Rolfe 1990, van Tuinen & Hadly 2004, Wikipedia MAK111106

Note: [1] David Marjanovic, who at the time of writing is working on a very large cladistic analysis, argues is a basal lepospondyl with ordinary lepospondyl tabulars (See Dinosaur Mailing List) MAK111109

Stem Amniota

Casineria and Westlothiana

Taxa on This Page

The first reptile

Descriptions