Body impressions of tetrapods are very rare
Fossils with skin and no bones are rarely found. Most belong to amphibians from the Carboniferous/Permian of Europe and North American. Not talking about footprints with drag marks here. A recent paper by Niedziedzki and Bojanowski (2012) describes foot, belly and tail imprints from the Lower Permian ascribed to a eupelycosaur, like Dimetrodon.
From their abstract
“We describe a new specimen of a supposed Paleozoic tetrapod body impression from the Lower Permian Slupiec Formation in the Intra-Sudetic Basin, Poland. The size, integument morphology of belly and part of tail imprints, and the morphology of a well-preserved pes track diagnose the specimen and readily distinguish it from other described specimens of body impressions of Paleozoic tetrapods. The eupelycosaur identity of this new specimen is based on the identification of the footprint Dimetropus leisnerianus (Geinitz, 1863), which is connected with the inferred body imprint. The morphology of integument impressions indicates the presence of the various-sized square or rectangular-shaped scales on the bottom part of the belly and tail of this eupelycosaurid trackmaker.”
The trackmaker evidently ‘sat down’ after taking a walk. (I have not seen the paper, yet, btw.)
Luckily the Eupelycosauria are still with us
Laurin and Reisz (1997) redefined the Eupelycosauria “to designate a clade of synapsids that includes most pelycosaurs, as well as all therapsids and mammals.” That was meant to include all pelycosaurs (sans caseaurids, which are not related anyway) and all their descendants. If that indeed included Varanopidae, then the large reptile tree indicates that the Eupelycosauria also includes all new Archosauromorpha diapsids, which includes crocs, dinos and birds. That unfortunate bit of news is valid because current members of the Eupelycosauria include the Varanopidae, which includes the Mycterosaurinae, which includes Heleosaurus and Mesenosaurus, which are currently the best diapsid ancestor candidates in the large reptile tree. Take those few taxa out of the definition and the original intention of the Laurin and Reisz (1997) definition remains.
But, back to those scales
Similar to its Early Permian ancestors, a modern eupelycosaur (Didelphis marsupialis) still retains a scaly tail (Fig. 1), giving us a nice peek into those ancient times 295 million years ago.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Laurin M and Reisz RR 1997. Autapomorphies of the main clades of synapsids – Tree of Life Web Project.
Niedziedzki G and Bojanowski M 2012. A Supposed Eupelycosaur Body Impression from the Early Permian of the Intra-Sudetic Basin, Poland. Ichnos 19(3):150-155. DOI:10.1080/10420940.2012.702549 link
Oppossum tail link