Estemmenosuchus skin and osteoderms

Estemmenosuchus skin

Photographs and diagram of the preserved skin of Estemmenosuchus uralensis, from Chudinov (1968).

The dinocephalian Estemmenosuchus is one of the only non-mammalian synapsids with known integument. Skin from multiple specimens of E. uralensis, from the middle Permian Ezhovo locality in Russia, was described by Peter Chudinov in a 1968 paper. Like many other old Russian papers, Chudinov (1968) was not previously available online and details about it were scarce. Fortunately, I got a copy of the English translation courtesy of Tim Sosa, which is linked in the references below. I am glad to finally make it accessible, since it contains a lot of valuable information for paleontologists and paleoartists alike. In summary, many layers and structures of the integument from the top of the head and neck are preserved.

The most notable structures are small spheres only 0.3-0.45 mm in diameter that Chudinov called “lenses”. He interpreted these lenses as being the remains of dermal glands which may have secreted mucus or oils. If they are glands, they would be the earliest known in the fossil record. Chudinov’s 1983 monograph on therapsids includes a section on integument, which has some additional info. In it he specifically compares the skin texture of Estemmenosuchus to hippos and proposes an amphibious lifestyle. He mentions that there was possibly preserved melanin/melanosomes in the darker areas between the lenses. At the time only one other example of this was known, an ichthyosaur described by Whitear (1956).

The gaps between the adjacent lenses are small (0.1-0.2 mm) and, apparently, the basal layer of the epidermis during the life of the animal lay at the level of the mouths of the lenses. The deeper areas are filled with dark brown or almost black calcite, the color of which may be partly due to the pigment melanin, as it is interpreted for ichthyosaurs (Whitear, 1956).

Estemmenosuchus osteoderm

Sacral osteoderm of Estemmenosuchus uralensis, from Chudinov (1983).

Chudinov (1983) also reported an osteoderm found associated with the sacrum of a partial skeleton that he referred to Anoplosuchus tenuirostris. I have translated his full description of the osteoderm below.1 Anoplosuchus is now considered a junior synonym of E. uralensis (Ivakhnenko, 2001). These osteoderms were likely embedded in the skin and are reminiscent of those from mylodontid ground sloths. They might have served as protection from predators or as stores for calcium and other minerals. 

“In the mass of osteological material, dermal armor ossifications were found in one case among the bones of the sacral region of the incomplete skeleton of Anoplosuchus tenuirostris Chudinov (PIN No. 1758/321). Therefore, based on taphonomic data, it is more likely that dermal ossifications are confined to this genus and species than to other dinocephalians of the Ezhovo locality. The dermal ossification (Fig. 72) is a lozenge-shaped formation with an indistinctly expressed discoid central area from which the asymmetrical rounded outgrowths extend. The maximum thickness of the central disc is about 15 mm. The peripheral rounded outgrowths are not uniform in size and are partially separated from the central disc by grooves or circular grooved constrictions. Apparently, the central disc and the peripheral outgrowths form a single bone formation, in which the traces of fusion are completely lost. Some outgrowths are noticeably elongated and carry additional constrictions that give the outgrowth the appearance of an articulated segment. It is possible that the constrictions and segmentation represent traces of fusion and grouping of small independent ossifications near larger ones, from which separate armor plaques are formed. In general, the ossification surface is smooth, devoid of a layer of compact bone tissue. On the fractures, the bone tissue is loose, permeated with a dense network of Haversian canals. The smooth spherical surface shows that the fused bone plaques were scattered in the skin and did not form an articulated bone armor. From a single find, it is difficult to judge the number of ossifications in the skin and their distribution on the surface of the body. Perhaps their rarity is due to the weak ability to fossilize and the difference in the conditions of burial compared to the bones of skeletons.”

Chudinov additionally listed all of the therapsid integumentary material in the collections of the Paleontological Institute (PIN) in Moscow, which I have again translated below. There are 7 specimens of E. uralensis with skin and 1 with an osteoderm. The skin of Archaeosyodon and Eotitanosuchus is unfortunately unfigured and undescribed.

Estemmenosuchus uralensis Chudinov. Coverings: PIN 1758/21 – skull; PIN 1758/22 – incomplete skeleton with skull; PIN 1758/279 – left upper outgrowth of parietal region; PIN 1758/280 – left supratemporal outgrowth;  PIN 1758/287 – supratemporal outgrowth; PIN 1758/290 – fragment of frontal bones with unpaired median outgrowth; PIN 1758/291 – skull. 
Eotitanosuchus olsoni Chudinov. Integument (holotype).
 Archaeosyodon praeventor Chudinov. Coverings: PIN 1758/118 – fragmentary skulls; PIN 1758/314 – skull fragment; PIN 1758/318 – left jaw bone.
Anoplosuchus tenuirostris Chudinov [=E. uralensis]. Ossification: PIN 1758/321 – fragmentary skeleton.”

Notes

1All of the excerpts from Chudinov (1983) in this post were translated using Yandex and Google Translate.

Addendum (8/23/2022)

Smith et al. (2022) published a brief description of Lystrosaurus mummies from the Early Triassic Katberg Formation of South Africa. The preservation of the skin in these specimens is similar to that of the Estemmenosuchus skin. This suggests that the Estemmenosuchus  carcasses may likewise have been desiccated before burial and that they could have died during a drought.

“Two of the juvenile L. murrayi specimens found lying some 4 m apart are both tightly enveloped by a thin layer of dark brown fibrous calcite with a distinctively mamillated surface texture that we interpret as the impression skin. The adherence of the coating to the abdominal skeleton with its pustular texture and the series of well-defined puncture holes exposing the apices of 12 successive neural spines strongly resembles that observed in modern mummified carcasses. Microscopic examination of this enveloping layer display amorphous micrite with patches of fibrous calcite orientated perpendicular to the lower surface.”

References

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