A lone plastron fragment exposes a hidden turtle species in Germany’s Messel Pit

In the oil shales of the Messel Pit, one of the world’s most celebrated fossil sites, a single fragment of turtle shell has quietly rewritten the census of a 47-million-year-old ecosystem. The specimen, catalogued as SMF ME-3495 and housed in the Senckenberg Museum in Frankfurt, is a partial anterior plastral lobe — the front portion of the bony belly shield that protects a turtle’s underside. Yet this modest piece of anatomy, described in a new study in the journal Fossil Record, carries a combination of features that matches none of the turtle species previously documented from the Middle Eocene deposits of Hesse, Germany.

The Messel Pit is what paleontologists call a Konservat-Lagerstätte, a deposit of exceptional preservation where sub-tropical plants and animals from roughly 47 million years ago were buried in black, oil-rich shale beneath the waters of a volcanic maar lake. Four turtle species are broadly accepted from the site: the side-necked Neochelys franzeni, the soft-shelled Palaeoamyda messeliana, the bizarre pig-nosed relative Allaeochelys crassesculpta, and the pond turtle Palaeoemys messeliana, which most researchers now treat as encompassing two previously named morphotypes. Beyond that consensus, the taxonomy of Messel’s geoemydid turtles has been contentious for decades, with proposed species counts ranging from one to four, largely because these turtles show enormous variation within single species.

Against that backdrop, SMF ME-3495 stands out. The specimen was recovered in September 1999 from sediments directly beneath a well-dated marker horizon, near the area of the quarry nicknamed Turtle Hill, and was collected under controlled conditions rather than picked up as loose float. Its fragmentary state, the authors emphasize, is therefore not the product of modern weathering but of decay and disarticulation before burial — a taphonomic clue that becomes important later in the story.

The diagnostic anatomy lies in the fine details of the plastron’s front lobe. The specimen preserves much of the paired epiplastra and the median entoplastron, the bones over which the gular and humeral scutes — the horny covering plates — once lay. Three features together set it apart. First, the gular scutes bear small tubercles, tiny bumps along their rear margin. Second, the visceral, or body-facing, side of the epiplastra carries a distinct raised shelf, an epiplastral lip spanning about seventy percent of the midline suture. Third, the gular scutes are longer than they are wide, lap clearly onto the entoplastron, and meet the humeral scutes along a sinuous, S-shaped sulcus that bows first into the gular and then into the humeral field, forming a parabolic arc across the entoplastron.

Each of these characters rules out the known Messel turtles. The soft-shelled Palaeoamyda and Allaeochelys are eliminated outright because their plastral bones lack scute sulci entirely. The side-necked Neochelys franzeni has extragular scutes and a single median gular. The pond turtle Palaeoemys, in both its messeliana and kehreri forms, has gulars that are broader than long, barely touch the entoplastron, and form a straight sulcus with the humerals — and its epiplastral lip is incipient or absent. The fragment simply does not fit any of them.

What it does resemble is a group of Eocene geoemydids informally known as ptychogasterids, hard-shelled pond turtles whose defining traits include exactly the kind of epiplastral lip and gular tubercles seen here. The closest named relatives in space and time are Merovemys ploegi from the Early Eocene of France and Geiselemys ptychogastroides from the roughly contemporaneous Geiseltal deposits of central Germany. Intriguingly, SMF ME-3495 sits morphologically between the two: its gular tubercles are more widely spaced than in Merovemys, but its epiplastral lip has not yet reached the extravagant length of Geiselemys. Despite this, the authors decline to name a new species. Ptychogasterids are notorious for intraspecific variation in precisely the character complex on display, and a single plastral fragment cannot support a robust diagnosis. The specimen is therefore identified, tentatively, as an indeterminate ptychogasterid — the first ever recorded from Messel.

The identification carries biogeographic weight. A survey of more than 250 turtle individuals across the three largest Messel collections shows a fauna dominated by the aquatic Allaeochelys and Palaeoemys, with the soft-shelled Palaeoamyda less common and the side-necked Neochelys vanishingly rare. Strikingly absent are the terrestrially adapted tortoises that are common in contemporaneous deposits across France and Germany, a pattern suggesting strong sampling bias toward aquatic turtles in the lake sediments. If ptychogasterids were terrestrial, as some evidence suggests, their absence from Messel until now would be exactly what that bias predicts.

There is, however, a competing ecological reading, and it comes from the specimen’s pathologies. The ventral surface bears a large lesion in the humeral scute region where the cortical bone has been uniformly corroded away, exposing porous cancellous bone beneath, with smooth boundaries and a distinctive light cream coloration. This damage is consistent with shell disease — informally, shell rot — caused by bacterial or fungal infections, and comparable lesions in other fossil turtles have been grouped into trace-fossil ichnotaxa. Smaller pits on the entoplastron even resemble Karethraichnus, a trace attributed to ectoparasites such as ticks or leeches. Crucially, the shell diseases characteristic of terrestrial turtles attack the epidermis and leave bone largely untouched, whereas bone-corroding lesions are typical of aquatic taxa. That points toward a semi-aquatic or aquatic habit for the Messel ptychogasterid, which would also explain the odd preservation: an isolated plastral lobe rather than a complete skeleton could reflect transport of a carcass into the lake from elsewhere.

The specimen also preserves a mystery in its coloration. A thin, patchy coat of cream and orangish material covers the visceral surface that once lined the body cavity, and similar light coloration appears on the corroded lesion where the scute had necrosed or flaked away. Comparable coloration is not seen on exposed visceral surfaces or preserved internal organs of other Messel turtles, and its taphonomic significance — whether it reflects different depositional conditions, behavior, or habitat — remains unresolved.

Whether SMF ME-3495 ultimately proves to be a wandering terrestrial species swept into the volcanic lake or an aquatic resident with a distinctive shell infection, the fragment demonstrates that the Messel turtle fauna is not yet fully inventoried. The authors note that other specimens with ambiguous characters may already sit in museum drawers, awaiting mechanical preparation or radiographic imaging of the epiplastral region to reveal their affinities. Until then, a single corroded piece of belly shell stands as the sole evidence that at least one more turtle species — a ptychogasterid among pond turtles — once lived in or around the Eocene forests of Messel.

Subject of Research: A fossil turtle plastron fragment from the Messel Pit indicating an additional, previously unrecorded Eocene turtle species

Article Title: A plastron fragment reveals a previously unrecorded turtle species in the Eocene of Messel Pit, Germany

Article References: Ascarrunz, E., & Joyce, W. G. (2024). A plastron fragment reveals a previously unrecorded turtle species in the Eocene of Messel Pit, Germany. Fossil Record, 27(2), 259-264. https://doi.org/10.3897/fr.27.132374

Image Credits: AI Generated

DOI: 10.3897/fr.27.132374

Keywords: Messel Pit, turtles, Eocene, paleontology, plastron, ptychogasterids, geoemydids, shell disease, Germany, Konservat-Lagerstätte, fragment, reveals