The history of the species is itself a tale of slow scientific assembly. Nopcsa recognized that rare Cretaceous turtles from both sides of the Atlantic, including the German Helochelys danubina and the North American Naomichelys speciosa, shared the unusual tuberculate shell surface, and he grouped them together. But because he never formally attached a species name to his genus, the combination Helochelydra nopcsai was not established until 1999, more than seventy years later, when Lapparent de Broin and Murelaga named the species on the basis of the same specimen.

The new German material comes from the Busche quarry near the village of Beckum, where mining operations have cut into a cave system of deep hydrothermal karst origin. The cavities are completely filled with sediments that palynological evidence dates to the late Barremian to early Aptian stages of the Early Cretaceous. The turtle remains were collected over the past twenty years from two fissures: a narrow passage no more than 4 meters wide and a large cavern reaching up to 35 meters in diameter.

The quarry has already yielded isolated theropod teeth, salamanders, sauropod bones, and mammals, but the turtle fauna, which preliminary analysis suggests also includes the species Chitracephalus dumonii, Peltochelys duchastelii, and a new pleurosternid, is only now being formally documented. To squeeze the most information out of the fragmentary bones, the research team scanned selected specimens, including a partial parietal skull bone, shell fragments, and granicones, using micro-computed tomography at the University of Fribourg. The scans, performed at voxel sizes as fine as 12 micrometers, generated thousands of image slices that were segmented into three-dimensional digital models, now freely available on MorphoSource.

The only skull element in the sample, a partial left parietal about 2.7 centimeters long, matches the equivalent region of the nearly complete skull from the Isle of Wight in size, but differs strikingly in showing clear sulci that outline at least twelve polygonal cranial scutes, whereas the English skull shows no evidence of such scutes at all. The shell bones themselves reveal a turtle of considerable size. One costal fragment with intact anterior and posterior margins measures 56 millimeters across, slightly exceeding the widest costal of the only known complete Naomichelys speciosa skeleton, which suggests that the Balve animal's carapace may have reached a length of more than 70 centimeters.

The carapace bones are thin, between 5 and 8 millimeters, and deep grooves decorated the contacts between the neural and costal plates along the rear half of the shell. A broad nuchal notch at the front of the shell, framed by the second peripherals, is typical of helochelydrids, and the plastral bones show thickened inguinal notches reaching up to 2.5 centimeters in the largest specimens. Perhaps the most anatomically significant finding concerns the visceral cavity of the shell.

In most turtles, the cavity is laterally bounded by V-shaped bridge peripherals, but the Balve peripherals demonstrate that in Helochelydra nopcsai the cavity extends continuously forward to the first peripheral and backward, through a series of finger-like pockets, all the way to the eleventh peripheral at the rear of the shell. This full-length expansion was previously documented in other helochelydrids such as Solemys, Naomichelys, and Aragochersis, and also in compsemydids and pleurosternids. Its distribution is not yet fully mapped, but it has been cited as evidence for the hypothesis that helochelydrids are a subclade of Paracryptodira, possibly closely related to pleurosternids, though a similar condition in some Triassic turtles raises the possibility that the feature is instead an ancient inheritance.

The granicones, the turtle's bony armor, form the centerpiece of the sample. These osteoderms consist of a flat base that anchored into the skin and an apex that protruded from the body, and their external surfaces mirror the tuberculate texture of the shell. Qualitative inspection distinguished three informal morphotypes: cone-shaped elements making up about 35 percent of the sample, hook-shaped elements with offset apices at about 43 percent, and flat plate-like elements at about 22 percent.

A principal components analysis of four measurements taken from the 136 best-preserved specimens showed that size explains 65.7 percent of overall variance, and that after correcting for size the morphotypes do not occupy discrete clusters but rather grade into one another, with only the plates separating weakly along the second shape axis. Five elements deviate from all three morphotypes, including compound granicones in which two or more conical apices were sutured together at a shared base, a feature not known in any other helochelydrid. Because all granicones were found in isolation, their exact position on the body is unknown, but the articulated skeleton of Naomichelys speciosa suggests that flat plates covered the outer surfaces of the lower limbs while spur-like hooks sat on the walking surfaces.

The authors conclude with confidence that Helochelydra nopcsai is the only turtle at Balve that possessed osteoderms, since every granicone displays the same surface texture regardless of shape or size. They also note that granicones remain surprisingly rare in the fossil record given how widespread helochelydrids were, likely because few continental sites are systematically screen-washed for small fossils and because such elements are easily overlooked or unreported when they are collected. The study stops short of resolving the turtle's lifestyle.

Helochelydrids are generally assumed to have been terrestrial, based on their occurrence in continental sediments, shell bone density resembling that of tortoises, and the association of osteoderms with land-adapted turtles. Yet the best-preserved helochelydrid skull, that of Helochelydra nopcsai itself, lacks the inflated nasal cavity typical of terrestrial turtles and instead shows broad triturating surfaces usually associated with aquatic feeding, leading to earlier suggestions that it may have crushed hard terrestrial food such as grubs and snails. The Balve fissure fauna mixes unambiguously aquatic and terrestrial taxa, and the fragile but complete condition of the turtle remains indicates only that they were not transported far, so taphonomy cannot settle the question.

Subject of Research: Fossil remains of the helochelydrid turtle Helochelydra nopcsai from Early Cretaceous fissure fills at Balve, Germany

Article Title: The helochelydrid turtle Helochelydra nopcsai from the Early Cretaceous (late Barremian – early Aptian) fissure fills of Balve, North Rhine-Westphalia, Germany, including a large sample of granicones

Article References: Joyce, W. G., Evers, S. W., Ren, S., Rollot, Y., & Schwermann, A. H. (2023). The helochelydrid turtle Helochelydra nopcsai from the Early Cretaceous (late Barremian – early Aptian) fissure fills of Balve, North Rhine-Westphalia, Germany, including a large sample of granicones. Fossil Record, 26(1), 117-133. https://doi.org/10.3897/fr.26.102128

Image Credits: AI Generated

DOI: 10.3897/fr.26.102128

Keywords: Helochelydra nopcsai, helochelydrid turtles, granicones, Early Cretaceous, fissure fills, Busche quarry Balve, paleontology, osteoderms, shell morphology, micro-CT, Paracryptodira, Germany