Elettra | TomoLab Micro-CT scanning sheds light on the cranial anatomy and unusual diet of the prehistoric crocodilian Acynodon adriaticus Just north of Trieste, the Villaggio del Pescatore fossil site stands out as one of the most important palaeontological sites in Europe, and the sole Italian locality where multiple, almost complete Mesozoic fossil vertebrates have been collected. In addition to the popular dinosaurs “Antonio” and “Bruno” (two individuals of the duck-billed dinosaur Tethysdhadros insularis), a species of small-bodied crocodilian was discovered and named Acynodon adriaticus. Known for various postcranial material and an exquisitely preserved skull with articulated jaw, the bluntsnouted A. adriaticus barely surpassed 1 meter in length and belonged to the enigmatic European family of primitive, oddly toothed crocodilians Hyalaeochampsidae. Despite the excellent preservation of the skull (MCSNT 57248), many details remained hidden due to the physical overlap between different bones and the still abundantly present hard limestone matrix, which was left during preparation to avoid damage to the specimen. The lack of finer bone details and impossibility to properly inspect the complete dentition significantly hindered our understanding of Acynodon’s anatomy and palaeoecology. Through 13 Micro-CT scanning sessions at the Elettra TomoLab covering the whole skull (Fig. 1 a), we were able to safely overcome these limits. Meticulous manual segmentation allowed to remove part of the matrix and to isolate every teeth – including unerupted ones. Among the previously undetectable morphologies of the skull osteology, tomography allowed to describe the inner surfaces of bones, location and size of individual foramina and to detect superficially invisible suture lines; these data render the skull of A. adriaticus as the best-known among hylaeochampsids (Fig. 1 b), providing a remarkable baseline for future taxonomic revisions and phylogenetic analyses of the family. The relatively minute orbit size suggests very small eyes and consequently reduced eyesight, a common phenomenon in aquatic vertebrates inhabiting murky environments. The brevirostrine and massive skull shows biomechanical adaptations for enhanced performance of the adductor muscles (involved in the biting force and jaw mobility), hinting at a very strong bite and high stress-load capacity, which likely goes hand by hand with the bizarre teeth. Indeed, the renewed definition of the isolated dentition highlights a very specialized condition. A. adriaticus lacks fang-like caniniform teeth as in other hylaeochampsids, featuring anterior chisel-like teeth and posterior hypertrophic globular molar-like teeth with extremely thick enamel, exhibiting complex grip-enhancing wrinkly ornamentations. Upper and lower teeth rows occlude in overbite on the anterior half of the jaw but display partial toothon-tooth occlusion between posterior molariforms (Fig. 1 c). The unerupted teeth still inside the alveoli suggest an accelerated replacement rate of the molariforms and a lower rate for the anterior ones, unlike modern predatory crocodiles which display the opposite pattern. Lower molariforms underwent an unusual oblique/subvertical early growth phase, possibly as a solution to space constraints in having such oversized teeth in a proportionately small mandible. Life history traits of the analysed specimen are also detectable; the strong rugosity of the skull and almost invisible sutures suggest that this individual had reached a mature or even senescent stage. At the time of death, the animal suffered from various pathologies. In addition to an already known healed fracture on the left forelimb, peculiar bony spurs and laminae on the craniomandibular joint region likely represent symptoms of jaw joint arthritis. Two separate molar-like teeth arising from the same alveolus in the left lower jaw, if not a postmortem damage, may represent a case of supernumerary teeth anomaly (Fig. 1 d). The palaeoecological inferences rendered possible by the anatomical assessment are immediate; quantitative analysis based on dental and cranial morphofunctional traits places A. adriaticus among other extinct and extant reptiles which specialise on durophagy – the consumption of hard-shelled molluscs like clams and snails. Most representatives of the Hylaeochampsidae seem to have exhibited an array of unusual dental adaptations, with possibly omnivorous, herbivorous and other durophagous species. Acynodon adriaticus appears to have one of the most obvious and dramatic sets of traits linked to durophagy, being probably one of the most specialized known crocodilians toward this diet, benefiting from an increase of molluscan diversity during the Upper Cretaceous. Our new understanding of this small crocodilian points to a likely a slow forager inhabiting turbid or densely vegetated shallow waters, gripping mollusc shells with its finely wrinkled teeth and crushing them with increased muscular power and optimized lever configuration. Thanks to this unusual dietary specialization Acynodon may have also avoided competition with other coeval, yet undiscovered crocodiles. As with modern durophages, A. adriaticus probably relied on relatively intense intra-oral food processing, an unusual strategy for living crocodilians but consistent with their former anatomical and ecological diversity. These remarkable results currently represent the first tomographic approach to the study of Villaggio del Pescatore’s fossil vertebrates, highlighting the great potential of further applications on other fossil specimens from the same locality. Elettra Highlights 2024–2025 62
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