Phytosaurs were formidable basal archosauriforms that superficially resembled modern crocodiles, with elongated snouts, armored bodies and a clearly semi-aquatic lifestyle, yet they were not closely related to crocodylians at all. From the late Carnian onwards they spread widely across the Northern Hemisphere, and their well-ossified skulls, skeletons and distinctive bony armor plates dominate many Late Triassic fossil localities. Their classic hunting grounds are the Middle and Upper Keuper sediments of the Germanic Basin, a vast epicontinental depositional realm that stretched across central Europe. But exactly when the last phytosaurs died out has long been disputed, with some researchers arguing the group faded away gradually through the Late Triassic and others insisting it persisted right up to the Triassic-Jurassic boundary.

The Bonenburg specimen, catalogued as WMNM P98442, was found in 2017 during the annual excavation campaign of the University of Bonn in collaboration with the LWL-Museum für Naturkunde in Münster. The clay pit, operated by the Lücking Brick Company about one kilometer north of the village of Bonenburg in the city of Warburg, first gained fame for the unique discovery of a Triassic plesiosaur skeleton, but most of its fossils come from several bone beds, condensation horizons packed with teeth, fish scales and bones. The new osteoderm came from Bone Bed 2, embedded in the dark marine mudstones of the Contorta Beds of the Exter Formation, seven meters above the base of those beds and 17.5 meters below the Triassic-Jurassic boundary exposed in the pit.

What makes this find so powerful is the precision of its dating. Dense sampling of the Bonenburg section and excellent palynomorph preservation allowed researchers to subdivide the standard palynozones of the North German Basin into finer subzones. Bone Bed 2 sits in the upper half of subzone RLb, the middle subzone of the Rhaetipollis-Limbosporites palynozone, which pins the bed to the late middle Rhaetian. By age interpolation on the Triassic chronostratigraphic chart, that translates to roughly 203.5 million years ago, about two million years before the end of the Triassic at 201.4 million years ago. The palynological age is independently corroborated by the presence of the conchostracan Euestheria brodieana immediately below the bone bed, a species known from the middle and late Rhaetian.

The osteoderm itself is a single, slightly damaged left paramedian plate, 64 millimeters long and 62 millimeters wide, with a maximum thickness of about 13 millimeters at its center. Its external surface is extensively sculptured while the internal side is completely flat and smooth. A rounded anteroposterior ridge dominates the exterior, offset medially, and subtle asymmetries in the ridge and the anterior margin reveal that the plate came from the left side of the body. In life, phytosaurs carried two paramedian rows of osteoderms along the neck and two more along the trunk and at least the base of the tail, plus a ventral gular shield, all partially overlapping. Because these plates were not attached to the skeleton, they frequently became scattered after death, which is exactly how the Bonenburg specimen was found, alone in the bone bed without any associated bones or matching osteoderms nearby.

Assigning the plate to a species proved difficult. The researchers compared it with virtually all relevant European phytosaur material, including the partial skeleton MfN MB.R. 2747 of Mystriosuchus sp. from Steinlah near Salzgitter, the only substantial Rhaetian phytosaur skeleton from Europe, as well as collections from Niederschönthal near Basel, Switzerland, and Halberstadt in central Germany, plus Norian osteoderms of Mystriosuchus and Nicrosaurus from southern Germany and Alpine marine deposits in Italy, Austria and Switzerland. None of these offered a convincing match. The Steinlah and Niederschönthal osteoderms are far larger and more coarsely sculptured, while the classic Norian forms show a distinctive pattern of elongate radial pits separated by thin ridges. Intriguingly, the closest resemblance is to two isolated osteoderms from the early Rhaetian of Halberstadt, which share a similar sculpture and a characteristic jagged margin with bone spicules extending into the former soft skin. The Bonenburg plate cannot be assigned to any named species, but its distinctiveness suggests that at least two, and possibly three, phytosaur taxa coexisted in Europe during the Rhaetian.

The fossil record of late phytosaurs is a famously messy affair, littered with dubious contenders for the title of last survivor. A partial skull impression of Machaeroprosopus from the Wingate Sandstone of Lisbon Valley, Utah, long called the last phytosaur, comes from rocks that are probably Rhaetian but not latest Rhaetian in age, and the exact position of the Triassic-Jurassic boundary within those eolian sandstones remains uncertain. Meanwhile, a rostral fragment from St. Audries Bay in Somerset, England, assigned to cf. Mystriosuchus and found in beds that may be either latest Rhaetian or earliest Hettangian, may not even be a phytosaur at all; new discoveries have shown that thalattosuchian crocodylomorphs, which have similar long snouts, likely originated in the Rhaetian or even Norian, making a marine reptile identity entirely plausible. Other claims of earliest Jurassic phytosaurs, such as teeth from a bone bed at Hettange-Grande in France, may be reworked from older sediments or misidentified. Against this uncertain backdrop, the Bonenburg osteoderm stands out because its stratigraphic position is pinned down by palynology with unusual rigor.

A critical question for any fossil from a bone bed is whether it was reworked from older strata. Bone beds are certainly time-averaged accumulations, and the possibility of reworking has been raised before for a temnospondyl humerus from the same bed at Bonenburg. The researchers cannot entirely exclude this scenario for the osteoderm, but they emphasize that the specimen is well preserved and shows no signs of abrasion, looking just like the other bones in the deposit, so there is no positive evidence for redeposition either. The sheer rarity of phytosaur remains at Bonenburg is also noteworthy: apart from this single plate, the site has yielded no phytosaur material at all, and the complete absence of phytosaur teeth is striking given that teeth are abundant in other Rhaetian European bone beds. That scarcity could hint at a declining population by the late middle Rhaetian, but an environmental explanation is equally likely, since several other faunal elements are similarly rare at the site.

Taken together, the evidence points to a clear conclusion: phytosaurs survived in Europe until at least the late middle Rhaetian, roughly 203.5 million years ago, and probably fell victim to the end-Triassic extinction event about two million years later. This runs counter to the view that terrestrial tetrapods experienced no dramatic losses at the Triassic-Jurassic boundary and instead supports the idea that phytosaurs were thriving shortly before a catastrophic end. The large body size of the Steinlah Mystriosuchus and the improved dating of Rhaetian finds from Switzerland and northern Germany reinforce the picture of a group still flourishing a few million years before the end of the Triassic, only to be eliminated abruptly.

The Bonenburg discovery also highlights how much remains to be learned from even fragmentary fossils. The researchers suggest that future work, including quantitative morphological approaches and machine-learning analyses of osteoderm sculpture, could refine the descriptive terminology of these armor plates and eventually allow isolated specimens like WMNM P98442 to be assigned to named taxa. For now, a modest plate of sculptured bone from a Westphalian clay pit holds the crown as the last well-dated phytosaur, a quiet witness to the final flourish of one of the Triassic’s great reptile lineages just before the world changed forever.

Subject of Research: Phytosaur survival into the late middle Rhaetian and implications for phytosaur extinction at the end of the Triassic

Article Title: A phytosaur osteoderm from a late middle Rhaetian bone bed of Bonenburg (North Rhine-Westphalia, Germany): Implications for phytosaur extinction

Article References: Sander, P. M., & Wellnitz, P. W. (2024). A phytosaur osteoderm from a late middle Rhaetian bone bed of Bonenburg (North Rhine-Westphalia, Germany): Implications for phytosaur extinction. Fossil Record, 27(1), 147-158. https://doi.org/10.3897/fr.27.114601

Image Credits: AI Generated

DOI: 10.3897/fr.27.114601

Keywords: phytosaurs, osteoderm, Rhaetian, Triassic-Jurassic boundary, Bonenburg, Germany, end-Triassic extinction, Exter Formation, bone bed, palynology, archosauriforms, Mystriosuchus