Phytosauria indet., 1986

Sander, P. Martin & Wellnitz, Paul 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), pp. 147-158 : 147

Phytosauria indet.

Material.

One single, slightly damaged left paramedian osteoderm, WMNM P98442.

Locality and horizon.

Clay pit #3 of Lücking Brick Company, 1 km north of the village of Bonenburg, City of Warburg, North Rhine-Westphalia, Germany (Fig. 1A ). The specimen derives from Bone Bed 2 in the dark marine mudstones of the Contorta Beds of the Exter Formation, 7 m in the section above the base of the Contorta Beds and 17.5 m below the Triassic-Jurassic boundary exposed in the pit (Fig. 1B ).

Morphological description.

The external side of the Bonenburg osteoderm WMNM P98442 is extensively sculptured, whereas the internal, or visceral, side is smooth (Fig. 2 ). The external side is dominated by a rounded ridge or keel, indicating the orientation of the osteoderm relative to the body long axis (Fig. 2A, B ). This anteroposterior ridge is offset medially as can be seen on articulated phytosaur specimens, providing the medial direction. One end of the ridge extends to the margin of the osteoderm, whereas the other does not. The latter asymmetry indicates anterior because the ridge does not reach the anterior osteoderm margin in phytosaur osteoderms. Together with the asymmetry of the location of the ridge, the location of the anterior margin indicates that the osteoderm is from the left side of the body. The thickness of the osteoderm decreases in anterior and lateral directions (Fig. 2C, D ). The osteoderm shows two distinct indentations, one on the lateral and one on the posterior margin (Fig. 2 ). The latter resulted from damage sustained during discovery. The bone shows no signs of abrasion.

In mediolateral direction, the osteoderm is 62 mm wide and in anteroposterior direction, it is 64 mm long. It shows a maximum thickness of about 13 mm at its centre. Except for the thick ridge, the external sculpture on the osteoderm is of relatively low relief (Fig. 2A, B ). Towards lateral, there are some indistinct pits, but there are no sharp crests or grooves. Only the region medial to the main ridge shows a deep sulcus.

The lateral part of the external surface of the osteoderm, that is not sculptured, shows a radial, fan-like structure on the surface of the bone (Fig. 2A, B ). This structure originates in the centre of the bone, right below the middle of the anteroposterior ridge. The fan structure also affects the silhouette of the lateral part of the bone. The internal surface of the osteoderm is completely flat and shows no sculpturing. There are multiple small hole-like structures on the medial part of the external surface of the bone (Fig. 2E, F ).

The general morphology of the Bonenburg osteoderm (Fig. 2 ) fits the description of phytosaur osteoderms in the literature ( Huene 1922; Gozzi and Renesto 2003; Scheyer et al. 2014) (Fig. 3 ). A more detailed investigation of the osteoderms of the Lombardian Mystriosuchus specimen MCSNB 10087 ( Gozzi and Renesto 2003) and other articulated and osteoderm-bearing phytosaur skeletons might help to further constrain the anatomical position of the Bonenburg osteoderm.

Figures

Figure 1. Locality and stratigraphy of Bonenburg clay pit # 3 of Luecking Brick Company. A. Location of the clay pit in the eastern part of the State of North Rhine-Westphalia, Germany; B. Simplified stratigraphy of the Norian to Hettangian section exposed at the Bonenburg clay pit, with a special emphasis on the location of the four bone beds and the plesiosaur skeleton in the section. The main part of the section is made up by the Exter Formation which is subdivided into the Postera Beds, Contorta Beds and Triletes Beds, in ascending order. The thick dashed line on top of the Contorta Beds indicates truncation by a low-angle fault. Note that the reddish mudstones above the dashed white line also belong to the Triletes Beds. Colours of the rock types in the main stratigraphic column approximate colours in fresh outcrop. General abbreviations: Bb, bone bed; Cly, claystone; Fss, fine-grained sandstone; Het., Hettangian; L. J., Lower Jurassic; Md, mudstone; Nor., Norian; PF, Psilonotenton Formation; Slt, siltstone. Abbreviations of palynozones: CE, Classopollis - Enzonalasporites palynozone; RL, Rhaetipollis - Limbosporites palynozone; RP, Riccisporites - Polypodiisporites palynozone; PiK, Pinuspollenites - Kraeuselisporites palynozone. Subzones are labelled alphabetically. Modified from Wintrich et al. (2017), palynostratigraphy is from Gravendyck et al. (2020).

Figure 2. Left paramedian phytosaur osteoderm WMNM P 98442 from the late middle Rhaetian Bone Bed 2 of the Contorta Beds of the Exter Formation of Bonenburg, North Rhine-Westphalia, Germany. A, B. Photograph and drawing of external view showing the typical phytosaur radiating sculpture and the central longitudinal ridge of a paramedian osteoderm. Note that the anterior part (left) is less sculptured and that the ridge does not extend to the anterior edge; C, D. Photograph and drawing of anterior view. Note the thinness of the edge which was underlapping the preceding osteoderm; E, F. Photograph and drawing of internal view, showing the flat and smooth surface. Abbreviations: a, anterior; m, medial.

Figure 3. Comparison of selected Rhaetian and Norian phytosaur osteoderms in external view. A. WMNM P 98442, left paramedian osteoderm from the Rhaetian Bone Bed 2, Exter Formation, Bonenburg, Germany. Note the resemblance of sculpture to C and D; B. MfN MB. R. 2747, five articulated paramedian osteoderms of the left side in association with anterior dorsal vertebrae of Mystriosuchus sp. (Butler et al. 2019), Exter Formation, Steinlah near Salzgitter, Germany. Note the resemblance of sculpture to F; C. Isolated gular osteoderm MfN MB. R. 4224 from the Rhaetian of Halberstadt (Germany) figured by Huene (1922, fig. 112) and assigned to ' Angistorhinops ruetimeyeri ' by him. Note the resemblance of sculpture to A and D; D. Isolated left paramedian osteoderm MfN MB. R. 4372.1 from the Rhaetian of Halberstadt, Germany. Note the resemblance of sculpture to A and C; E. Isolated left paramedian caudal osteoderm MfN MB. R. 4374.1 from the Rhaetian of Halberstadt (Germany) figured by Huene (1922, fig. 87) and assigned by him to a small indeterminate phytosaur; F. Right paramedian osteoderm NHMB N. B. 14 of ' Angistorhinops ruetimeyeri ' from the Rhaetian bone bed of Niederschoenthal (northern Switzerland) figured by Huene (1911 b, Pl. VIII, fig. 2). Note the resemblance of sculpture to B; G. Right paramedian osteoderm MfN MB. R. 4219 from the Rhaetian of Halberstadt (Germany) figured by Huene (1922, fig. 113) and assigned to ' Angistorhinops ruetimeyeri ' by him; H. Left paramedian osteoderm SMNS uncatalogued of Mystriosuchus sp. from the middle Norian Stubensandstein of Aixheim, southwestern Germany; I. Right paramedian osteoderm SMNS 4063 / 7 of Nicrosaurus kapffi from the middle Norian Stubensandstein of Heslach near Stuttgart, south-western Germany. Abbreviations: a, anterior; m, medial; o, osteoderm; v, vertebra.