Metriorhynchidae, Fitzinger, 1843

Madzia, Daniel, Szczygielski, Tomasz & Wolniewicz, Andrzej S., 2021, The giant pliosaurid that wasn’t-revising the marine reptiles from the Kimmeridgian, Upper Jurassic, of Krzyżanowice, Poland, Acta Palaeontologica Polonica 66 (1), pp. 99-129 : 114-117

Metriorhynchidae indet.

Figs. 7 , 8 .

Material.— MZ VIII Vr-72, partial jaws and dentition from an unknown locality and age (see Discussion below); reported by Tyborowski and Błażejowski (2019a) as originating from the light yellow marl, upper Kimmeridgian of Krzyżanowice, Poland ( Dąbrowska 1957; Borsuk-Białynicka and Młynarski 1968).

Tyborowski and Błażejowski (2019a, b) reported a limestone block allegedly preserving a left premaxilla and right dentary of Pliosaurus ( Tyborowski and Błażejowski 2019a) , collectively catalogued as MZ VIII Vr-72 ( Fig. 7 ). As already noted, the same catalogue number ( MZ VIII Vr-72) has been previously assigned to incomplete jaws originating from the Oxfordian of Załęcze Wielkie that were described and illustrated by Maryańska (1972: pl. 1: 2) and referred to Peloneustes sp. More recently the specimen was proposed to represent a possible teleosaurid thalattosuchian ( Ketchum and Benson 2011: appendix S1, p. 11).

Description.—The interpretation of MZ VIII Vr-72 by Tyborowski and Błażejowski (2019a) is ambiguous, both morphologically and taxonomically, and the description contains numerous errors and misinterpretations. The “left premaxilla” (inconsistently referred to as the “right premaxilla” in other parts of their manuscript; e.g., see Tyborowski and Błażejowski 2019a: 745) was described as bearing 10 alveoli, with the first alveolus being supposedly reduced. The presumed posterior margin of the “premaxilla” (the premaxilla-maxilla contact) was considered to form a “zig-zag” suture, and the teeth preserved in the alveoli were described as trihedral. These purportedly observed characters (specifically the reduced first alveolus, “zig-zag” suture at the premaxilla-maxilla contact, and presence of trihedral teeth) then served as the basis to assign the specimen to the globally distributed Late Jurassic pliosaurid taxon Pliosaurus .

In their description of the “right dentary”, in turn, Tyborowski and Błażejowski (2019a: 746) stated that “[a]ll of the dentary teeth are preserved” and that “[t]he crowns of all teeth are robust, conical, gently recurved lingually and display gently longitudinal ridges throughout”, with the “longitudinal ridges” being “regularly and evenly spaced throughout”. Tyborowski and Błażejowski (2019a: 746) further noted that “[t]he preserved dentary teeth share trihedral morphology” which, again, supposedly supports the identification of the specimen as Pliosaurus . However, an imprint of a tooth crown, preserved on the same block as the “premaxilla” and the “dentary” was said to show a subtrihedral morphology because it was “conical, curved and its cross-section has flattened labial and lingual surfaces” ( Tyborowski and Błażejowski 2019a: 746); its “distal surface bears prominent, apicobasally-oriented carinae” and its “labial surface […] bears many apicobasal enamel ridges”. Both the “premaxillary” and “dentary” dentitions were further characterized as being heterodont and anisodont though the two terms just appear to be used interchangeably ( Tyborowski and Błażejowski 2019a: 745, 746).

Even though the “left premaxilla” is badly preserved, the observed tooth count makes it highly unlikely that it is indeed a premaxilla. The element likely represents either a right maxilla or a left dentary (see below). Furthermore, the dental morphology and the distribution of the outer enamel structural elements clearly show that the specimen cannot be referred to Pliosaurus . In all species currently assigned to that taxon, the premaxillae have typically five or six trihedral or subtrihedral teeth (Knutsen 2012; Benson et al. 2013; Zverkov et al. 2018). Although Tyborowski and Błażejowski (2019a) noted that the teeth of MZ VIII Vr-72 were all trihedral (except for a single tooth, illustrated in their figure 6, that was characterized as subtrihedral), this is incorrect. In trihedral teeth, their cross section is nearly triangular. The labial surface of tooth crowns is almost flat and the flat segment is bordered by prominent carinae see e.g., Zverkov et al. 2018: fig. 2A). Subtrihedral teeth, in contrast, have their cross sections somewhat transitional between trihedral and subcircular, their labial surfaces mostly lack ridges, and are only slightly flattened. The term “subtrihedral” was introduced by Benson et al. (2013) to describe the teeth of Pliosaurus kevani Benson, Evans, Smith, Sassoon, Moore-Fay, Ketchum, and Forrest, 2013 , though it has been used for other taxa as well (for a detailed overview, see the supplementary information in Zverkov et al. 2018: table S1).

Our personal examination of MZ VIII Vr-72 reveals that the cross sections of all the “premaxillary” and “dentary” teeth are actually subcircular, or very indistinctly labiolingually compressed at most. None of the “premaxillary” nor dentary” crowns show “gently longitudinal ridges” similar to those of plesiosaurs. Two carinae are present, one placed mesially and one distally. In addition, only subtle structural elements, unlike the well-developed apicobasal ridges in pliosaurid teeth, can be observed. The basal half of the crowns is ornamented with short, apicobasally oriented elements resembling ridglets (sensu Zverkov et al. 2018; see Fig. 8 ). The ornamentation becomes finer apically and forms a vermicular pattern.

The apical and basal structural elements are separated by a short segment of smoother enamel ( Fig. 8A 1 and A 2 ), although this is not a rule as in some teeth the ridglets and the vermicular pattern meet at mid-section ( Fig. 8A 3 ).

Other discussed features are problematic or misinterpreted as well. There is no evidence indicating that the first “premaxillary” alveolus was substantially reduced. The anterior part of the “premaxilla” is incomplete and the first preserved tooth (possibly originating from that region) is markedly displaced (see Fig. 7A 1 ). Nothing suggests that either the “premaxillary” or “dentary” dentition is heterodont (variable with respect to shape) or clearly anisodont variable with respect to size), though minor differences in size can be observed. Furthermore, the “dentary” certainly does not preserve the complete set of teeth (as stated; see above). It is also apparent that the posterior portion of the premaxilla” does not indicate the presence of the “zig-zag” suture as observable in Pliosaurus (see e.g., Benson et al. 2013: 3 A). The element is irregularly broken, posteriorly, and the broken surface only superficially resembles a “zig-zag” shape.

Tyborowski and Błażejowski (2019b) also noted that the “ Pliosaurus ”remainsdiscoveredatKrzyżanowiceindicated the presence of a taxon comparable in size to Pliosaurus funkei Knutsen, Druckenmiller, and Hurum, 2012b from the middle Volgian (Tithonian) of Svalbard, Norway, and Pliosaurus kevani from the lower Kimmeridgian of Dorset, United Kingdom ( Benson et al. 2013). Despite the fact that both taxa are mostly known from non-overlapping parts of their skeletons, which makes comparisons between them difficult, both Pliosaurus funkei and Pliosaurus kevani had skulls approximately 2 m long and their total body lengths could have reached, and possibly exceeded, 10 m ( Knutsen et al. 2012b; Benson et al. 2013). However, Tyborowski and Błażejowski (2019b) did not specify how their estimates for the total body size of MZ VIII Vr-72 were obtained. In their first paper on the assemblage from Krzyżanowice, the authors noted that their study was preliminary ( Tyborowski and Błażejowski 2019a: 741) and that they reported “only a small part of the enormous collection of marine reptile bones from the Krzyżanowice site” ( Tyborowski and Błażejowski 2019a: 743), but they also noted that the material they described comprises some of the most informative specimens ( Tyborowski and Błażejowski 2019a: 743). We assume, therefore, that their size estimates are based on the published “premaxilla” and “dentary”. If so, our reassessment of the material does not indicate the taxon could have approached the size of the aforementioned pliosaurids. The heights of the largest tooth crowns, the only elements that could be used to infer some size-related information, reach up to 40 mm. Even though the crowns of Pliosaurus kevani are up to 50 mm high, the size of pliosaurid teeth would be a weak proxy for their total body length. For example, whereas the tooth crowns of Pliosaurus kevani are apicobasally short comparable to the teeth of some other species referred to Pliosaurus (see Fischer et al. 2017: supplemental information data S3; Zverkov et al. 2018: fig. 6B), the reconstructed skull length of that taxon suggests it represented one of the largest Late Jurassic pliosaurids ( Benson et al. 2013).

We propose that, rather than a 10-meter-long representative of Pliosaurus, MZ VIII Vr-72 actually represents a metriorhynchid thalattosuchian (Crocodylomorpha) that probably did not reach half the size (see e.g., Young et al. 2011). The teeth of MZ VIII Vr-72 share a number of characteristics present in dentitions of members of both major metriorhynchid subclades, the metriorhynchines and the geosaurines; though not developed in all of them. These include lingually curved and subcircular to indistinctly labiolingually compressed cross sections near the base of the crowns; indistinctly blunt apices; well-developed and continuous carinae; no constriction at the crown/root junction; and ornamentation that includes a combination of structural elements restricted to particular segments of the tooth crowns. When metriorhynchids with such dental morphology and general characteristics of enamel structural elements are considered, MZ VIII Vr-72 shares the presence of short, apicobasally oriented ridges/ridglets limited to the basal part of their crowns and changing apically into an indistinct to conspicuous vermicular pattern with some taxa, such as the geosaurines Suchodus brachyrhynchus ( Eudes-Deslongchamps, 1868b) , Torvoneustes spp. , and Tyrannoneustes lythrodectikos Young, de Andrade, Brusatte, Sakamoto, and Liston, 2013a , and the metriorhynchines Gracilineustes leedsi Andrews, 1913 , and Metriorhynchus superciliosus ( Blainville, 1853) ( Andrade et al. 2010; Young et al. 2013a, b). All of these taxa, however, appear to lack the smoother mid-section that is present in most teeth of MZ VIII Vr-72, in which the lingual surface is exposed. Also, as in Suchodus spp. and T. lythrodectikos , but unlike in the other metriorhynchids mentioned above, the crown ornamentation in MZ VIII Vr-72 is well-developed on the lingual side but near-absent labially ( Fig. 8 ). The lingual ornamentation on the crowns of G. leedsi and M. superciliosus is better developed than that on the labial side as well but it is also well-noticeable labially.

It is worth noting that some teleosauroids, such as the machimosaurins Lemmysuchus obtusidens ( Andrews, 1909) , Machimosaurus spp. , and Yvridiosuchus boutilieri ( Eudes-Deslongchamps 1868a) , show teeth similar in certain features of their crown ornamentation to those of MZ VIII Vr-72 as well (Young et al. 2014; Johnson et al. 2018, 2020). Specifically, as in MZ VIII Vr-72, the tooth crowns of the machimosaurins show ridged basal halves of the crowns that gradually change into a vermicular pattern at the apex. However, unlike the ornamentation in the crowns of MZ VIII Vr-72, the enamel structural elements in the machimosaurins are of high relief. The tooth crowns of L. obtusidens , Machimosaurus spp. , and Y. boutilieri further differ from those of MZ VIII Vr- 72 in their morphology; they are conical, without labiolingual compression, and rather robust, and their apices are conspicuously blunt (see Young et al. 2014: figs. 9, 11, 21, 28, 29, 31–35; Johnson et al. 2018: fig. 12; Johnson et al. 2020: figs. 4L, 8D–F).

We were not given permission to examine the microscopic anatomy of the dentition of MZ VIII Vr-72 and so were unable to describe its carinal morphology, which is also used in thalattosuchian taxonomy (e.g., Andrade et al. 2010) and could possibly provide additional data important for determining the taxonomic affinity of the specimen.

Figures

Fig. 7. Jaw fragments of an indeterminate metriorhynchid (MZ VIII Vr-72) from an unknown locality and age, preserved on the opposite sides of a limestone block: the “premaxilla” (A1) and the “dentary” (A2) of Tyborowski and Błażejowski (2019a, b).

Fig. 8. Selected tooth crowns of an indeterminate metriorhynchid (MZ VIII Vr-72) from an unknown locality and age, showing observable features of the enamel: “premaxillary” teeth in lingual view (A1, A2), “dentary” teeth (A3, A4), showing the only tooth crown accessible from lingual side (A3) and the smooth labial surface (A4). Note the absence of the smooth enamel band at the mid-section (A3). Abbreviations: dc, distal carina; rgl, ridglets; se, horizontal band of smooth enamel; vp, vermicular pattern. Scale bars 10 mm.

Fig. 1. Geographic distribution of Jurassic marine reptile fossil occurrences in Poland. See Table 1 for details. Silhouettes obtained from phylopic.org: Geosaurinae (Dmitry Bogdanov, CC BY 3.0), Ichthyosauria, Metriorhynchinae, and Teleosauroidea (Gareth Monger, CC BY 3.0), indeterminate reptile clade (public domain), Plesiosauroidea (Adam Stuart Smith, CC BY-SA 3.0), Pliosauridae (Nobu Tamura, CC BY-SA 3.0), and Testudinata (public domain).

Fig.2.The thalassochelydian turtle Craspedochelys? sp., MZVIII Vr-71,from the Kimmeridgian of Krzyżanowice.Central part of the carapace (Fig.6A:1), in external (A1, A2) and visceral (A3, A4) views.A1, A3, photographs; A2, A4, explanatory drawings.

Fig. 3. The thalassochelydian turtle Craspedochelys? sp., MZ VIII Vr-71, from the Kimmeridgian of Krzyżanowice, parts of the carapace and plastron likely belonging to a single individual.A. Anterior part of the carapace and plastron (Fig. 6A: 2) in dorsal (A1) and ventral (A2) views.B. Posterior part of the cara- pace and plastron (Fig. 6A: 3) in dorsal (B1) and ventral (B2) views.The fragments likely belong to the same individual as the material shown in Fig. 4A–C.