Friday, September 13, 2019

[Paleontology • 2019] The Mystery of Mystriosaurus: Redescribing the poorly known Early Jurassic Teleosauroid Thalattosuchians Mystriosaurus laurillardi and Steneosaurus brevior

Mystriosaurus laurillardi Kaup, 1834

in Sachs, Johnson, Young & Abel, 2019. 

The genus Mystriosaurus, established by Kaup in 1834, was one of the first thalattosuchian genera to be named. The holotype, an incomplete skull from the lower Toarcian Posidonienschiefer Formation of Altdorf (Bavaria, southern Germany), is poorly known with a convoluted taxonomic history. For the past 60 years, Mystriosaurus has been considered a subjective junior synonym of Steneosaurus. However, our reassessment of the Mystriosaurus laurillardi holotype demonstrates that it is a distinct and valid taxon. Moreover, we find the holotype of “Steneosaurus” brevior, an almost complete skull from the lower Toarcian Whitby Mudstone Formation of Whitby (Yorkshire, UK), to be a subjective junior synonym of M. laurillardiMystriosaurus is diagnosed in having: a heavily and extensively ornamented skull; large and numerous neurovascular foramina on the premaxillae, maxillae and dentaries; anteriorly oriented external nares; and four teeth per premaxilla. Our phylogenetic analyses reveal M. laurillardi to be distantly related to Steneosaurus bollensis, supporting our contention that they are different taxa. Interestingly, our analyses hint that Mystriosaurus may be more closely related to the Chinese teleosauroid (previously known as Peipehsuchus) than any European form.

 Key words: Thalattosuchia, Teleosauroidea, Mystriosaurus, Jurassic, Toarcian Posidonienschiefer Formation, Whitby Mudstone Formation, Germany, UK.

Fig. 2. Teleosauroid thalattosuchian Mystriosaurus laurillardi Kaup, 1834 (NHMUK PV OR 14781, holotype of Steneosaurus brevior Blake, 1876, lower Toarcian of Whitby (Yorkshire, UK); skull in lateral, dorsal, and ventral views.

Teleosauroid thalattosuchian Mystriosaurus laurillardi Kaup, 1834:
HLMD V946-948, holotype, lower Toarcian of Altdorf (southern Germany); skull in dorsal view.
NHMUK PV OR 14781, holotype of Steneosaurus brevior Blake, 1876, lower Toarcian of Whitby (Yorkshire, UK); skull in dorsal view.
Reconstruction of Mystriosaurus laurillardi Kaup, 1834 cranium and mandible in dorsal view.

Systematic palaeontology

Thalattosuchia Fraas, 1901
Teleosauroidea Geoffroy Saint-Hilaire, 1831

Genus Mystriosaurus Kaup, 1834

Type species: Mystriosaurus laurillardi Kaup, 1834,
Altdorf bei Nürnberg, lower Toarcian, Lower Jurassic.

Mystriosaurus laurillardi Kaup, 1834

1834 Mystriosaurus laurillardi gen. et sp. nov.; Kaup 1834: p. 28 [not figured].
1876 Steneosaurus brevior sp. nov.; Blake 1876: pl. 1:1, 2.

 Holotype: HLMD V946-948, an incomplete cranium and mandible.

Type locality: Altdorf bei Nürnberg, Nürnberger Land district, Bavaria, southern Germany. Type horizon: Posidonienschiefer Formation, Schwarzjura Group, probably Harpoceras serpentinum Sub-Mediterranean Ammonite Zone, lower Toarcian, Lower Jurassic (Page 2003; German Stratigraphic Commission 2016).

Emended diagnosis.— Teleosauroid crocodylomorph with the following unique combination of characters (proposed autapomorphic characters indicated by an asterisk): robust skull with well-developed and extensive ornamentation on the premaxillae, maxillae, nasals*, frontal, prefrontal, lacrimal, jugal*,postorbital and squamosal*; frontal ornamentation composed of small sub-circular to elongate pits that are closely spaced or that fuse and become a ridge-groove pattern; mesorostrine snout (preorbital length is approximately 66% of skull length), which is broad near the orbits but narrows and becomes slightly more slender anteriorly; slight constriction of the snout in front of the orbits; ...

Photograph of teleosauroid thalattosuchian specimen (UH 7), lower Toracian of Holzmaden (southwestern Germany), which was described by Mueller-Töwe (2006) as “Steneosaurusbrevior Blake, 1876, and which we herein refer to tentatively as ?Mystriosaurus sp.

The presented combination of characters clearly demonstrates that Mystriosaurus is a distinct and valid genus. Recognition of this taxon, and its presence in England and Germany, furthers our understanding of European teleosauroid biodiversity during the Toarcian. From lower Toarcian deposits of England, Luxembourg, and Germany, a diverse array of teleosauroids have been recovered: the lateral-orbited and small-bodied longirostrine species Steneosaurus gracilirostris; the large-bodied longirostrine form Steneosaurus bollensis; the mesorostrine and robust toothed taxon Mystriosaurus laurillardi, and the heavily armoured and longirostrine Platysuchus multiscrobiculatus (Westphal 1962; Johnson et al. 2018; Young and Steel in press). Although these taxa are found in many of the same formations, their geographical distributions need to be re-examined based on our developing understanding of their comparative anatomies. Nevertheless, our re-description of Mystriosaurus further highlights that teleosauroids had already achieved ecological diversity through niche partitioning, and large body-size by the early Toarcian.

Sven Sachs, Michela M. Johnson, Mark T. Young, and Pascal Abel. 2019.  The Mystery of Mystriosaurus: Redescribing the poorly known Early Jurassic Teleosauroid Thalattosuchians Mystriosaurus laurillardi and Steneosaurus brevior. Acta Palaeontologica Polonica. 64(3); 565–579. DOI:  10.4202/app.00557.2018