Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography

Feiko Miedema,Stephan N F Spiekman,Jelle W F Reumer,Vincent Fernandez,Torsten M Scheyer

doi:10.1038/s41598-020-68912-4

Feiko Miedema, Stephan N F Spiekman + Show 3 more

Open Access

https://doi.org/10.1038/s41598-020-68912-4

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Abstract

The genus Macrocnemus is a member of the Tanystropheidae, a clade of non-archosauriform archosauromorphs well known for their very characteristic, elongated cervical vertebrae. Articulated specimens are known from the Middle Triassic of Alpine Europe and China. Although multiple articulated specimens are known, description of the cranial morphology has proven challenging due to the crushed preservation of the specimens. Here we use synchrotron micro computed tomography to analyse the cranial morphology of a specimen of the type species Macrocnemus bassanii from the Besano Formation of Monte San Giorgio, Ticino, Switzerland. The skull is virtually complete and we identify and describe the braincase and palatal elements as well the atlas-axis complex for the first time. Moreover, we add to the knowledge of the morphology of the skull roof, rostrum and hemimandible, and reconstruct the cranium of M. bassanii in 3D using the rendered models of the elements. The circumorbital bones were found to be similar in morphology to those of the archosauromorphs Prolacerta broomi and Protorosaurus speneri. In addition, we confirm the palatine, vomer and pterygoid to be tooth-bearing palatal bones, but also observed heterodonty on the pterygoid and the palatine.

Highlights

The genus Macrocnemus is a member of the Tanystropheidae, a clade of non-archosauriform archosauromorphs well known for their very characteristic, elongated cervical vertebrae
Macrocnemus fuyuanensis Li, Zhao and Wang, 2007 13, is known from the Middle Triassic of China. Both the holotype and referred specimens are articulated and contain both cranial and postcranial elements[13,14]. This species is distinguished from other species of Macrocnemus based on limb bone ratios in which its humerus is significantly longer than its radius and its femur is relatively longer than its tibia-fibula in contrast to M. bassanii[13]
A specimen from the Besano Formation of Switzerland has been assigned to Macrocnemus aff. fuyuanensis based on the limb ratios and interclavicle morphology of the specimen, considered closer to that of the Chinese taxon than to that of M. bassanii[12]

Summary

Introduction

The genus Macrocnemus is a member of the Tanystropheidae, a clade of non-archosauriform archosauromorphs well known for their very characteristic, elongated cervical vertebrae. We use synchrotron micro computed tomography to analyse the cranial morphology of a specimen of the type species Macrocnemus bassanii from the Besano Formation of Monte San Giorgio, Ticino, Switzerland. Macrocnemus fuyuanensis Li, Zhao and Wang, 2007 13, is known from the Middle Triassic of China Both the holotype and referred specimens are (semi-) articulated and contain both cranial and postcranial elements[13,14]. This species is distinguished from other species of Macrocnemus based on limb bone ratios in which its humerus is significantly longer than its radius and its femur is relatively longer than its tibia-fibula in contrast to M. bassanii[13]. We were able to identify and describe much of the cranial morphology of the specimen and to reconstruct the cranium of this species in a three-dimensional model

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