Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography

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. The genus Macrocnemus was described in the early twentieth century by Baron von Nopcsa based on a fragmentary specimen from Monte San Giorgio 1,2. After subsequent expeditions to the Besano Formation, which crops out in the Monte San Giorgio area, resulted in the discovery of additional specimens, the taxon was more properly described 3. Unfortunately, the holotype, stored in Milan, has since been lost as it was destroyed during the Second World War 4. The phylogenetic affinities of Macrocnemus were initially unclear. Early researchers observed similarities in the shape of the cervical vertebrae with contemporaneous taxa and erected the taxon Protorosauria 5,6. Notable members of this formerly recognised 'superorder' were Tanystropheus, Protorosaurus, Macrocnemus and Prolacerta, and all these taxa have elongated cervical vertebrae. The elongation of the cervical vertebrae is expressed most extremely in the genus Tanystropheus, of which the neck is three times the length of its trunk 7. Researchers were unsure whether to place the clade closer to the Lepidosauria or to the Archosauromorpha (e.g. 7,8). Currently, the members of the Protorosauria are all considered non-archosauriform archosauromorphs (e.g. 9-11). However, the clade itself has been observed to be paraphyletic or even polyphyletic in recent phylogenetic analyses 9 and references therein. Nonetheless, the recent analyses noted above have all recovered a monophyletic Tanystropheidae at the base of Archosauromorpha that includes Macrocnemus as well as Tanystropheus, Amotosaurus, Langobardisaurus, and Tanytrachelos, with Jesairosaurus often recovered as sister taxon to the clade.

The cranial morphology ofTanystropheus hydroides(Tanystropheidae, Archosauromorpha) as revealed by synchrotron microtomography

“…A similar postnarial process is also present in Prolacerta broomi, Azendohsaurus madagaskarensis, and Teyujagua paradoxa in which this process forms a simple articulation with the maxilla (Flynn et al, 2010;Pinheiro, Simão-Oliveira & Butler, 2019;Spiekman, 2018). The premaxilla of Macrocnemus bassanii also has an elongate postnarial process, but additionally bears a posteromedial process, and these two processes form a complicated articulation with the maxilla (Miedema et al, 2020). Since the medial surface of the premaxilla cannot be observed for any known specimen, it is unclear whether Tanystropheus longobardicus possessed a similar posteromedial process.…”

“…The prenarial process of Tanystropheus longobardicus and Macrocnemus spp. is also incipient, and has been reduced completely in rhynchosaurs, Teyujagua paradoxa, and the allokotosaurs Azendohsaurus madagaskarensis, Pamelaria dolichotrachela, and Shringasaurus indicus among early archosauromorphs (Dilkes, 1998;Flynn et al, 2010;Miedema et al, 2020;Nosotti, 2007;Pinheiro, Simão-Oliveira & Butler, 2019;Sengupta, Ezcurra & Bandyopadhyay, 2017). In contrast, the prenarial process is well-established and elongate in Protorosaurus speneri, Prolacerta broomi, Dinocephalosaurus orientalis, and Pectodens zhenyuensis (Gottmann-Quesada & Sander, 2009;Li et al, 2017;Modesto & Sues, 2004;Rieppel, Li & Fraser, 2008).…”

“…Tanystropheidae represents a particularly ecomorphologically diverse group that includes terrestrial (e.g. Macrocnemus bassanii and Langobardisaurus pandolfii), largely aquatic (Tanytrachelos ahynis and Tanystropheus hydroides), and possibly even fully marine (Dinocephalosaurus orientalis) taxa (Liu et al, 2017;Miedema et al, 2020;Olsen, 1979;Renesto & Dalla Vecchia, 2000;Rieppel, Li & Fraser, 2008;Spiekman et al, 2020). The clade had a likely worldwide distribution and occurred between the Early and Late Triassic (De Oliveira et al, 2018Formoso et al, 2019;Pritchard et al, 2015;Sennikov, 2011;Spiekman & Scheyer, 2019).…”

“…However, due to the generally poor and fragmentary preservation of specimens, our understanding of tanystropheids is limited, and information on skull morphology in particular is sparse. Nevertheless, tanystropheids likely exhibited widely diverse cranial morphologies, as can be deduced from their ecological disparity and the diversity of their dentitions, which range from small conical teeth in for instance Macrocnemus bassanii and Amotosaurus rotfeldensis, to the 'fish-trap' type dentition of Tanystropheus hydroides, to the partially tricuspid dentition of Tanystropheus longobardicus and Langobardisaurus pandolfii (Fraser & Rieppel, 2006;Li et al, 2017;Miedema et al, 2020;Rieppel, Li & Fraser, 2008;Spiekman et al, 2020). The dental morphology of Langobardisaurus pandolfii in particular is peculiar, as the premaxilla was likely edentulous and the posteriormost teeth of both the upper and lower jaw were modified into large and flat tooth plates used for crushing, thus representing a unique dental system among tetrapods (Renesto & Dalla Vecchia, 2000;Saller, Renesto & Dalla Vecchia, 2013).…”

“…Synchrotron radiation X-ray micro computed tomography (SRµCT) has recently revealed the cranial morphology of the tanystropheid taxa Macrocnemus bassanii and Tanystropheus hydroides in previously unachievable detail, providing much improved cranial reconstructions (Miedema et al, 2020;Spiekman et al, 2020). This has shown that the cranial morphology of the terrestrial Macrocnemus bassanii is remarkably similar to that of Prolacerta broomi and that Macrocnemus bassanii possessed many characters that are likely plesiomorphic to Archosauromorpha and Tanystropheidae (Miedema et al, 2020). In contrast, Tanystropheus hydroides exhibits a highly derived cranial morphology that bears several adaptations indicating that it was an aquatic ambush predator (Spiekman et al, 2020).…”