Osteology of a New Specimen of Macrocnemus aff. M. fuyuanensis (Archosauromorpha, Protorosauria) from the Middle Triassic of Europe: Potential Implications for Species Recognition and Paleogeography of Tanystropheid Protorosaurs

Abstract

Over the past two decades, a wealth of marine and terrestrial reptiles, including protorosaurian archosauromorphs, has been described from Triassic shales and limestone layers in southern China. Recovered from the eastern margin of the Tethys Ocean, these forms often show remarkable similarities to taxa that were previously known and described from Europe, i.e., the western Tethyan margin. One protorosaurian that is known from the western and the eastern Tethyan province is the genus Macrocnemus, with currently three recognized species: (1) Macrocnemus bassanii from the Middle Triassic Besano Formation and Meride Limestone (late Anisian-early Ladinian), UNESCO World Heritage Site Monte San Giorgio, Ticino, Switzerland; (2) Macrocnemus obristi from the Prosanto Formation (early Ladinian) of the Ducan area, Grisons, Switzerland; and (3) Macrocnemus fuyuanensis from the Falang Formation (Ladinian), Yunnan Province, southern China. Recently a new specimen, PIMUZ T 1559, from the upper Besano Formation at Meride, Ticino, Switzerland, was prepared, revealing a disarticulated skeleton which includes most of the cranium and lower jaw, pre-caudal vertebral column and ribs, the forelimbs, and girdle elements. Unambiguously assignable to the genus Macrocnemus, it evinces particularly gracile elongated cervical ribs, as well as a humerus/radius ratio that is comparable only to that of M. fuyuanensis from southern China. Based on this feature we tentatively recognize the new specimen as M. aff. fuyuanensis from Europe. The position and exquisite preservation of the clavicle and interclavicle in this specimen allows a revision of the shoulder girdle of Macrocnemus when articulated, which also has implications for closely related protorosaurian taxa, Jaquier et al.Osteology and Paleogeography of Macrocnemus such as the long-necked Tanystropheus. Furthermore, differences in the shape and morphology of the interclavicle including pointed wing-like lateral processes and a short, fusiform caudal process represent rare discrete characters that allow separation of the specimens of M. bassanii from the new specimen of M. aff. fuyuanensis.

A taxonomic revision of the genus Tanystropheus (Archosauromorpha, Tanystropheidae)

“…Nevertheless, these findings indicate a very close association between the faunas from both ends of the Tethys Ocean, as has also been established for other marine reptile clades; e.g., Sauropterygia (Li, 2006;Jiang et al, 2008;Wang et al, 2019) and Ichthyopterygia (Jiang et al, 2006). Furthermore, even for the genus Macrocnemus, a taxon generally considered to be terrestrial, a very close association has been established for specimens occurring at both sides of the Tethys (Jaquier et al, 2017).…”

“…T. longobardicus (Li et al, 2004;Li, 2007;Li et al, 2007;Rieppel et al, 2008;Rieppel et al, 2010;Jiang et al, 2011), as well as potentially Fuyuansaurus acutirostris and Pectodens zhenyuensis Li et al, 2017). These findings greatly enlarged the known geographic range of the clade to the eastern part of the Triassic Tethys Ocean, including that of Tanystropheus spp., which were previously only known from its western margin Rieppel et al, 2010;Jaquier et al, 2017). Two Tanystropheus specimens from China are currently described, both comprising postcranial skeletons lacking the skulls and originating from the Zhuganpo Member of the Falang Formation (latest Ladinian to earliest Carnian) near Xingyi, Guizhou Province, China.…”

“…The vomer of the small morphotype on the other hand is an anteroposteriorly straight bone ( Figure 4C), similar to, but considerably shorter relatively, than the vomer of Macrocnemus spp. (Jaquier et al, 2017). The vomerine tooth row is straight and bears small teeth.…”

“…In recent years the known diversity of tanystropheids has increased substantially with the discovery of new species assigned to new genera; e.g., Amotosaurus rotfeldensis, Langobardisaurus pandolfii, and Augustaburiania vatagini, Dinocephalosaurus orientalis, and Fuyuansaurus acutirostris (Renesto, 1994;Li, 2003;Fraser and Rieppel, 2006;Sennikov, 2011;; and new species of previously established genera; e.g., Macrocnemus obristi and Macrocnemus fuyuanensis Jiang et al, 2011;Fraser and Furrer, 2013;Jaquier et al, 2017); as well as isolated remains that can very likely be attributed to the clade; e.g., the Hayden Quarry tanystropheid material (Pritchard et al, 2015) and the isolated cervical vertebrae from the Sanga do Cabral Formation (De Oliveira et al, 2018), indicating that strongly elongated cervical vertebrae occurred in a more diverse set of early archosauromorphs than previously appreciated. Therefore, the assignment of Tanystropheus antiquus to the genus Tanystropheus should be reconsidered taking specifically into consideration the cervical morphology of other tanystropheid taxa not assigned to the genus Tanystropheus.…”

A new archosauromorph from South America provides insights on the early diversification of tanystropheids

“…Overall, the tanystropheid bauplan is regarded as evidence of a semiaquatic or even completely aquatic lifestyles [10][11][12][13]. However, recent studies failed to support a fully aquatic habit for tanystropheids, demonstrating that Macrocnemus was presumably terrestrial, whereas the lifestyle of the enigmatic Tanystropheus, the largest and most bizarre of all tanystropheids, remains enigmatic [9,14]. The fossil record of tanystropheids and related forms mostly come from the Middle/Late Triassic of Asia, Europe and North America [5,15], and the clade is exceptionally rare in Lower Triassic rocks (see [2,5,8,16,17]).…”