Mysterious Eusaurosphargis-like Ribs Found in Anisian Triassic Netherlands

Remarkable dorsal ribs with distinct uncinate processes from the early Anisian of the Germanic Basin (Winterswijk, The Netherlands)

Authors:

Klein et al

Abstract:

Isolated dorsal ribs of tetrapods from the Lower Muschelkalk (early Anisian, Middle Triassic) of Winterswijk, The Netherlands, with a distinct posteriorly located broad uncinate process are described. Dorsal ribs with posteriorly directed uncinate processes are known from various marine reptile groups such as the placodont Paraplacodus, the Saurosphargidae, the enigmatic genus Eusaurosphargis, and the eusauropterygian Bobosaurus. Their rib morphology is compared in detail with the ribs from Winterswijk presented herein. Some of the dorsal ribs from Winterswijk described resemble the morphology of dorsal ribs of Eusaurosphargis, which is known from the Besano Formation (Grenzbitumenhorizont, Anisian-Ladinian boundary, Middle Triassic) of Northern Italy. The morphology of one of the ribs is unique among the compared taxa. The slenderness of the rib, the gracile and constricted rib head, the shape and extension of the uncinate process, and its restriction to the proximal part of the rib excludes saurosphargid or Eusaurosphargis affinities but does also not fit to the morphology of dorsal ribs characteristic for Paraplacodus or any other taxon known so far. Different anatomical positions can also be excluded. Hence, this rib indicates the presence of an additional yet unknown taxon in Winterswijk.