A large new Middle Jurassic ichthyosaur shows the importance of body size evolution in the origin of the Ophthalmosauria

Abstract

The Middle Jurassic is an important time period for the evolutionary history of marine reptiles as it represented a transitional phase for many clades. Notably, in ichthyosaurs, many early parvipelvian taxa went extinct. The Middle Jurassic saw the emergence of the derived Ophthalmosauria, ultimately becoming the dominant ichthyosaurian clade by the end of the epoch. Even though this is an important period in the evolutionary history of Ophthalmosauria, our understanding remains limited in terms of morphology and taxonomy due to the scarcity of vertebrate-bearing strata. Here we present a large new ichthyosaur from the Bajocian of Switzerland, represented by an almost complete skull with 3D-preserved bones, the (inter)clavicles and a large portion of the postcranial skeleton. After CT- and surface scanning, we reconstructed the 3D in vivo morphology. Our morphological observations and phylogenetic analyses show that the new taxon named Argovisaurus martafernandezi is nested at the base of the Ophthalmosauria. The holotype and only known specimen of Argovisaurus likely represents an adult individual. Bajocian members of the Ophthalmosauria (Mollesaurus and Argovisaurus) were large-bodied animals, a trait typically associated with the more derived Platypterygiinae. This hints at the importance of a large body size early in ophthalmosaurian evolution.LSID: urn:lsid:zoobank.org:act:C3312628-1544-4B87-BBE3-B12346A30BE3LSID: urn:lsid:zoobank.org:act:23C2BD71-8CF0-4D99-848A-0D631518415B

Oldest record of Machimosaurini (Thalattosuchia, Teleosauroidea): teeth and scavenging traces from the Middle Jurassic (Bajocian) of Switzerland

“…The large size of the ichthyosaur and the inferred thick layer of soft tissue overlying the ribs make scavenging a more plausible source for these traces rather than predation. Taphonomic interpretation suggests that the carcass lay exposed on the seafloor for a prolonged period prior to burial [ 10 ]. Furthermore, only posterior thoracic ribs bear the bite marks, which suggests that the scavenging machimosaurin(s) accessed the tissues from behind like many other scavengers today [ 39 , 40 ].…”

“…The ichthyosaur skeleton (PIMUZ A/III 5279 [ 10 ]) and the three Machimosaurini teeth (two larger tooth crowns, PIMUZ A/III 5281a and PIMUZ A/III 5281b, associated with a lacrimal and the left angular of the ichthyosaur skeleton, respectively; PIMUZ A/III 5281c, a smaller tooth crown associated with the axis neural arch) were found in the Oberegg quarry, Auenstein, Canton Aargau, Switzerland ( figure 1 a ). They were recovered from the Lower Acuminata beds of the Hauptrogenstein Formation ( subfurcatum/niortense zone), which are late Bajocian ( figure 1 b ) in age [ 20 ].…”

“…Thus, early Middle Jurassic reptiles constitute rare finds, with only a few valid species described globally (e.g. [8][9][10]).…”

“…The purpose here is to describe three tooth crowns from the Bajocian, which constitute the oldest record of the Machimosaurini tribe. These teeth are considered a rare find indeed, as they were recovered in association with the skeletal remains of a new Middle Jurassic ichthyosaur [ 10 ]. Finally, the bite marks on the ichthyosaur skeleton are identified as potential scavenging traces left by the teeth of the Machimosaurini.…”

Anatomy and size of Megateuthis, the largest belemnite

“…Neither was Argovisaurus (Miedema et al, 2024) the largest ichthyosaur of the Jurassic, nor were the ammonites the largest of all times (Stevens, 1988). Nevertheless, the middle European region is remarkable in the co-occurrence of several big animal species.…”