The First Metriorhynchid Crocodylomorph from the Middle Jurassic of Spain, with Implications for Evolution of the Subclade Rhacheosaurini

Abstract

BackgroundMarine deposits from the Callovian of Europe have yielded numerous species of metriorhynchid crocodylomorphs. While common in English and French Formations, metriorhynchids are poorly known from the Iberian Peninsula. Twenty years ago an incomplete, but beautifully preserved, skull was discovered from the Middle Callovian of Spain. It is currently the oldest and best preserved metriorhynchid specimen from the Iberian Peninsula. Until now it has never been properly described and its taxonomic affinities remained obscure.Methodology/Principal FindingsHere we present a comprehensive description for this specimen and in doing so we refer it to a new genus and species: Maledictosuchus riclaensis. This species is diagnosed by numerous autapomorphies, including: heterodont dentition; tightly interlocking occlusion; lachrymal anterior process excludes the jugal from the preorbital fenestra; orbits longer than supratemporal fenestrae; palatine has two non-midline and one midline anterior processes. Our phylogenetic analysis finds Maledictosuchus riclaensis to be the basal-most known member of Rhacheosaurini (the subclade of increasingly mesopelagic piscivores that includes Cricosaurus and Rhacheosaurus).Conclusions/SignificanceOur description of Maledictosuchus riclaensis shows that the craniodental morphologies that underpinned the success of Rhacheosaurini in the Late Jurassic and Early Cretaceous, as a result of increasing marine specialization to adaptations for feeding on fast small-bodied prey (i.e. divided and retracted external nares; reorientation of the lateral processes of the frontal; elongate, tubular rostrum; procumbent and non-carinated dentition; high overall tooth count; and dorsolaterally inclined paroccipital processes), first appeared during the Middle Jurassic. Rhacheosaurins were curiously rare in the Middle Jurassic, as only one specimen of Maledictosuchus riclaensis is known (with no representatives discovered from the well-sampled Oxford Clay Formation of England). As such, the feeding/marine adaptations of Rhacheosaurini did not confer an immediate selective advantage upon the group, and it took until the Late Jurassic for this subclade to dominate in Western Europe.

Postcranial elements of "Maledictosuchus riclaensis" (Thalattosuchia) from the Middle Jurassic of Spain

“…The results of the analysis maintain the phylogenetic position of Maledictosuchus riclaensis, and the strict consensus is identical to that of Parrilla-Bel et al (2013) (Fig. 7).…”

“…The apomorphy of metriorhynchids is state (1) instead of state (0) as was said in Young et al (2012) and Parrilla-Bel et al (2013).…”

“…The analysis was based on the matrix of Parrilla-Bel et al (2013), only adding the vertebrae data (5 characters were recoded). It included 74 taxa coded for 240 characters, with the non-crocodylomorph pseudosuchian archosaur Postosuchus kirkpatricki used as the outgroup taxon.…”

“…(Ruiz-Omeñaca et al, 2010) and lately reassigned to cf. Plesiosuchus manselii (Young et al, 2012), and a skull and ver-tebrae described as Maledictosuchus riclaensis (Parrilla-Bel et al, 2013). This taxon comes from the Ágreda Formation (Middle Callovian) in Ricla (Zaragoza).…”

Revision of the Late Jurassic crocodyliform Alligatorellus, and evidence for allopatric speciation driving high diversity in western European atoposaurids

“…Wellnhofer (1971) regarded this feature as diagnostic of Alligatorellus. However, it may be a synapomorphy of all atoposaurids: in other crocodyliforms with divided external nares, this division is formed by a sagittal projection of the premaxillae, e.g., the metriorhynchid Maledictosuchus (Parilla-Bel et al, 2013), whereas the external nares are fully open or only partially divided posteriorly in eusuchians (e.g., Delfino et al, 2008). A pair of small, slit-like antorbital fenestrae are present and are entirely enclosed by the nasals, a feature absent in A. beaumonti, but present within all specimens of Theriosuchus for which the snout is preserved; as such we consider this feature to be locally diagnostic of A. bavaricus within non-Theriosuchus atoposaurids.…”