Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications

Gabriele Bindellini; Andrzej S Wolniewicz; Feiko Miedema; Torsten M Scheyer; Cristiano Dal Sasso

doi:10.7717/peerj.11179

Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications

PeerJ. 2021 May 6:9:e11179. doi: 10.7717/peerj.11179. eCollection 2021.

Authors

Gabriele Bindellini¹, Andrzej S Wolniewicz², Feiko Miedema^{3

4}, Torsten M Scheyer⁴, Cristiano Dal Sasso⁵

Affiliations

¹ Dipartimento di Scienze della Terra "Ardito Desio", Università degli Studi di Milano, Milano, Italy.
² Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.
³ Staatliches Museum für Naturkunde Stuttgart, Stuttgart, Germany.
⁴ Paläontologisches Institut und Museum, Universität Zürich, Zürich, Switzerland.
⁵ Sezione di Paleontologia dei Vertebrati, Museo di Storia Naturale di Milano, Milano, Italy.

Abstract

Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 was described on the basis of a single fossil excavated near Besano (Italy) nearly three decades ago. Here, we re-examine its cranial osteology and assign five additional specimens to B. leptorhynchus, four of which were so far undescribed. All of the referred specimens were collected from the Middle Triassic outcrops of the Monte San Giorgio area (Italy/Switzerland) and are housed in various museum collections in Europe. The revised diagnosis of the taxon includes the following combination of cranial characters: extreme longirostry; an elongate frontal not participating in the supratemporal fenestra; a prominent 'triangular process' of the quadrate; a caudoventral exposure of the postorbital on the skull roof; a prominent coronoid (preglenoid) process of the surangular; tiny conical teeth with coarsely-striated crown surfaces and deeply-grooved roots; mesial maxillary teeth set in sockets; distal maxillary teeth set in a short groove. All these characters are shared with the holotype of Mikadocephalus gracilirostris Maisch & Matzke, 1997, which we consider as a junior synonym of B. leptorhynchus. An updated phylogenetic analysis, which includes revised scores for B. leptorhynchus and several other shastasaurids, recovers B. leptorhynchus as a basal merriamosaurian, but it is unclear if Shastasauridae form a clade, or represent a paraphyletic group. The inferred body length of the examined specimens ranges from 1 m to about 8 m. The extreme longirostry suggests that B. leptorhynchus primarily fed on small and elusive prey, feeding lower in the food web than an apex predator: a novel ecological specialisation never reported before the Anisian in a large diapsid. This specialization might have triggered an increase of body size and helped to maintain low competition among the diverse ichthyosaur fauna of the Besano Formation.

Keywords: Besano Formation; Cranial anatomy; Ichthyosauria; Longirostry; Marine reptiles; Middle Triassic; Monte San Giorgio; Osteology; Phylogeny; Shastasauridae.

Grants and funding

Comparative data collection by Andrzej S. Wolniewicz was funded by a Natural Environment Research Council (NERC) Ph. D. Studentship (cohort grant NE/L501530/1) carried out at the Department of Earth Sciences, University of Oxford (2013–2017). Torsten M. Scheyer was supproted by the Swiss National Science Foundation (grant no. 31003A_179401). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.