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.
Highlights
Shastasaurids were important components of Triassic marine ecosystems and represented, along with Cymbospondylidae, one of the earliest groups of medium to large-bodied ichthyosaurs
Multiplanar reconstructions (MPR) and volume rendering reconstructions (VR) allowed to inspect the bones hidden under other ones within the matrix, otherwise impossible to study without damaging the fossil (Fig. 10)
Subsequent assignment of the Paläontologisches Institut und Museum der Universität Zürich (PIMUZ) shastasaurid material to Mikadocephalus gracilirostris (i.e., PIMUZ T 4376; Maisch & Matzke, 2000) must be rejected, and the valid taxon name for all the referred material remains by priority Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996, according to the ICZN (1999)
Summary
Shastasaurids were important components of Triassic marine ecosystems and represented, along with Cymbospondylidae, one of the earliest groups of medium to large-bodied ichthyosaurs. As defined by Ji et al (2016), Shastasauridae include six genera of long-bodied (presacral count > 55) forms: Shastasaurus, Besanosaurus, Guanlingsaurus, Guizhouichthyosaurus, Shonisaurus and ‘Callawayia’ wolonggangense (Camp, 1980; Dal Sasso & Pinna, 1996; Maisch & Matzke, 1997a; Li & You, 2002; Nicholls & Manabe, 2004; Maisch et al, 2006b; Pan, Jiang & Sun, 2006; Chen, Cheng & Sander, 2007; Shang & Li, 2009; Sander et al, 2011; Ji et al, 2013). Some of the specimens previously referred to Pessopteryx nisseri are morphologically very similar to Besanosaurus leptorhynchus (McGowan & Motani, 2003), which indicates their shastasaurid affinity, but these specimens comprise postcranial material only and will not be discussed here
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