Cranial morphology in Mollisquama sp. (Squaliformes; Dalatiidae) and patterns of cranial evolution in dalatiid sharks

Abstract

Dalatiid sharks are members of a family of predominantly small, midwater meso- and bathypelagic chondrichthyans. The family is notable for both its number of monotypic genera and high morphological disparity. Three of the seven dalatiid genera are known only from holotype specimens (Mollisquama parini) or from only a handful of specimens (Euprotomicroides zantedeschia, Heteroscymnoides marleyi), with the only detailed anatomical work consistent across all taxa being studies of dentition. Here, we present detailed anatomical description of the second-ever specimen of Mollisquama (Mollisquama sp.) covering chondrocranial, jaw, dental, and muscular anatomy, derived from a phase-contrast synchrotron microtomographic scan. Mollisquama sp. is unique among dalatiids in possessing a deep carinal process, extending ventrally from the bar between the subethmoid region and basal angle in squaloid sharks, containing a large fenestra infiltrated by the suborbitalis muscle. Mollisquama sp. also exhibits additional possibly diagnostic features, including a planar configuration of the labial cartilages and the absence of labial folds; a pad-like orbital process on the palatoquadrate; and the origination of the suborbitalis muscle solely on the carina, rather than the intraorbital wall. Character optimization of anatomical data onto a phylogeny of dalatiid sharks suggests Mollisquama sp. to be among the most specialized in the family, expanding the existing dalatiid morphospace. However, the functional significance of such transformations remains unclear. Synchrotron-derived data, which do not require chemical pretreatment of specimens, may elucidate soft-tissue functional correlates in future studies of undersampled taxa, such as dalatiids.

A new Western North Atlantic Ocean kitefin shark (Squaliformes: Dalatiidae) from the Gulf of Mexico

“…parini Dolganov 1984(Del Moral-Flores et al 2015 Mollisquama parini Dolganov 1984(Weigmann 2016 Mollisquama sp. (Denton et al 2018) Mollisquama parini Dolganov 1984(de Figueiredo Petean & de Carvalho 2018 Holotype: -TU 203676, central Gulf of Mexico, NOAA Ship PISCES survey 7620101001, station 053, 4 February 2010, 142.0 mm total length, 14.6 g, immature male.…”

“…Lighter pigmentation around and within mouth. Additional diagnosis of features and morphometric values are found in Grace et al (2015) and Denton et al (2018), and specimen photographs taken just after capture are found in Grace et al (2015). Dentition.…”

“…For lower teeth the root height is approximately 50% of the total tooth height and most cusp tips curve toward jaw corners. For lower teeth the basal sulcus is absent or weakly present (Denton et al 2018). Dental formula 10-1-10/15-1-15 (Denton et al 2018).…”

“…For lower teeth the basal sulcus is absent or weakly present (Denton et al 2018). Dental formula 10-1-10/15-1-15 (Denton et al 2018). Habitat.…”

“…1A, B; illustrations previously from Grace et al 2015). Phylogenetic analysis of DNA sequence data (mitochondrial NADH2 gene) places Mollisquama in Dalatiidae as sister to Isistius (Grace et al 2015;Denton et al 2018).…”