Of teeth and trees: A fossil tip‐dating approach to infer divergence times of extinct and extant squaliform sharks

Abstract

Fossil tip‐dating allows for the inclusion of morphological data in divergence time estimates based on both extant and extinct taxa. Neoselachii have a cartilaginous skeleton, which is less prone to fossilization compared to skeletons of Osteichthyans. Therefore, the majority of the neoselachian fossil record is comprised of single teeth, which fossilize more easily. Neoselachian teeth can be found in large numbers as they are continuously replaced. Tooth morphologies are of major importance on multiple taxonomic levels for identification of shark and ray taxa. Here, we review dental morphological characters of squalomorph sharks and test these for their phylogenetic signal. Subsequently, we combine DNA sequence data (concatenated exon sequences) with dental morphological characters from 85 fossil and extant taxa to simultaneously infer the phylogeny and re‐estimate divergence times using information of 61 fossil tip‐dates as well as eight node age calibrations of squalomorph sharks. Our findings show that the phylogenetic placement of fossil taxa is mostly in accordance with their previous taxonomic allocation. An exception is the phylogenetic placement of the extinct genus †Protospinax, which remains unclear. We conclude that the high number of fossil taxa as well as the comprehensive DNA sequence data for extant taxa may compensate for the limited number of morphological characters identifiable on teeth, serving as a backbone for reliably estimating the phylogeny of both extinct and extant taxa. In general, tip‐dating mostly estimates older node ages compared to previous studies based on calibrated molecular clocks.

Dental morphology and microstructure of the Prickly Dogfish Oxynotus bruniensis (Squaliformes: Oxynotidae)

“…Squaliform teeth are well represented in the fossil record (Ledoux, 1970; 1972; Welton, 1981; Suzuki, 2008; Cappetta, 2012). Accordingly, several authors present phylogenies of the order Squaliformes based heavily or, in some cases, exclusively on dental characters of extinct and Recent species (Adnet and Cappetta, 2001; Kriwet and Klug, 2009; Klug and Kriwet, 2010; Flammensbeck et al ., 2018). Adnet and Cappetta (2001) place the genus Oxynotus within the Somniosinae at the base of the clade [[[ Centroscymnus + Scymnodon ] + Scymnodalatias] + Oxynotus ].…”

“…crochardi, based on two Meckelian teeth and proposes it as the sister taxon to O. bruniensis. Welton's (1981) description does not describe the teeth of O. bruniensis in great detail or with respect to their functional significance, but it proved sufficient for some researchers to postulate, on the basis of shared dental characters with O. bruniensis, that dietary specialization in Oxynotidae may have evolved more than 20 million years ago (Welton, 1981;Flammensbeck et al, 2018). Cappetta (2012) offers an overview of oxynotid tooth morphology and arrangement remarking on the extreme heterodonty exhibited by oxynotids, but he does not address O. bruniensis explicitly.…”

The Sawshark Redemption: Current knowledge and future directions for sawsharks (Pristiophoridae)

“…The ancestral pristiophorids of the family Pristiophoridae are thought to have evolved during the Jurassic period (~200–145 Ma) (Flammensbeck, Pollerspöck, Schedel, Matzke, & Straube, 2018; Keyes, 1982; Sorenson, Santini, & Alfaro, 2014; Underwood, 2006). However, the earliest fossil remains for this family are located in deposits dating from the Late Cretaceous (~86–66 Ma) (Engelbrecht, Mörs, Reguero, & Kriwet, 2017; Underwood, 2006).…”

“…However, the earliest fossil remains for this family are located in deposits dating from the Late Cretaceous (~86–66 Ma) (Engelbrecht, Mörs, Reguero, & Kriwet, 2017; Underwood, 2006). Recent phylogenetic studies utilizing molecular and fossil data indicate that the crown pristiophorid group had arisen and diversified towards the end of the Cretaceous period (Flammensbeck et al., 2018; Sorenson et al., 2014).…”

“…However, it is suggested that this represents a functional rather than phylogenetic relationship between sawsharks and batoids (Jambura et al., 2020). Phylogenies based on oral tooth morphology indicate a sister relationship between sawsharks and angel sharks, with these taxa sharing a common ancestor with bramble sharks (Family: Echinorhinidae; Adnet & Cappetta, 2001; Flammensbeck et al., 2018). These studies, however, did not include the oral teeth of batoids in their analyses.…”