Ontogenetic Tooth Reduction in Stenopterygius quadriscissus (Reptilia: Ichthyosauria): Negative Allometry, Changes in Growth Rate, and Early Senescence of the Dental Lamina

Abstract

We explore the functional, developmental, and evolutionary processes which are argued to produce tooth reduction in the extinct marine reptile Stenopterygius quadriscissus (Reptilia: Ichthyosauria). We analyze the relationship between mandible growth and tooth size, shape, and count, to establish an ontogenetic trend. The pattern in S. quadriscissus is consistent with hypotheses of tooth size reduction by neutral selection, and this unusual morphology (a functionally edentulous rostrum) was produced by a series of different evolutionary developmental changes that are known for other taxa showing tooth reduction and loss. Specifically, this species evolved functional edentulism by evolutionary changes in the growth allometry of the dentition and by altering growth rates through ontogeny. This observation supports previous hypotheses that S. quadriscissus underwent ontogenetic tooth reduction. Tooth reduction in S. quadriscissus may be caused by unique selective pressures resulting from prey choice and feeding behavior, expanding our current understanding of the mechanisms producing tooth reduction.

Trophic niche ontogeny and palaeoecology of early Toarcian Stenopterygius (Reptilia: Ichthyosauria)

“…In S . quadriscissus , ontogenetic tooth reduction and tooth crown shape change increases the possibility of a concomitant niche shift (McGowan ; Dick & Maxwell ). If dental guild is correlated with diet (Massare ), such a change in shape implies a functional component: juvenile S. quadriscissus are best described as Pierce II Guild (Massare ), having relatively tall conical teeth with a sharp apex and no visible wear (Dick & Maxwell ).…”

“…quadriscissus , ontogenetic tooth reduction and tooth crown shape change increases the possibility of a concomitant niche shift (McGowan ; Dick & Maxwell ). If dental guild is correlated with diet (Massare ), such a change in shape implies a functional component: juvenile S. quadriscissus are best described as Pierce II Guild (Massare ), having relatively tall conical teeth with a sharp apex and no visible wear (Dick & Maxwell ). Conversely, adult S. quadriscissus possess teeth more accurately fitting the Smash guild (Massare ), having relatively short, robust teeth with a blunt apex, or are functionally edentulous, with teeth too small to have been erupted (Dick & Maxwell ).…”

“…If dental guild is correlated with diet (Massare ), such a change in shape implies a functional component: juvenile S. quadriscissus are best described as Pierce II Guild (Massare ), having relatively tall conical teeth with a sharp apex and no visible wear (Dick & Maxwell ). Conversely, adult S. quadriscissus possess teeth more accurately fitting the Smash guild (Massare ), having relatively short, robust teeth with a blunt apex, or are functionally edentulous, with teeth too small to have been erupted (Dick & Maxwell ). Reconsidering Massare's () original suggestions from the perspective of niche ontogeny, such a shift in tooth guild is predicted here to correlate with a shift in diet.…”

First report of Leptonectes (Ichthyosauria: Leptonectidae) from the Lower Jurassic (Pliensbachian) of Asturias, northern Spain

“…Nevertheless, the dentition pattern of MUJA 4095 is consistent with that of Leptonectidae, particularly with MUJA 3687 and differs from that of Wahlisaurus massarae characterized by recurved crowns and large, bulbous infolded roots (Lomax, 2016). On the other hand, teeth of MUJA 4095 are inconsistent with tooth morphology of the other Lower Jurassic ichthyosaurs such as Temnodontosaurus, Ichthyosaurus and Stenopterygius, even considering ontogenetic change (Dick and Maxwell, 2015).…”

“…The taxonomic value of tooth morphology (size and shape) of Early Jurassic ichthyosaurs has been questioned. Recent contributions have shown that size and shape of teeth change during ontogeny, at least in Stenopterygius (Dick and Maxwell, 2015;Dick et al, 2016). In the case of Ichthyosaurus communis and I. intermedius, historically differentiated on the basis of tooth morphology, Massare and Lomax (2017) recognized different morphologies, although they pointed out that tooth features were not reliable to distinguish these two species.…”