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.

Fig 1

Image showing A) juvenile Stenopterygius quadriscissus (SMNS 54026) with relatively large, unreduced teeth (specimen mandible length: 368.67 mm). B) Large Adult S. quadriscissus (SMNS 53001), showing highly reduced, non-functional teeth (too short to protrude beyond the dental groove) (specimen mandible length: 523.2 mm). C) Close-up of the teeth seen in the box inset in B. Scale bars: A = 1 cm, B = 5 cm, and C = 1 cm.

Fig 2. Diagram showing measurements used throughout the analysis.

Fig 3. Graph showing RMA regression of logMandible vs. logCrownHeight.

All values in log mm. Symbols: red-colored upward pointing triangles = embryos, red-colored downward pointing triangles = pregnant females.

Fig 4. Dental growth in S. quadriscissus modelled using the von Bertalanffy growth model.

Symbols: red-colored upward pointing triangles = embryos, red-colored downward pointing triangles = pregnant females. Values are raw measurements.

Fig 5. Plot showing the allometric slopes of the different tooth quadrants included in the One-way ANCOVA.

Each vertical group of four points is a single specimen. Values = log mm.

Fig 6. RMA analysis of tooth count through ontogeny in Stenopterygius quadriscissus.

Fig 7. RMA analysis of tooth crown shape through ontogeny in Stenopterygius quadriscissus.

Tooth Shape calculated as per Massare (1987).

Fig 8. An example of a belemnite with a shattered phragmocone, interpreted as a bite mark (SMNS 60863).

Scale bar = 1 cm.