Comparative Studies On The Teeth Of Keichousaurus And Other Typical Marine Reptiles From The Triassic Of South China

The end-Permian mass extinction and the subsequent recovery of ecological environment are major events in the evolutionary history of earth.In the Triassic,marine ecosystems underwent great transformations,and several groups of reptiles adapted the marine environments,represented by the Ichthyosauria,Thalattosauria and Saurpterygia(including pachypleurosaurs,placodonts,nothosaurs and plesiosaurs).The Triassic marine strata are widely distributed in South China,with numerous fossils of marine reptiles preserved.These marine reptiles are taxonomically rich,providing a good basis for studying the composition,food web and trophic level of marine ecosystems at th at time.Keichousaurus hui,a small-sized saurpterygian,was one of the most abundant marine reptiles from the late Middle Triassic Yangtze Sea in Guizhou and Yunnan,South China.The previous studies of Keichousaurus mainly focused on its osteology,ontogeny,sexual dimorphism,reproductive stratiagy,movement mode,etc.,and little was dealed with the internal structure,replacement mode and functional form of its teeth.In this contribution,the external shape,implantation and replacement of the teeth of Keichousaurus were studied in detail through the observations and measurements of a large quantity of specimens of Keichousaurus collected in the past several decades with multiple ways of preservation.The tooth attachment and implantation have been further reveale d by X-ray computed microtomography.Addtionally,the dental function and dietary preference of Keichousaurus were discussed.Like other typical vertebrates,the teeth of Keichousaurus composed of dense enamel,looser dentin and large pulp cavity,are characterized by a cylindrical root inserted into jawbones,a conical crown exposed out in mouth and a waist-shaped neck connected between the crown and root.The tooth roots are deeply inserted into the U-shaped alveolar bone,accounting for about 1/2 to 2/3 of the teeth.This suggests that the thecodonts of Keichousaurus could grow firmly on the jawbone and they did not fall off easily.The tooth necks are usually presented as a circular depression,implying a gum attachment,which helps to stabilize the tooth and to resist the stimulation of occlusion.The enameled crown has a sharp and slightly curved cusp with a sculptured surface of the apicobasal ridges ex t en di n g t o th e t op.T h es e ap i co b as al r id ge s co ul d h el p p i e r c e smo ot h,struggling scaly prey,promote blood discharge and prevent the prey from escaping.By analyzing the cross section of the premaxillary teeth,this paper first studied the internal tissue structure of teeth of Keichousaurus.The transverse section at the base of the crown shows that the peripheral wall of the large dental pulp cavity has a thin dense enamel layer,a transparent spherical dentin layer and a dark circular dentin band(plicidentine,presenting as uneven longitudinal ridges and grooves on the tooth surface).In other words,the teeth of Keichousaurus may be specialized: large dental pulp cavity can improve sensitivity and penetration,and the adverse effects of thin enamel-dentine wall on tooth strength can be improved by plicidentine and the good development of gum.Considering the similarities of dental shape and structure between Keichousaurus and ancient and modern piscivorous predators(e.g.,nothosaurs),Keichousaurus may mainly feed on small fish from the same ecosystem and some relatively soft-bodied invertebrates(e.g.,mysidacean shrimps).Considering the replacement tooth situated inside the pulp cavity of functional tooth,it is inferred that the replacement tooth of Keichousaurus adopts the iguanid replacement type.Initially,the small replacement teeth only with loose dentine but lacking enamel were exposed near the mesial-lingual side of the pulp cavity of functional teeth.As replacement tooth extends anterodorsally from the posteroventral edge of the root to the mesial-labial margin,the ratio of replacement tooth to pulp cavity of functional tooth changes from less than 1/2 to more than 1/2.Finally,the replacement tooth occupies almost the entire pulp cavity of the functional tooth,evenly bearing enamel on its crown.Additionally,this paper also makes a comparative study on the morphological structure,replacement and attachment of teeth of other typical marine reptiles from Triassic of South China.The “fish trap” teeth of Nothosaurus are most similar to those of Keichousaurus.Their heterodont dentition consists of the enlarged,and procumbent anterior teeth,a pair of expanded caniniform teeth,and small fangs.Their crown index range is concentrated,which implies the specificity of piercing function and their picky diet.However,the teeth of Mixosaurus,Psephochelys polyosteoderma,Paraplacodus luopingensis,Pectodens,Yunguisaurus and Anshunsaurus are quite different from those of Keichousaurus.Blunt and round crushing teeth exist in the durophagous Mixosaurus,Psephochelys polyosteoderma,and Paraplacodus luopingensis.The smashing teeth with smooth dental surface are respectively present at Pectodens and Anshunsaurus.The conical teeth of Yunguisaurus and Qianichthyosaurus have both piercing and crunching functions.The teeth in the former are with slightly backward curved cusp,and smooth dental surface;while those in the latter are upright,and the longitudinal ridge on the dental surface only extends to the middle of the crown.The replacement teeth of Anshunsaurus are different from those of Keichousaurus,erupting on the interdental location,which may adopt the varanid replacement type.As for the attachment mode of teeth,the roots of Nothosaurus,Yunguisaurus,Anshunsaurus and the roots of the anterior teeth of Paraplacodus luopingensis are similar to those of Keichousaurus;they insert into the separate U-shaped deep grooves,which implies the attachment type of thecodonty.The roots of the anterior teeth of Mixosaurus,the roots of Pectodens and Qianichthyosaurus are attached to the longitudinal and continuous groove,of which the labial wall is higher than the lingual wall,suggesting the attachment type of subthecodonty.The roots of teeth of Psephochelys polyosteoderma,roots of posterior teeth of Mixosaurus and Paraplacodus luopingensis are implanted into the shallow dental sockets,implying the attachment way of ankylosed thecodonts.