Abstract
Placodontia is a clade of durophagous, near shore marine reptiles from Triassic sediments of modern-day Europe, Middle East and China. Although much is known about their primary anatomy and palaeoecology, relatively little has been published regarding their life history, i.e. ageing, maturation and growth. Here, growth records derived from long bone histological data of placodont individuals are described and modelled to assess placodont growth and life-history strategies. Growth modelling methods are used to confirm traits documented in the growth record (age at onset of sexual maturity, age when asymptotic length was achieved, age at death, maximum longevity) and also to estimate undocumented traits. Based on these growth models, generalized estimates of these traits are established for each taxon. Overall differences in bone tissue types and resulting growth curves indicate different growth patterns and life-history strategies between different taxa of Placodontia. Psephoderma and Paraplacodus grew with lamellar-zonal bone tissue type and show growth patterns as seen in modern reptiles. Placodontia indet. aff. Cyamodus and some Placodontia indet. show a unique combination of fibrolamellar bone tissue regularly stratified by growth marks, a pattern absent in modern sauropsids. The bone tissue type of Placodontia indet. aff. Cyamodus and Placodontia indet. indicates a significantly increased basal metabolic rate when compared with modern reptiles. Double lines of arrested growth, non-annual rest lines in annuli, and subcycles that stratify zones suggest high dependence of placodont growth on endogenous and exogenous factors. Histological and modelled differences within taxa point to high individual developmental plasticity but sexual dimorphism in growth patterns and the presence of different taxa in the sample cannot be ruled out.
Highlights
Placodontia are a basal group of Sauropterygia, a diverse radiation of diapsid marine reptiles that ranged from the late Lower Triassic until the end of the Cretaceous [1,2,3]
The Triassic radiation of Sauropterygia consisting of the shallow marine Placodontia, Pachypleurosauria, and Nothosauria as well as the more open marine Pistosauria; the Jurassic and Cretaceous seas were ruled by the open marine Plesiosauria
Placodontia lived in shallow marine environments in the Germanic Basin and along the coasts of the epicontinental seas of the western and eastern Tethys [1,16,17]
Summary
Placodontia are a basal group of Sauropterygia, a diverse radiation of diapsid marine reptiles that ranged from the late Lower Triassic until the end of the Cretaceous [1,2,3]. The Triassic radiation of Sauropterygia consisting of the shallow marine Placodontia, Pachypleurosauria, and Nothosauria as well as the more open marine Pistosauria; the Jurassic and Cretaceous seas were ruled by the open marine Plesiosauria (figure 1). The latter had a global distribution, whereas most Triassic representatives were largely restricted to the epicontinental seas of the Tethys. Placodontia lived in shallow marine environments in the Germanic Basin and along the coasts of the epicontinental seas of the western and eastern Tethys (modern-day Europe/Middle East and South China) [1,16,17]
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