Abstract
The scutes of the carapace of extant turtles exhibit common elements in a narrow range of topographical arrangements. The typical arrangement has remained constant since its origin in the clade Mesochelydia (Early Jurassic), after a period of apparent greater diversity in the Triassic. This contribution is a review of the development and evolutionary history of the scute patterns of the carapace, seen through the lens of recent developmental models. This yields insights on pattern variations in the fossil record. We reinterpret the “supracaudal” scute and propose thatProganochelyshad five vertebral scutes. We discuss the relationship between supramarginal scutes and Turing processes, and we show how a simple change during embryogenesis could account for origin of the configuration of the caudal region of the carapace in mesochelydians. We also discuss the nature of the decrease in number of scutes over the course of evolution, and whether macroevolutionary trends can be discerned. We argue that turtles with complete loss of scutes (e.g., softshells) follow clade-specific macroevolutionary regimes, which are distinct from the majority of other turtles. Finally, we draw a parallel between the variation of scute patterns on the carapace of turtles and the scale patterns in the pileus region (roof of the head) of squamates. The size and numbers of scales in the pileus region can evolve over a wide range, but we recognized tentative evidence of convergence towards a typical configuration when the scales become larger and fewer. Thus, typical patterns could be a more general property of similar systems of integumentary appendages.
Highlights
A conspicuous feature of amniotes is the diversity of skin appendages that cover their bodies, such as hair in mammals, feathers in birds, and scales in reptiles
We argue that turtles with complete loss of scutes follow clade-specific macroevolutionary regimes, which are distinct from the majority of other turtles
There is substantial intraspecific variation of pholidotic patterns in turtles (Zangerl and Johnson 1957; Bujes and Verrastro 2007; Zimm et al 2017; Szczygielski et al 2018; Horváth et al 2020; Maffucci et al 2020; Bentley et al 2021). In this contribution we integrate a series of recent studies in palaeontology and developmental biology, including a model based on reaction-diffusion processes (Moustakas-Verho et al 2014), to shed new light on ancient fossils and the constructional factors that have shaped the evolution of the carapace since the earliest known testudinatans
Summary
A conspicuous feature of amniotes is the diversity of skin appendages that cover their bodies, such as hair in mammals, feathers in birds, and scales in reptiles (including the legs of birds). In this contribution we integrate a series of recent studies in palaeontology and developmental biology, including a model based on reaction-diffusion processes (Moustakas-Verho et al 2014), to shed new light on ancient fossils and the constructional factors that have shaped the evolution of the carapace since the earliest known testudinatans.
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