Turtles creep through the pages of The Anatomical Record

Testudinata (turtles) as a clade have highly unusual, and therefore intrinsically fascinating, anatomy. From their iconic shell to a scapula that resides inside the equivalent of their ribcage to a lack of a muscular partition separating the thorax and abdomen, turtle anatomy is inherently remarkable. As such, it is no wonder that they have been the subject of extensive study in The Anatomical Record (AR) over the years. In this Special Issue of the AR, we celebrate the turtle. Specifically, the volume brings together a collection of papers initially presented at the 2021 Turtle Evolution Symposium (TES), on topics ranging from turtle paleontology to the development of extant turtles. The issue has been expertly guest edited by two giants in the field of turtle paleontology, Drs. Juliana Sterli and Evangelos Vlachos (Figure 1). Juliana is a Researcher from the Argentinean Council of Research (CONICET) at Museo Paleontol ogico Egidio Feruglio in Trelew, Argentina. She completed her education in La Plata National University, Argentina. Her research focuses on the early evolution of turtles, specifically their anatomy and phylogeny, especially stem turtles, meiolaniforms (horned turtles) and pan-chelids. Evan is a Greek-Argentine paleontologist, currently a Researcher at CONICET and Museo Paleontol ogico Egidio Feruglio. He completed his education in the Aristotle University in Greece, and since 2015, he has been living and working in Patagonia. His research focuses on the evolution of testudinids and on the diversity and extinction events of turtles both within South America and at a global scale. We wish to thank Drs. Sterli and Vlachos from the outset for their excellent work organizing the TES (Figure 2) and for their contribution to this issue. The first turtles appeared in the fossil record approximately 230 million years ago, and while they have subsequently diversified and evolved, the general turtle Bauplan has remained remarkably constant. Their unique anatomical adaptations have allowed turtles to be quite evolutionary successful, even surviving the Cretaceous–Paleogene (K–Pg) mass extinction event that wiped out the non-avian dinosaurs. Over the years, research in AR has explored details of some of the turtle's most unusual anatomical features. This introductory editorial will briefly recount some of the most significant prior publications on turtle anatomy, physiology, and evolution in the AR. Perhaps the most notable and diagnostic aspect of a turtle's anatomy is its shell. The embryonic development of shell bones in Podocnemis unfilis was evaluated by Lima et al. (2011). As in other pleurodires, the carapace (the dorsal part of the shell) starts to form simultaneously with the ribs and plastron (ventral part of the shell), while the costal and neural plates originate as expansions of the ribs and neural arches (Lima et al., 2011). Threedimensional morphometrics of the epiplastron (anteriormost bone in the plastron) have been shown to distinguish among species of the fossil geoemydid genera Echmatemys and Bridgeremys (Adrian et al., 2022). Adaptations and development of turtle limbs have also been discussed in AR. The normal pattern of skeletogenesis in Kinosternidae (mud and musk turtles) was described, in which chondrification and ossification of the limb elements were identified to embryonic stage (dos Santos Braga et al., 2021). Krahl et al. (2019) combined dissection with microcomputed tomography (microCT) and finite element analysis (FEA) to evaluate compressive stress distribution and muscle force vectors in the cheloniid foreflipper. They determined that muscular support is essential for minimizing bending moments and maintaining homogenous compressive loadings during upstroke and downstroke (Krahl et al., 2019). The work by Lorenzo Alibardi and colleagues has revealed developmental and histological details of the turtle beak (Alibardi, 2021a) and claws (Alibardi, 2021b). Turtles lack teeth, and instead, their jaws are covered with a corneous rhamphotheca, or “beak. ” During development, placodes arise from the maxillary and mandibular epidermis that merge with horny laminae via cell proliferation (Alibardi, 2021a). Nearby, the palate of turtles has both keratinized and nonkeratinized regions, with the former concentrated near the rhamphotheca (Heiss et al., 2008). Similarly, turtle claws arise from proliferation and protein synthesis in the dorsal epidermis of the digits (Alibardi, 2021b). Turtles use their claws for swimming, digging, and in reproduction (Evans, 1952). The respiratory anatomy of turtles reflects various adaptations to an aquatic habitus, including diving. The lungs of the loggerhead sea turtle (Caretta caretta) Received: 2 March 2023 Accepted: 3 March 2023