Morphology and Histology of the Femora of Salamanders of the Genus Kiyatriton (Caudata) from the Middle Jurassic and Early Cretaceous of Western Siberia

This paper describes the morphology and histological structure of the femora of salamanders of the genus Kiyatriton K. krasnolutskii from the Middle Jurassic (Bathonian) Berezovsk Quarry locality in Krasnoyarsk krai and K. leshchinskiyi from the Lower Cretaceous (Aptian) Shestakovo 1 locality in the Kemerovo Region. The femora of Kiyatriton are characterized by the presence of a large spur-like trochanter, a developed high trochanteric crest, a large ventral fossa (fossa trochanterica), and a bean-shaped proximal end in cross-section. The histological structure is characterized by the presence of a thick, almost avascular cortex formed by both the primary periosteal bone and secondary endosteal bone; a thick periosteal cortex, which consists of a parallel-fibred bone matrix and bears cyclical growth marks (annuli and LAGs), including double LAGs; a thick endosteal cortex consisting of a lamellar bone matrix; the presence of a small medullary cavity; the presence of the Katschenko’s line; and the absence of a calcified cartilage and bone remodeling (erosion bays, secondary osteons, and resorption of the walls of the medullary cavity). The similarities in the morphological and histological structure of the femora of Kiyatriton and small-sized modern metamorphosing salamanders (large spur-like trochanter of the femur, deep ventral fossa, rounded proximal end of the femur, avascular periosteal cortex with growth marks, thick layer of endosteal bone forming the inner part of the cortex, and small medullary cavity) indicate that members of the genus Kiyatriton were metamorphosing salamanders with a terrestrial adult stage. The similarity in the structure of the femora of different-aged Middle Jurassic and Early Cretaceous representatives of Kiyatriton indicates that the biological features (growth pattern and the presence of metamorphosis) have not changed for about 40 million years.