Long Bone Morphology and Histology of the Stem Salamander Kulgeriherpeton ultimum (Caudata, Karauridae) from the Lower Cretaceous of Yakutia

The morphology and histological structure of the humerus and femora of the stem karaurid salamander Kulgeriherpeton ultimum Skutschas et al., 2018, from the Lower Cretaceous Teete locality, Republic of Sakha (Yakutia) is described. The microanatomical and histological structure of K. ultimum is characterized by the presence of a thick compact primary cortex formed by a parallel-fibered bone; the absence (in the humerus) or presence of a small medullary cavity in the mid-diaphysis; the presence of a medullary cavity expanding towards the epiphyses, which continues in the proximal and distal parts as a complex network of branching canals, partially replaced by erosion bays; the presence of primary vascular canals and growth marks in the primary cortex; the presence of remnants of unresorbed cartilage and the Kashchenko’s line; active secondary remodeling with the formation of erosion bays similar to those in large-sized salamanders (other stem karaurid salamanders and cryptobranchids). Skeletochronological analysis of the humerus of K. ultimum showed that, at the time of the animal’s death, its individual age was 13–16 years, and the absence of a reduction in the distance between cyclic growth marks in the peripheral part of the cortex indicates that it belonged to an actively growing individual that had not reached its maximum possible size. The similarity in the morphology of the humerus and femur of K. ultimum and extant aquatic neotenic salamanders (absence of a dorsal crest on the humerus for the attachment of m. subcoracoscapularis, lower, forward-displaced trochanter of the femur, and shallow ventral fossa (fossa trochanterica) on the femur), as well as the presence of remnants of cartilage and preservation of Kashchenko’s line in the internal structure of limb bones, confirm conclusions about aquatic life style and neotenic nature of stem karaurid salamanders.