Petra Frýdlová, Vendula NutilováJan, DudákJan Žemlička, Pavel Němec, Petr Velenský, Tomáš Jirásek & Daniel Frynta (2017)
Patterns of growth in monitor lizards (Varanidae) as revealed by computed tomography of femoral growth plates.
Zoomorphology 136 (1): 95-106
Growth plate cartilage (GPC) is responsible for the growth of long bones due to endochondral ossification, which is the main mechanism of longitudinal skeletal growth in tetrapods. Degradation of GPC is a sign of determinate growth as it arrests the growth irreversibly. By contrast, indeterminate growth requires the persistence of GPC throughout the entire life. Monitor lizards (Varanidae) were previously reported to exhibit a dual type of growth, determinate as well as indeterminate. To reinvestigate this highly unexpected finding, we examined 13 species of varanids and their close relatives (Shinisauridae and Helodermatidae). In order to visualize GPC on the proximal part of the femur, we employed micro-radiography and micro-computed tomography. In large-bodied species, an extended capability of longitudinal growth was demonstrated; GPC was preserved for most of their lives. On the other hand, GPC senescence with complete disappearance of cartilage was found in adults of small-bodied varanids. These results confirm previous finding and, together with the absence of GPC in the outgroup species, challenge the universality of indeterminate growth in squamates. Moreover, we observed disappearance of GPC in an extremely old Varanus indicus, implying that GPC degradation is not entirely absent but only delayed to senescence in this large-bodied species. These findings raise the intriguing possibility that it is the timing, rather than other underlying mechanisms, what sets apart determinate from indeterminate growth. We therefore suggest that this dual type of growth represents an extreme case of heterochrony and is a consequence of strong sexual selection pressure to large-bodied varanids.
Lichtig, Asher J. & Lucas, Spencer G. (2017)
Sutures of the shell of the Late Cretaceous-Paleocene baenid turtle Denazinemys.
Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 283(1): 1-8
The sutures of the Late Cretaceous-Paleocene turtle Denazinemys are described in detail and sexual dimorphism recognized based on historical specimens and newly studied, unfused shells. The carapace is remarkable in the possession of an unfused paired nuchal. While it has been observed in turtle embryology that this bone has two ossification centers, this is the first adult turtle known to show this morphology. Furthermore, the pustules on the surface of the carapace of Denazinemys do not align with the sutures, crossing them with no change. Conversely, the pustules are clearly interrupted by the sulci. Histological examination reveals that the pustules on the dorsal surface are composed of an expanded external cortex with no noticeable expansion of the cancellous bone layer. The plastron of Denazinemys is particularly similar to Boremys in the intersection of the gular-humeral sulcus at the widest point of the entoplastron. Overall, the sutures are consistent with what would be expected from the proposed sister group relationship ofDenazinemys with Boremys based on sulci and skull anatomy.
Caitlin Brown, Emily Curd & Anthony Friscia (2017)
An actualistic experiment to determine skeletonization and disarticulation in the La Brea tar seeps.
PALAIOS 32(3): 119-124
The famous Rancho La Brea tar seeps of Southern California trapped thousands of Pleistocene and early Holocene vertebrates, preserving them as jumbled columns of millions of disarticulated bones. Previous work has contributed to a hypothetical entrapment scenario, however, it lacks detail in the period between the time the animals perished and the permeation of their bones with tar. Additionally, previous work has shown that skeletal elements moved apart from each other at least 1–3 meters but it is unclear whether this movement occurred near the surface of the tar, soon after submersion, or later after burial by sediment and compaction. To help answer these questions of disarticulation and transport, we conducted an actualistic experiment to record the progress of microbial succession and skeletonization of specimens in tar. We submerged dismembered bobcat (Lynx rufus) carcasses in an undisturbed tar seep and recorded the progress of microbial faunal changes and tissue decay. Microbial communities differed between tar environments and tissue decay, and changes in microbial communities across the stages of decay indicate rapid microbial succession, with the microbes most involved in the decay likely originating from the liquid surface tar. The minimum time to achieve clean bone was 40 days, from which we conclude that a surficial process may have been responsible for the movement of bones in the La Brea tar seeps.