Shape and size variation in elapid snake fangs, and the effects of phylogeny and diet

Published online: 9 October 2023Recent studies have found correlations between the shape of snake teeth/fangs and diet. These studies were done at a very broad phylogenetic scale, making it desirable to test if correlations are still detectable at a narrower evolutionary scale, specifically within the family Elapidae. To this end, we studied fang shape in a dense selection of elapids representing most genera worldwide (74%). We used three-dimensional geometric morphometrics to analyse fang diversity and evaluate possible correlations between fang shape, fang size, and diet. We detected only weak phylogenetic signal in our dataset for both shape and size, and no significant evolutionary allometry when correcting for phylogeny. Overall, the distribution of elapid fangs in morphospace was found to be surprisingly conservative, with only a few outliers. The only two dietary categories that were found to have a significant effect on fang shape are fish and snakes, while mammals have a significant effect on absolute but not relative fang size. Our results show that there are disparate patterns in fang-diet relationships at different evolutionary scales. Across all venomous snakes, previous work found that fangs are strongly influenced by diet, but within elapids our study shows these same associations are weaker and often non-significant. This could result from limitations in these types of studies, or could reflect the fact that elapids are a relatively young clade, where recent extensive divergences in diet have yet to be mirrored in fang shape, suggesting a lag between changes in ecology and dental morphology.Alessandro Palci, Michael S. Y. Lee, Jenna M. Crowe, Riddell, Emma Sherrat

Cretaceous Blind Snake from Brazil Fills Major Gap in Snake Evolution

Blind snakes (Scolecophidia) are minute cryptic snakes that diverged at the base of the evolutionary radiation of modern snakes. They have a scant fossil record, which dates back to the Upper Paleocene-Lower Eocene ( 56 Ma); this late appearance conflicts with molecular evidence, which suggests a much older origin for the group (during the Mesozoic: 160–125 Ma). Here we report a typhlopoid blind snake from the Late Cretaceous of Brazil, Boipeba tayasuensis gen. et sp. nov, which extends the scolecophidian fossil record into the Mesozoic and reduces the fossil gap predicted by molecular data. The new species is estimated to have been over 1 m long, much larger than typical modern scolecophidians (<30 cm). This finding sheds light on the early evolution of blind snakes, supports the hypothesis of a Gondwanan origin for the Typhlopoidea, and indicates that early scolecophidians had large body size, and only later underwent miniaturization.Thiago Schineider Fachini, Silvio Onary, Alessandro Palci, Michael S.Y. Lee, Mario Bronzati, and Annie Schmaltz Hsio

Novel vascular plexus in the head of a sea snake (Elapidae, Hydrophiinae) revealed by high-resolution computed tomography and histology

Novel phenotypes are often linked to major ecological transitions during evolution. Here, we describe for the first time an unusual network of large blood vessels in the head of the sea snake Hydrophis cyanocinctus. MicroCT imaging and histology reveal an intricate modified cephalic vascular network (MCVN) that underlies a broad area of skin between the snout and the roof of the head. It is mostly composed of large veins and sinuses and converges posterodorsally into a large vein (sometimes paired) that penetrates the skull through the parietal bone. Endocranially, this blood vessel leads into the dorsal cerebral sinus, and from there, a pair of large veins depart ventrally to enter the brain. We compare the condition observed in H. cyanocinctus with that of other elapids and discuss the possible functions of this unusual vascular network. Sea snakes have low oxygen partial pressure in their arterial blood that facilitates cutaneous respiration, potentially limiting the availability of oxygen to the brain. We conclude that this novel vascular structure draining directly to the brain is a further elaboration of the sea snakes' cutaneous respiratory anatomy, the most likely function of which is to provide the brain with an additional supply of oxygen.Alessandro Palci, Roger S. Seymour, Cao Van Nguyen, Mark N. Hutchinson, Michael S. Y. Lee and Kate L. Sander

Evolutionary morphology of Pareiasaurs

SIGLEAvailable from British Library Document Supply Centre-DSC:D064156 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Geometric morphometrics, homology and cladistics: review and recommendations

Geometric morphometric (GM) data has a long and contentious history in phylogenetic analyses. Often associated with phenetics, GM has been considered by many to be unable to provide meaningful information on phylogenetic relationships. However, the concepts of primary and secondary homology as developed for discrete characters can be readily extended to GM data: raw similarity in aligned landmark positions represents primary homology, and similarity ascribable to common ancestry represents secondary homology. We review fundamental concepts from the literature and provide a series of practical guidelines for the use of GM data in phylogenetics: (i) alignments that minimize linear distances between landmarks (or their approximation) perform better in highlighting apomorphic traits; (ii) Type I, Type II and linear semi-landmarks are preferable to Type III and surface semi-landmarks; (iii) excluding bilateral landmarks after, rather than before, alignment will prevent artefactual mediolateral displacement of midsagittal landmarks; (iv) phylogenetic analyses should employ linear rather than squared-change parsimony analysis of landmark displacements; (v) optimization of shape changes across a tree can be improved with methods that re-align the landmark configurations based on the results of the phylogenetic analysis; and (vi) GM data are no substitute for traditional morphological characters, but rather a complementary descriptor of shape diversity.Alessandro Palcia, b, and Michael S. Y. Le