Osteology of the enigmatic threadsnake species Epictia unicolor and Trilepida guayaquilensis (Serpentes, Leptotyphlopidae) with generic insights

Abstract

Since the rearrangement of all leptotyphlopid species previously known as "Leptotyphlops" in Adalsteinsson et al. (2009)'s paper, several taxa have remained untested regarding their generic identity and have been assigned to different genera based on phenotypic data or, in some cases, without any clear justification. Most of the difficulties in assigning some leptotyphlopid taxa are due to their conserved external morphology, summed with a relatively small number of available specimens, which complicate the recognition of a unique combination of characters for their reasonable generic allocation. On the other hand, recent osteological studies-especially those on the skull-provide relevant data combinations for species assignment and even for the recognition of new genera. In this work, we have attempted to determine the generic allocation of Epictia unicolor and Trilepida guayaquilensis-both species currently known only by their holotypebased on a detailed description of cranial and post-cranial osteology. We confirm the assignment of E. unicolor to the genus Epictia despite the divergent configuration of cephalic shields. Based mainly on data from the skull, suspensorium, and cervical vertebrae but supported by external morphology as well as a redescription of the holotype, we propose the assignment of Trilepida guayaquilensis to the genus Epictia. This study provides not only detailed data on the osteology of Epictia, but also a first approach to the putative combination of osteological characters for the genus.

Picking up the threads: Comparative osteology and associated cartilaginous elements for members of the genus Trilepida Hedges, 2011 (Serpentes, Leptotyphlopidae) with new insights on the Epictinae systematics

“…(see Uetz et al, 2020; including specimens previously described in Pinto et al, 2015 andSalazar-Valenzuela et al, 2015), although only 11 species were considered for description, excluding Trilepida guayaquilensis . While examining μCT images of the T. guayaquilensis holotype (ZMB 4508), we noticed that osteological data, allied to external morphology evidence pointed out by Pinto et al (2010), suggested that the taxon most likely pertains to another genus, and thus will not be described herein but will be briefly mentioned in the Discussion section (but see Koch et al, 2021). Considering the genus Trilepida, we have not described osteological data for Trilepida anthracina, Trilepida brevissima, and Trilepida dugandi.…”

Evolutionary treasures hidden in the West Indies: Comparative osteology and visceral morphology reveals intricate miniaturization in the insular genera Mitophis Hedges, Adalsteinsson, & Branch, 2009 and Tetracheilostoma Jan, 1861 (Leptotyphlopidae: Epictinae: Tetracheilostomina)

“…Even though the first studies regarding the leptotyphlopid internal morphology are dated back from the 19th century (e.g., Duméril & Bibron, 1844), the first mention to the utility of osteological data for the diagnosis of genera and even species within leptotyphlopids only arose a few decades ago (Broadley & Broadley, 1999). The past few years have witnessed the increase of contributions on the osteology of leptotyphlopids (e.g., Koch et al, 2019;Koch et al, 2021;Martins et al, 2021;Salazar-Valenzuela et al, 2015), reinforcing the utility of these data in the systematics of the family. Similarly, studies on snake visceral morphology and topography represent relevant sources of systematic data, being historically applied to the proposition of new supraspecific taxa, as well as employed in species delimitation (e.g., Underwood, 1967;Wallach and Ineich, 1996;Wallach and Gunther, 1997;Wallach and Smith, 1992).…”

“…This seems to be true for Mitophis and Tetracheilostoma, with M. leptepileptus exhibiting a distinct configuration of snout elements in comparison with other leptotyphlopids. In leptotyphlopids, the dorsal lamina of the septomaxilla inflects medially to contact the nasal septum dorsally (Koch et al, 2021;Martins et al, 2021;Pinto et al, 2015;Rieppel et al, 2009). In M. leptepileptus, besides the medial contact with the nasal septum, the septomaxilla projects dorsally into a laterally compressed flange that contacts the frontals dorsally.…”

“…The genus Tetracheilostoma, on the other hand, does not exhibit any trace of a distinct supraoccipital. We hypothesize that the supraoccipital is fused to the parietal in Tetracheilostoma given that in all Epictinae the parietal ends in an approximately slightly convex suture, contacting the prootics, and supraoccipitals (Koch et al, 2019(Koch et al, , 2021List, 1966;Martins et al, 2021;Martins, Koch, et al, 2019;Pinto et al, 2015;Rieppel et al, 2009). As the fusion of skull elements is very common in miniaturized species (Yeh, 2002), we suggest herein that the possible fusion of the parietal with the supraoccipitals into a wide plate may be due to miniaturization effects, although one may not disregard the association of such a fusion as a direct result of fossoriality.…”

“…A few Epictia spp. might also exhibit a reduced or absent intercentrum I (Martins, 2016;Koch et al, 2019Koch et al, , 2021, and thus such a feature is not exclusive to Tetracheilostomina. As in squamates the neural arch ossifies first, with several other vertebral regions (e.g., synapophyses, zygapophyses, and zygosphenes) chondrifying later in the development as outgrowths of the neural arches (Boughner et al, 2007;Winchester & Bellairs, 1977), the truncation of the intercentrum I development in these snakes might be accomplished by the retention of embryonic stage in adults (i.e., paedomorphosis), reflecting heterochronic implications in vertebral morphology.…”