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)

Abstract

The genera Mitophis and Tetracheilostoma comprise two extant lineages of small-sized threadsnakes that exclusively inhabit several islands of the West Indies. Even though leptotyphlopids are known for their extremely reduced size, miniaturization has only been hypothesized to reflect insular dwarfism for the genus Tetracheilostoma. Herein, we aim to describe the comparative osteology and visceral morphology of both genera, investigating and discussing their several internal morphological simplifications and novelties. Our results indicate that these taxa exhibit several autapomorphies mostly concentrated in the dorsoposterior skull elements and maxillae, as well as in their axial skeleton and viscera. These novelties and simplifications are most likely a result of extreme miniaturization driven by the evolutionary constraints or ecological opportunities possibly imposed by the "island rule." Both Mitophis and Tetracheilostoma distinguish from all other Epictinae in lacking a dentigerous process in the maxillae, by having the prootic fused to the otooccipital, and by the lack (except in comparison to a few Epictia) of a cervical vertebrae intercentrum I. Additionally, Mitophis can be distinguished from other Epictinae by the participation of the unpaired supraoccipital in the dorsal border of the foramen magnum, by the absence of the pleurapophyses in the caudal vertebrae, by a higher number of liver segments, and by the extreme degeneration of the pelvic rudiments. Tetracheilostoma differs from other Epictinae by lacking a distinct supraoccipital, which is fused to the parietal. Thus, our results reinforce that morphological

Cranial osteology of Hypoptophis (Aparallactinae: Atractaspididae: Caenophidia), with a discussion on the evolution of its fossorial adaptations

“…A functional analogue of the alethinophidian central rod design in the anomalepidids, whose frontal is devoid of medial flanges, is achieved via a ventral contact between the nasal medial vertical lamina and the subolfactory process of the frontal (Rieppel et al, 2009 ). In leptotyphlopids, the nasals and prefrontals usually establish a dorsal and lateral contact with the dorsal horizontal and orbital laminae of the frontals, respectively, while the septomaxillae contacts the frontal subolfactory process ventrally (Koch et al, 2021 ; Martins et al, 2021 ; Rieppel et al, 2009 ). Hence, leptotyphlopid crania incorporate features from both the outer shell and the central rod designs to transmit the loading stress (Rieppel et al, 2009 ).…”

Ontogenetic mechanisms of size change: implications for the Lilliput effect and beyond

“…As such, miniaturization and gigantism are also “phylogenetic statements” and indicate dramatic shifts in body size and mode of life with reference to other members of a clade (Haken and Wake 1993). Cases of miniaturization and gigantism are plentiful, especially among living taxa, including miniature threadsnakes of the West Indies (Martins et al 2021); Brookesia chameleons, including the exceptionally small Brookesia nana (Glaw et al 2012); kinosternine turtles (Cordero 2021); extinct varanid lizards (Erickson et al 2003); baleen whales (Slater et al 2017); and amphiumid salamanders (Bonett et al 2009).…”