Ontogenetic skull variation in a shovel-headed amphisbaenian species

HOHL; LEANDRO; AZORIT, CONCEPCIÓN; VASSALLO AI; CASINOS, ADRIÀ; ROCHA BARBOSA, OSCAR

Journal of Morphology

Wiley

Año: 2023

Leposternon microcephalum is a species belonging to the Amphisbaenia, a group of burrowing reptiles. The Amphisbaenia present various morphological and physiological adaptations that allow them to penetrate the ground and live underground, through a system of galleries and permanent chambers that they build themselves. Among the morphological adaptations those of the skull stand out, this serving as the main excavation tool. Four basic skull shapes are recognized: rounded, keeled, sword-shaped, and spade-shaped. The skull of L. microcephalum belongs to this last type, which is considered the most specialized. The species inhabits soils that are highly compacted and difficult to penetrate. Among the species of the genus Leposternon present in South America, L. microcephalum is the one with the widest distribution, being found in all Brazilian biomes and neighboring countries such as Bolivia, Argentina, Paraguay, and Uruguay. The analysis of the skull of this species was carried out using 3D Geometric Morphometry (GM), a technique that allows comparative analysis, through robust statistical methods, of the shape and its variations, using Cartesian coordinate data from a configuration of homolog reference points. The technique allows the size and shape components of a structure to be analyzed separately. From an ontogenetic point of view, this methodology had also been used to investigate variations in Cynisca leucura, a member of the family Amphisbaenidae with a rounded head. Our hypothesis is that the patterns of morphological differentiation in the skull, mainly in the intermediate and occipital regions, are similar in different Amphisbaenia species. Therefore, the objective of this study was to analyze cranial morphological variations in an ontogenetic series of L. microcephalum using 3D GM technique. CT micrographs of 13 specimens were analyzed: juveniles (N = 8) and adults (N = 5), based on 20 reference points that described the skull. Principal components and regression analyses between shape (dependent variable) and size (independent variable) showed a clear difference between the cranial morphological pattern of Page 2 of 25 John Wiley & Sons Journal of Morphology 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60For Peer Review Hohl 3 juvenile individuals and that of adult individuals. For instance, comparing with adults, young specimens tend to have a dorsoventrally wide neurocranium, with the tip of the snout more anteriorly oriented and its dorsal boarder subtly curved. Dorsally, the parietal region is thicker and smoothly dome-shaped. As in C. leucura, the variation was strongly correlated with the size change from juvenile to adult, indicating a dominant role for ontogenetic allometry in determining skull shape.