Moving beyond the surface: Comparative head and neck myology of threadsnakes (Epictinae, Leptotyphlopidae, Serpentes), with comments on the ‘scolecophidian’ muscular system

Abstract

Studies on the cephalic myology of snakes provide a series of relevant data on their biology and systematics. Despite the great amount of descriptive studies currently available for the group, much of the knowledge remains obscure for most scolecophidian taxa. This study aimed to describe in detail the cephalic (head and neck) myology of members of the tribe Epictinae, Leptotyphlopidae. We provide the first report of the presence of extrinsic ocular muscles, and a double Musculus pterygoideus acessorius in Leptotyphlopidae. A well-developed M . levator anguli oris is exclusive to the subtribes Renina and Epictina, being reduced in Tetracheilostomina species. Both inter- and intraspecific variations are reported for the head and neck muscles, and such results provide additional data and raise an interesting discussion on the neck-trunk boundaries in snakes. We also provide a discussion on the terminology of a few head muscles in Leptoyphlopidae in comparison to the other lineages of ´Scolecophidia´ (Anomalepididae and Typhlopoidea).

A century of waiting: description of a newEpictiaGray, 1845 (Serpentes: Leptotyphlopidae) based on specimens housed for more than 100 years in the collection of the Natural History Museum Vienna (NMW)

“…Premaxilla roughly rectangular in anterior view and hexagonal in ventral view, edentulous, pierced by six foramina (two in anterior view and four in ventral view); transverse process of premaxilla absent and vomerian process single; premaxilla with internal septum composed by two laminae that support the septum nasii dorsally, expanding posteriorly to fit medially in the septomaxilla (internally); nasals paired, approximately rectangular in dorsal view, being pierced by a pair of foramina in lateral border of contact with prefrontals (foramen for the apicalis nasi ); an additional pair of foramina pierce the medial contact within both nasals; a single additional foramen pierces the anterior-medial region of the nasal dorsal lamina; nasal septum descending as double medial vertical flanges that contact the premaxilla, septomaxilla and vomer ventrally (internally); prefrontals paired, subtriangular in dorsal view, in contact with septomaxilla and maxilla ventrally; septomaxillae paired, complex in shape, expanding dorsally into the naris; conchal invagination absent; ascending process of premaxilla pierced by single large foramen; internally, dorsal surface of each septomaxilla pierced by a foramen, and with a medial deep sulcus that extends from its posterior to anterior regions; vomers paired, located midventral to vomeronasal cupola, bearing transversal arms, and with short posterior arms in contact with each other posteriorly; a pair of foramina pierce the ventral lamina of the vomer; frontals paired, nearly rectangular in dorsal view, the left element bearing short anterolateral projections to attach to prefrontals; frontal pillars absent; optic nerve restricted to lateral descending surface of frontals; maxilla edentulous, irregular in shape, pierced by four large foramina in lateral view, two in the dentigerous process of maxilla; posterior process of maxilla reaching the level of the optic nerve foramen; posterior orbital element absent; parietal single, wide, representing the largest bone of braincase; parietal internal pillars ( sensu Martins, 2016) absent; parabasisphenoid arrow-like, with tapered anterior tip lying dorsally to palatine, and fitting in medial line of vomeronasal cupola; in ventral view, parabasisphenoid bearing posterior-lateral projections to provide insertion for the neck muscles (Martins, Passos & Pinto, 2019); parabasisphenoid with shallow pituitary fossa and lateral sulcus; anterior opening for the palatine artery indistinct or absent in parabasisphenoid dorsal (internal) surface, internal carotid artery foramen and abduscens nerve foramen present; opening for the palatine ramus of the facial nerve formed by the lateral edge of the parabasisphenoid and the ventral edge of the parietal; basioccipital single and approximately pentagonal in ventral view, bearing lateral process to attach tendons for the neck muscles (A Martins, 2016, unpublished data); basioccipital does not participate in the formation of the foramen magnum; supraoccipitals paired, approximately rectangular in dorsal view, pierced medially (internally) by a large endolymphatic foramen; prootics paired and triangular in lateral view; prootics forming the trigeminal nerve foramen together with the parietal; prootics pierced medially (internally) by two acoustic nerve foramina, and an additional foramen ventral to the former; statolythic mass in cavum vestibuli absent; stapedial footplate apparently not co-ossified with prootic; otooccipitals paired and irregular in dorsal view, descending to contact each other ventrally to exclude the basioccipital in the formation of the foramen magnum and forming a short but distinct atlantal process ( sensu Cundall & Irish, 2008); medial surface (internal) of otooccipitals pierced by an internal opening for the recessus scalae timpani and a wide foramen that forms the internal opening for the vagus nerve foramen; a reduced foramen pierces the posterior (externa...…”

An evolutionary paradox on threadsnakes: Phenotypic and molecular evidence reveal a new and remarkably polymorphic species of Siagonodon (Serpentes: Leptotyphlopidae: Epictinae) from Amazonia

“…Although molecular evidence has helped in determining their higher-level relationships (Adalsteinsson et al 2009), such data have rarely been useful for unambiguous diagnosis among genera (Martins et al 2019a), which were defined based on general resemblance (i.e., external morphology) of traditional species groups (see Orejas-Miranda 1967;Peters and Orejas-Miranda 1970). On the other hand, several leptoptyphlopid studies have brought magnificent evidence derived from the internal morphological systems (i.e., bones, cartilages, muscles, glands, visceral anatomy and genitalia), contributing decisively to re-positioning many taxa in the tree of life (Martins et al 2019a(Martins et al , 2019b(Martins et al , 2021a(Martins et al , 2021bKoch et al 2021). In addition, a few taxonomic decisions based on the external morphology characters (e.g., Pinto and Fernandes 2012) have been also…”

“…6; n = 3; holotype condition indicated with asterisk). Premaxilla wide, expanded anterolaterally, roughly rectangular (n = 2*) or trapezoidal (n = 1) in anterior view; and triangular in ventral view, edentulous, pierced by seven (n = 2) or eight (n = 1*) foramina; nasal process of premaxilla reduced and tapered toward apex, transverse process absent and vomerian process tapered and single; premaxilla with internal septum composed of two laminae that support the septum nasi dorsally, expanding posteriorly and contacting septomaxilla ventrally; nasals paired, elongated, expanded anterolaterally, with convex anterior limit, and undulated dorsoposterior suture with frontals; being pierced by pair of foramina in posterolateral border of contact with prefrontals and frontals (foramen for the apicalis nasi); additional pair of foramina pierce on the medial contact within both nasals; nasal septum descending as double medial vertical flanges that contact premaxilla and septomaxilla ventrally (internally); prefrontals paired, irregular in shape, in contact with septomaxilla midventrally, and separated by short gap (n = 2*) from maxilla ventrolaterally or in contact with it (n = 1); each prefrontal perforated by two (n = 2*) or three (n = 1*) foramina; septomaxillae paired, complex in shape, expanding dorsoanteriorly and comprising posterior limit of naris; conchal invagination absent; ascending process of premaxilla pierced by single large foramen (n = 2*) or by two foramina (n = 1); internally, dorsal surface of each septomaxilla pierced by single foramen (n = 3 sides*) or two foramina (n = 3 sides*), and with medial inconspicuous sulcus (n = 2*), or sulcus absent (n = 1); vomers paired, located midventral to vomeronasal cupola, transversal arms absent, with posterior arms in contact with each other posteriorly (n = 2*) or not (n = 1); pair of foramina pierce the ventral lamina of each vomer; frontals paired, elongated, approximately twice longer than wide, both elements bearing short anterolateral projections to attach to prefrontals; frontal pillars absent; optic nerve foramen paired (n = 2*) or single (n = 1), restricted to lateral descending surface of frontals; maxilla edentulous, irregular in shape, pierced by four (n = 1*) or six (n = 2) foramina; posterior process of maxilla reaching (n = 1) or not reaching (n = 2*) level of anteriormost optic nerve foramen in lateral view; posterior orbital element absent; parietal single, wide, representing largest bone of braincase; parietal internal pillars (sensu Martins et al 2021a) present; parabasisphenoid arrow-like, with tapered anterior tip lying dorsally to palatine, and fitting in medial line of vomeronasal cupola; posterior limit of parabasisphenoid convex medially; internal (dorsal) face of parabasisphenoid with inconspicuous lateral sulci; anterior opening for palatine artery indistinct or absent, internal carotid artery foramen and abducens nerve foramen present; opening for palatine ramus of facial nerve formed by lateral edge of parabasisphenoid and ventral edge of parietal; basioccipital single and approximately triangular in ventral view, lateral process for attachment of tendons for nuchal muscles (Martins et al 2019b) absent; basioccipital does not participate in formation of foramen magnum; supraoccipitals fused into single unit, approximately twice wider than long; prootics fused to otooccipitals; prootics+otooccipitals forming trigeminal nerve foramen together with parietal and parabasisphenoid (on one side of one specimen parabasisphenoid does not participate in it); prootics pierced medially (internally) by single acoustic nerve foramen; two statolythic masses present in cavum vestibuli (n = 2*) or statolythic mass absent; stapedial footplate apparently not co-ossified with prootic; otooccipitals dorsally fo...…”

“…7; n = 3; holotype condition indicated with asterisk). Dentary supports series of five long teeth ankylosed to inner surface of medial margin of dental concha; mental foramen nearly under fifth tooth; dorsoposterior process of dentary with rounded enlargement (n = 1), possibly to provide attachment for Musculus ceratomandibularis and/or to Musculus cervicomandibularis (Martins et al 2019b) or not as above (n = 2*); splenial conical, visible in lateral view, representing smallest bone in lower jaw, extending from level of third tooth to contact angular posteriorly; anterior mylohyoid foramen absent on splenial; posterior mylohyoid foramen on ventral surface of angular; angular conical, extending posteriorly to level of posterior surangular foramen; compound bone pierced by two foramina on surangular lamina, posterior elliptical and wide, anterior small and rounded; anterior surangular foramen located slightly anterior to coronoid; foramen for chorda tympani of hyomandibular ramus of facial nerve (VII) present (n = 2*) or indistinct (n = 1); foramen of retroarticular process absent; prearticular lamina of compound bone presenting dorsal process to support coronoid (n = 1) or not (n = 2*); coronoid wide, approximately triangular in shape, about as wide as tall, pierced by foramen in its medial side (n = 2*) or not (n = 1); quadrate long and slender, about 50% of skull length, posterior process absent (sensu Martins 2016); dorsal foramen in anterior half of quadrate absent; dorsoanterior lamina of quadrate with shallow sulcus that extends throughout half its extension (n = 2*) or not (n = 1).…”