Zoosyst. Evol. 100 (2) 2024, 597-602 | DOI 10.3897/zse.100.118068 > PENSUFT. fis Neti yee BERLIN Osteology of the skull of the blind snake Helminthophis flavoterminatus (Peters, 1857) (Serpentes, Anomalepididae) Fidélis Janio Marra Santos! 1 Pontificia Universidade Catolica do Rio Grande do Sul (PUCRS), Laboratory of Vertebrate Systematics. Av. Ipiranga, 6681 Partenon; 90619-900, Porto Alegre, Rio Grande do Sul, Brazil https://zoobank. org/E DE9AA BA-4D35-426C-B837-056E865E97F 2 Corresponding author: Fidélis Junio Marra Santos (fidelismarra@gmail.com) Academic editor: Johannes Penner # Received | January 2024 # Accepted 8 April 2024 Published 17 May 2024 Abstract The blind snake Helminthophis flavoterminatus (Peters, 1857) is a species of Anomalepididae with distribution throughout northwestern Venezuela and western Colombia. Its osteology is poorly known, and thus this study presents information on its skull structure based on High-Resolution X-Ray Computed Tomography. The absence of the supraoccipital bone can be used for the determination of H. flavoterminatus in relation to its congeners, Helminthophis frontalis (Peters, 1860) and Helmintho- Dhis praeocularis Amaral, 1924. Key Words Helminthophis sp., HRXCT, reptiles, snakes, systematics, taxonomy Introduction Within the family Anomalepididae Taylor, 1939, the genus Helminthophis Peters, 1860, is a group of small blind snakes, distributed in the Neotropics, from lower Central America and northwestern South America: Costa Rica, Panama, Colombia, and Venezuela. Helminthophis is currently composed of three species (McDiarmid et al. 1999; Marra Santos and Reis 2018; Boundy 2021): Hel- minthophis flavoterminatus (Peters, 1857); Helmintho- phis frontalis (Peters, 1860); and Helminthophis prae- ocularis Amaral, 1924. The osteology of the skull of some species of snakes Anomalepididae, for example Anomalepis aspinosus Taylor, 1939 and Liotyphlops albirostris (Peters, 1857), has already been very well studied and presented by other researchers (Tihen 1945; Haas 1964, 1968; List 1966), and, in recent years, the use of High-Resolution X-Ray Computed Tomography (HRXCT) has increased the knowledge of the osteology of Anomalepididae, for example Anomalepis colombia Marx, 1953; Liotyphlops albirostris (Peters, 1857); Liotyphlops anops (Cope, 1899): Liotyphlops palauophis Marra Santos, 2023; Lio- typhlops taylori Marra Santos & Reis, 2018; Liotyphlops ternetzii (Boulenger, 1896); and Typhlophis squamosus (Schlegel, 1839) (Rieppel et al. 2009; Marra Santos and Reis 2018; Marra Santos and Reis 2019; Marra Santos 2023; Szyndlar and Georgalis 2023). An interesting and important aspect of HRXCT is that, in important ways, it permits replication of many aspects of the traditional techniques, and the resulting digital models allow for three-dimensional and rotational investigation similar to traditional observations of dry skulls or skeletons in hand (Bell et al. 2021). In this paper, the osteology of the skull of Hel- minthophis flavoterminatus is presented based on HRXCT data. This study is the first detailed description of the cranial osteology of He/minthophis, adding new information to the knowledge of the skull anatomy of Anomalepididae snakes. Copyright Marra Santos, F.J. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 598 Materials and methods The head of H. flavoterminatus (AMNH R 59407) was studied by HRXCT at the CT facility at Pontificia Uni- versidade Catolica do Rio Grande do Sul (PUCRS) us- ing a SkyScan 1173. Additional information will be available on MorphoBank (http://morphobank.org/per- malink/?P5113). The skull measures 4.6 mm in length, 2.4 mm in width, and 1.3 mm in height. The dataset is compared with HRXCT datasets for Helminthophis fron- talis and Helminthophis praeocularis. The specimen of H. praeocularis (AMNH R 38125) was also scanned at PUCRS. The head of Helminthophis flavotermina- tus (AMNH R 59407) and the head of H. praeocularis (AMNH R 38125) were scanned at 46 kV and 55 uA, and each reconstructed slice represents a thickness and spacing of 7.16 um. The head of H. frontalis (MCZ R-55117) was scanned at the CT facility of the Univer- sity of Texas at Austin using an Xradia microCT scanner. This specimen was scanned at 80 kV and 10 W, and each reconstructed slice represents a thickness and spacing of 3.58 um. The datasets were rendered in three dimensions using CTVox v. 3.2 (Bruker MicroCT, Inc., Billerica, MA) for Windows. The terminology used for bones fol- lows Rieppel et al. (2009) and Marra Santos (2023). Results Description of skull High-Resolution X-Ray Computed Tomography of skull bones in Figs 1-5. Neurocranium Main body of premaxilla on ventral surface of snout. Maxilla-premaxilla contact widely separated. Lateral maxillary foramina absent. Maxilla alveolar row oriented transversely. Nasal fused. Nasal-frontal boundary convex posteriorly in a shallow W-shaped suture. Prefrontal sepa- rated from nasal. Prefrontal moveably articulated to fron- tal. Postorbital element present. Posterior orbital margin incomplete. Frontals gradually tapering anteriorly. Fron- tal fused. Frontal-parietal contact (dorsal aspect) anterior- ly concave, 1.e., frontals extending posteriorly into broad median embayment in parietals. Parietal paired. Posterior border of parietal in contact with otico-occipital. Supra- occipital absent. Supratemporal present. Posteromedial flange of septomaxilla short, not contacting frontal. Sep- tomaxilla with lateral flange contributing to posterior bor- der of external naris. Fenestra for duct of Jacobson’s or- gan (fenestra vomeronasalis) posteroventrally positioned. Palatine not in contact with vomer, maxilla, or pterygoid. Ectopterygoid present. zse.pensoft.net Marra Santos, F.J.: Osteology of the skull of Helminthophis flavoterminatus Mandible and dentition Splenial not present as discrete element. Coronoid and angular separated by prearticular portion of compound bone. Retroarticular process long, longer than articular facet. Teeth present in maxilla and dentary, but lacking in premaxilla, palatine, and pterygoid. Discussion The osteology of the skull of Helminthophis reveals to us a character that readily distinguishes this genus from the other three belonging to Anomalepididae, which is the fused frontal (Figs 1B, 4A—C). The fron- tal paired in Anomalepis Jan, 1860; Liotyphlops Peters, 1881; and Zyphlophis Fitzinger, 1843, has already been presented and discussed by other researchers (Haas 1964, 1968; List 1966; Rieppel et al. 2009; Marra San- tos and Reis 2018; Marra Santos and Reis 2019; Marra Santos 2023). In addition to the frontal fused, which 1s a bony character of Helminthophis that distinguishes it from the other genera of Anomalepididae, H. flavo- terminatus is readily differentiated from H. frontalis and H. praeocularis by the absence of supraoccipital (Figs 1B, 4A, 5A). An important observation is that, if it were not for the presence of the fused frontal, the skull of Helminthophis could easily be confused with the skull of some of the species of Liotyphlops that also do not have the supraoccipital (for example, Lioty- phlops schubarti, Liotyphlops taylori, Liotyphlops ter- netzii, and Liotyphlops wilderi) or even with the skull of Typhlophis squamosus. With the exception of the fused frontal, H. flavoter- minatus presents other skull characters shared by other taxa Anomalepididae: (1) the nasal bone fused (Figs 1B, D, 4A); (2) absence of the lateral maxillary foramina (Fig. 1A); (3) absence of a prefrontal-nasal contact (Figs 1B, C, 4A); (4) presence of a discrete ossification (postor- bital element) (Figs 1A—D, 2A, B); and (5) by having an ectopterygoid (Figs 1A, C, 2B, C). One of the most interesting and novel consequences of HRXCT is that it has opened the door to the develop- ment and documentation of detailed data on the internal anatomy even of type specimens, which otherwise have historically remained immune to internal anatomical investigations that were, of necessity, invasive and de- structive (Bell et al. 2021). In this sense, the informa- tion on the osteology of the skull of H. flavoterminatus presented here, thanks to the use of HRXCT, can contribute to the knowledge of the anatomy and sys- tematics of snakes, especially extant non-caenophidian snakes. It is important to note that the data presented here on the osteology of the skull of H. flavotermina- tus will contribute to future publications focused on the taxonomy of Helminthophis. Zoosyst. Evol. 100 (2) 2024, 597-602 599 Figure 1. Three-dimensional reconstruction of the skull of Helminthophis flavoterminatus (AMNH R 59407), based on HRXCT data. A. Lateral view; B. Dorsal view; C. Ventral view with the lower jaw partially digitally removed; D. Anterior view; E. Posterior view. Scale bar: 1 mm. Anatomical abbreviations: a, angular; bo, basioccipital; cb, compound bone; cbp, compound bone prearticu- lar component; cbs, compound bone surangular component; co, coronoid; d, dentary; ec, ectopterygoid; en, external naris; f, frontal; fo, foramen; fv, fenestra vomeronasalis; m, maxilla; mf, mental foramen; n, nasal; oc, occipital condyle; ooc, otico-occipital (fused prootic + opisthotic + exoccipital); p, parietal; pa, palatine; pbs, parabasisphenoid; pe, postorbital element; pf, prefrontal; pg, pterygoid; pm, premaxilla; q, quadrate; rp, retroarticular process; sf, surangular foramen; sm, septomaxilla; st, supratemporal; tf, trigeminal foramen; v, vomer; vf, vomerine foramen. zse.pensoft.net 600 Marra Santos, F.J.: Osteology of the skull of Helminthophis flavoterminatus Figure 2. Three-dimensional reconstruction of the skull of Helminthophis flavoterminatus (AMNH R 59407), based on HRXCT data. A. Transversal view; B. Frontal view; C. Sagittal view. Scale bar: 1 mm. Anatomical abbreviations: a, angular; an, acoustic nerve foramen; bo, basioccipital; cb, compound bone; cbp, compound bone prearticular component; cbs, compound bone surangu- lar component; ci, conchal invagination; co, coronoid; ev, cavum vestibuli; d, dentary; dp, descensus parietalis; ec, ectopterygoid; f, frontal; fl, frontal laterally descending flange; fo, foramen; fpb, facial nerve palatine branch foramen; fs, frontal subolfactory process; fv, fenestra vomeronasalis; m, maxilla; Mc, Meckel’s canal; n, nasal; nl, nasal lateral flange; ns, medial nasal septum; oc, occipital condyle; on, optic nerve foramen; ooc, otico—occipital (fused prootic + opisthotic + exoccipital); p, parietal; pa, palatine; pbe, parabasal (Vidian) canal; pbs, parabasisphenoid; pe, postorbital element; pf, prefrontal; pg, pterygoid; pm, premaxilla; pvs, posterior vertical semicircular canal; q, quadrate; rstm, recessus scalae tympani medial aperture; s, stapes; sm, septomaxilla; sml, septomaxilla lateral flange; tre, trigeminofacialis chamber; v, vomer; vc, vomeronasal cupola; vn, vomeronasal nerve passage. zse.pensoft.net Zoosyst. Evol. 100 (2) 2024, 597-602 601 Figure 3. Three-dimensional reconstruction of the lower jaw of Helminthophis flavoterminatus (AMNH R 59407), based on HRXCT data. A. Lateral view; B. Medial view; C. Dorsal view; D. Ventral view; E. Anterior view; F. Posterior view. Scale bar: 1 mm. Anatomical abbreviations: a, angular; arf, articular fossa; cb, compound bone; ecbp, compound bone prearticular component; cbs, compound bone surangular component; co, coronoid; d, dentary; Mc, Meckel’s canal; mf, mental foramen; rp, retroarticular process; sf, surangular foramen. Figure 4. Incomplete dorsal view (some bones are not shown) of the three-dimensional reconstruction of the skulls of A. He/mintho- Dhis flavoterminatus (AMNH R 59407); B. Helminthophis frontalis (MCZ R-55117); and C. Helminthophis praeocularis (AMNH R 38125), showing the absence of supraoccipital in H. flavoterminatus. Scale bars: 1 mm. Anatomical abbreviations: f, frontal; n, nasal; p, parietal; pf, prefrontal; ooc, otico-occipital (fused prootic + opisthotic + exoccipital); so, supraoccipital. Figure 5. Transversal views of three-dimensional reconstruction of the posterior part of the skulls of A. Helminthophis flavoter- minatus (AMNH R 59407); B. Helminthophis frontalis (MCZ R-55117); and C. Helminthophis praeocularis (AMNH R 38125), showing the contribution of the supraoccipital to the internal sidewall of the neurocranium in H. frontalis and H. praeocularis. Scale bars: 1 mm. Anatomical abbreviations: bo, basioccipital; p, parietal; ooc, otico-occipital (fused prootic + opisthotic + exoccipital); q, quadrate; rp, retroarticular process; so, supraoccipital. zse.pensoft.net 602 Acknowledgements I am grateful to all curators, collection managers, and their respective institutions for the loan of specimens and for permission to examine specimens in their care: David A. Kizirian and Lauren Vonnahme (American Museum of Nat- ural History, New York) and José Rosado (Museum of Com- parative Zoology, Harvard University). I thank Christopher J. Bell, Jessie Maisano, and Patrick Stafford of the Univer- sity of Texas at Austin for their support in the acquisition of specimens and CT scanning. I thank Adolpho Augustin of the Pontificia Universidade Catolica do Rio Grande do Sul for help with CT scanning of specimens. I thank the Muse- um ftir Naturkunde, Berlin, for their support in the publi- cation of this manuscript. I thank Johannes Penner, Polina Petrakieva, Vassil Peev (Zoosystematics and Evolution edi- tors) and the reviewers, Van Wallach and Claudia Koch, for valuable suggestions during the review process. References Amaral A (1924) Helminthophis. Proceedings of the New England Zo- ological Club 9: 25-30. Bell CJ, Daza JD, Stanley EL, Laver RJ (2021) Unveiling the elusive: X-rays bring scolecophidian snakes out of the dark. 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Copeia 1945(4): 204-210. https://doi.org/10.2307/1438352 Helminthophis frontalis. Costa Rica. San José: MCZ R-55117. Helminthophis praeocularis. Colombia. Tolima, Honda: AMNH R 38125.