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 Helminthophis praeocularis Amaral, 1924.

Helminthophis sp., HRXCT, reptiles, snakes, systematics, taxonomy

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): Helminthophis flavoterminatus (Peters, 1857); Helminthophis frontalis (Peters, 1860); and Helminthophis praeocularis 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; Liotyphlops 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 Helminthophis flavoterminatus is presented based on HRXCT data. This study is the first detailed description of the cranial osteology of Helminthophis, adding new information to the knowledge of the skull anatomy of Anomalepididae snakes.

The head of H. flavoterminatus (AMNH R 59407) was studied by HRXCT at the CT facility at Pontifícia Universidade Católica do Rio Grande do Sul (PUCRS) using a SkyScan 1173. Additional information will be available on MorphoBank (http://morphobank.org/permalink/?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 frontalis and Helminthophis praeocularis. The specimen of H. praeocularis (AMNH R 38125) was also scanned at PUCRS. The head of Helminthophis flavoterminatus (AMNH R 59407) and the head of H. praeocularis (AMNH R 38125) were scanned at 46 kV and 55 μA, and each reconstructed slice represents a thickness and spacing of 7.16 μm. The head of H. frontalis (MCZ R-55117) was scanned at the CT facility of the University 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 μm. The datasets were rendered in three dimensions using CTVox v. 3.2 (Bruker MicroCT, Inc., Billerica, MA) for Windows. The terminology used for bones follows Rieppel et al. (2009) and Marra Santos (2023).

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 frontal paired in Anomalepis Jan, 1860; Liotyphlops Peters, 1881; and Typhlophis Fitzinger, 1843, has already been presented and discussed by other researchers (Haas 1964, 1968; List 1966; Rieppel et al. 2009; Marra Santos and Reis 2018; Marra Santos and Reis 2019; Marra Santos 2023). In addition to the frontal fused, which is a bony character of Helminthophis that distinguishes it from the other genera of Anomalepididae, H. flavoterminatus 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, Liotyphlops schubarti, Liotyphlops taylori, Liotyphlops ternetzii, and Liotyphlops wilderi) or even with the skull of Typhlophis squamosus.

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 prearticular 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.

With the exception of the fused frontal, H. flavoterminatus 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 (postorbital element) (Figs 1A–D, 2A, B); and (5) by having an ectopterygoid (Figs 1A, C, 2B, C).

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 surangular component; ci, conchal invagination; co, coronoid; cv, 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; pbc, 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; trc, trigeminofacialis chamber; v, vomer; vc, vomeronasal cupola; vn, vomeronasal nerve passage.

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; cbp, 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. Helminthophis 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 flavoterminatus (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.

One of the most interesting and novel consequences of HRXCT is that it has opened the door to the development 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 destructive (Bell et al. 2021). In this sense, the information 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 systematics 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. flavoterminatus will contribute to future publications focused on the taxonomy of Helminthophis.

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 Natural History, New York) and José Rosado (Museum of Comparative Zoology, Harvard University). I thank Christopher J. Bell, Jessie Maisano, and Patrick Stafford of the University of Texas at Austin for their support in the acquisition of specimens and CT scanning. I thank Adolpho Augustin of the Pontifícia Universidade Católica do Rio Grande do Sul for help with CT scanning of specimens. I thank the Museum für Naturkunde, Berlin, for their support in the publication of this manuscript. I thank Johannes Penner, Polina Petrakieva, Vassil Peev (Zoosystematics and Evolu­tion editors) and the reviewers, Van Wallach and Claudia Koch, for valuable suggestions during the review process.

Examined specimens:

Helminthophis flavoterminatus . Venezuela. Distrito Capital: AMNH R 59407.

Helminthophis frontalis . Costa Rica. San José: MCZ R-55117.

Helminthophis praeocularis . Colombia. Tolima, Honda: AMNH R 38125.