Scolecophidia Dumeril & Bibron, 1844

Szyndlar, Zbigniew & Georgalis, Georgios L., 2023, An illustrated atlas of the vertebral morphology of extant non-caenophidian snakes, with special emphasis on the cloacal and caudal portions of the column, Vertebrate Zoology 73, pp. 717-886 : 717

Scolecophidia Dumeril & Bibron, 1844

General information.

Commonly called "worm snakes" (as their name in Greek literally translates [σκώληξ + ὀφίδια]) or "blind snakes", Scolecophidia was for a long time considered a monophyletic assemblage of basal snakes (e.g., Underwood 1967; Rage 1984). Certain 19th century workers even included them within lizards and not snakes (e.g., Bonaparte 1839; Gray 1845). Notably, a similar view was held also more than one century after, when McDowell and Bogert (1954) suggested that Scolecophidia do not belong to snakes, and instead represent a lineage of limbless lizards that became convergent to snakes, a view accepted also by few others ( Robb 1960; Goin and Goin 1962)! Nevertheless, the true ophidian nature of scolecophidians is currently universally accepted, though their monophyly has been seriously challenged, primarily based on molecular data ( Vidal and Hedges 2005; Wiens et al. 2008; Vidal et al. 2010; Pyron and Burbrink 2012; Pyron et al. 2013; Hsiang et al. 2015; Reeder et al. 2015; Figueroa et al. 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Streicher and Wiens 2017; Miralles et al. 2018; Burbrink et al. 2020), as well as total evidence analyses ( Zaher et al. 2023). More specifically, recent analyses have suggested that anomalepidids either represent the sister group of alethinophidians, being thus not sharing an exclusive common ancestor with the remaining scolecophidians, or lie more basal to leptotyphlopids and typhlopoids ( Wiens et al. 2008; Pyron and Burbrink 2012; Miralles et al. 2018; Burbrink et al. 2020; Zaher et al. 2023).

The fossil record of scolecophidians is poor, consisting exclusively of vertebrae spanning across a small number of Cenozoic localities ( Szyndlar 1991a; Mead 2013; Georgalis et al. 2017; Ivanov 2022; Smith and Georgalis 2022) and a potential Cretaceous occurrence ( Fachini et al. 2020; but see Head et al. 2022 for an alternative interpretation of the latter Cretaceous form). In the vast majority of cases, scolecophidian fossil vertebrae cannot be referred more precisely to the family level, and this is especially the case for the Paleogene and Neogene remains ( Szyndlar 1991a; Mead 2013; Georgalis et al. 2017; Ivanov 2022; Smith and Georgalis 2022). Nevertheless, some Quaternary fossil vertebrae have been identified to the family or even the genus level, aided mainly by a geographic rationale (e.g., Bochaton et al. 2015; Syromyatnikova et al. 2021; Peralta and Ferrero 2023; Ramello et al. 2023). Despite this very poor fossil record, divergence date estimates suggest that scolecophidians have split from other snakes around the Early Cretaceous ( Pyron and Burbrink 2012; Zheng and Wiens 2016; Miralles et al. 2018; Fachini et al. 2020).

A considerable amount of skeletal studies in scolecophidians has focused primarily on their cranial anatomy already since the 19th century (e.g., Müller 1831; Jan and Sordelli 1860-1866; Boulenger 1893; Waite 1918b; Mahendra 1936; Dunn 1941; Dunn and Tihen 1944; List 1966; Wallach et al. 2007; Broadley and Wallach 2009; Rieppel et al. 2009; Pinto et al. 2015; Kraus 2017; Chretien et al. 2019; Marra Santos and Reis 2019; Linares-Vargas et al. 2021; Lira and Martins 2021; Chuliver et al. 2023; Graboski et al. 2023; Martins et al. 2023) revealing indeed important differences among the different families. In contrast, their vertebral morphology has received very little attention (e.g., Mookerjee and Das 1933; Mahendra 1936; List 1966; Fabrezi et al. 1985), though there seems to be growing interest recently, substantially aided by the usage of non-invasive technologies, such as μCT scanning (e.g., Martins et al. 2019, 2021a, 2021b; Herrel et al. 2021; Koch et al. 2019, 2021; Lira and Martins 2021).

All scolecophidians (leptotyphlopids, typhlopoids, and anomalepidids) seem to display a very simple and relatively homogenous vertebral morphology, which at most times renders it almost impossible to differentiate members of particular families based on postcranial osteology. They are all characterized by an elongate centrum, depressed cotyle and condyle, depressed neural arch, presence of relatively long prezygapophyseal accessory processes, the direction of the major axis of the prezygapophyseal articular facets approximating the direction of the major axis of the vertebra, absent or very shallow median notch of the neural arch, absent haemal keels in mid- and posterior trunk vertebrae, vestigial neural spine (present only in the anterior trunk vertebrae) shifted posteriorly, absence of any subcentral structures in the cloacal and caudal portion of the column, and very low number of caudal vertebrae.

Vertebral distinction among scolecophidian families.

Despite the morphological homogeneity of scolecophidian families, certain features have been addressed in the literature to distinguish them or of possessing potential diagnostic utility for family level determination. These are summarized and assessed in this section.

Mookerjee and Das (1933) and Mahendra (1935) highlighted the presence of a single subcentral foramen in trunk vertebrae of typhlopids, with the latter author regarding it as unique among extant snakes. Since then, the diagnostic utility of the subcentral foramen in typhlopids has been repeatedly highlighted ( List 1966; Lee and Scanlon 2002; Wallach 2009, 2020; Fachini et al. 2020). Wallach (2020) even suggested that this foramen was unusual among typhlopids and could be used as a characteristic of Indotyphlops braminus (Daudin, 1803). Fachini et al. (2020) regarded "the presence of asymmetrical subcentral foramina" as diagnostic of scolecophidians. We acknowledge the presence of such single large subcentral foramen in typhlopids, which, as it can be seen in the figures, it is not constantly present in all vertebrae and in all species. In our leptotyphlopid sample, this feature was not observed, but, judging from the published literature, we have to note that these foramina can be present in all trunk and cloacal vertebrae of certain leptotyphlopid taxa (e.g., Trilepida Hedges, 2011; see Pinto et al. 2015) as well as in anomalepidids, where they are also intracolumnarily variable (i.e., present in the anterior trunk vertebrae of Anomalepis Jan, 1860 in Jan and Sordelli 1860-1866 but absent posteriorly in this taxon [ Dunn 1941] and in the posterior trunk vertebrae of Liotyphlops Peters, 1881 [ Dunn and Tihen 1944]). It should be noted also that such foramina can be variably present in aniliids, uropeltids, and calabariids as well (see the respective entries below). Certain fossorial lizards also possess distinctive subcentral foramina in their vertebrae (e.g., amphisbaenians, see Georgalis et al. 2018 and Xing et al. 2018; dibamids, see figs in Xing et al. 2018). This feature should therefore be more properly and quantitatively assessed across many different scolecophidian genera, before it can obtain any diagnostic utility for taxonomic determinations.

In his monographic treatise, List (1966) noted that some species of Typhlopidae possessed a peculiar rodlike ossification associated with the fused terminal vertebrae (not occurring in other snakes), which he named as urostyle, but besides he did not find any identifying vertebral characters.

List (1966) also stated that zygosphenes and zygantra of leptotyphlopids are well dorsal to the level of the zygapophyseal joint. Holman (2000) stated that vertebrae of leptotyphlopids are more elongate than those of typhlopids. We consider that these features are variable and more widespread across scolecophidians, and so they cannot be used for determination.

One interesting feature that was addressed by Pinto et al. (2015) is that leptotyphlopids seem to possess a higher number of caudal vertebrae than typhlopids and anomalepidids, a fact further highlighted subsequently by Martins et al. (2021a). Based on our observations and the extensive literature overview presented below, we here tentatively concur with this statement, but we have to emphasize that data on caudal vertebral counts are missing from most typhlopid and leptotyphlopid species, in particular the African taxa. It would be interesting indeed to see how this number of caudal vertebrae ranges in the genera Rhinoleptus ( Leptotyphlopidae ) and Letheobia ( Typhlopidae ), which possess the highest total vertebral counts among all extant snakes (see below).

Fachini et al. (2020) further highlighted two additional characters, the "high position of the paradiapophyses (synapophyses in their terminology), which are located dorsal to the ventral margin of the cotyle" and the "smooth ventral margin of the centrum" as characteristic of typhlopids (and scolecophidians in general), considering these features as absent in all other snakes and therefore exclusively present for scolecophidians. We confirm the former feature as present (and sometimes distinct) in scolecophidians, but we have to note that this is also (intracolumnarily) variable present in few vertebrae of aniliids and uropeltids. As for the latter feature, we confirm that typhlopids (and scolecophidians as a whole) all possess a smooth centrum devoid of subcentral structures in their trunk vertebrae.

Finally, one feature that was highlighted by Herrel et al. (2021) is that (at least certain) typhlopids possess more robust cotyles, condyles, prezygapophyses, and postzygapophyses in their anterior trunk vertebrae, compared to those of leptotyphlopids and anomalepidids, which was explained as an adaptation for higher pushing forces during burrowing of the former group. This would of course prove to be of importance for taxonomic determination but its utility should be further quantified and studied across a high number of taxa.