Tropidophiidae Brongersma, 1951, 1816

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

Tropidophiidae Brongersma, 1951

General information.

Commonly known as "dwarf boas", they were for long lumped into an expansive " Boidae " (e.g., Cope 1887; Stull 1935; Romer 1956; Dowling 1959; Guibé 1970; Rage 1984). They were initially assembled together under the name Ungaliidae by Cope (1894) (subsequently spelled as Ungaliidae by the same author; Cope 1898), however, the typifying genus Ungalia Gray, 1842, is a junior synonym of Tropidophis Bibron in Ramón de la Sagra, 1838-1843, and Brongersma (1951) coined the name Tropidophiidae (see also Smith 1969). McDowell (1987) placed Tropidophiidae into their own superfamily, Tropidopheoidea , and divided them into two subfamilies, Tropidophinae (for Tropidophis and Trachyboa Peters, 1860) and Ungaliopheinae (for Exiliboa Bogert, 1968, and Ungaliophis Müller, 1880). More recently, though, it was suggested, based on external morphology and myology ( Zaher 1994), that they represented a paraphyletic assemblage. This polyphyly was later further strongly demonstrated by molecular evidence, with phylogenetic analyses recovering Tropidophis and Trachyboa nested instead closer to aniliids, while Ungaliophis and Exiliboa were nested within Booidea ( Wilcox et al. 2002; Lawson et al. 2004; Vidal et al. 2007, 2009; Wiens et al. 2008, 2012; Reynolds et al. 2014; Streicher and Wiens 2016; Zheng and Wiens 2016; Burbrink et al. 2020; Ortega-Andrade et al. 2022). This has resulted in a radically updated topology for Tropidophiidae , where Tropidophis and Trachyboa constitute its only extant members, confined to Tropical Americas, representing the sister group of Aniliidae ( Smith and Georgalis 2022). Ungaliophiidae in turn is placed into “true” boas, being close to Charinaidae ( Pyron et al. 2014; Reynolds and Henderson 2018; Georgalis and Smith 2020). Such distinction between Tropidophiidae and Ungaliophiidae is further supported by important differences in their cranial anatomy ( Bogert 1968a; Smith and Georgalis 2022) as well as internal anatomy ( Brongersma 1951).

A number of fossil forms from the Paleogene and Neogene of Europe, the Paleogene of Africa, the Paleogene and Neogene of southwestern Asia, and the Quaternary of the Americas have been referred to tropidophiids ( Szyndlar and Rage 2003; Szyndlar et al. 2008; Rage and Augé 2010; McCartney and Seiffert 2016; Syromyatnikova et al. 2019, 2021; Georgalis et al. 2021c; see Smith and Georgalis 2022). Considering our current understanding of the Tropidophiidae concept, among the Paleogene and Neogene taxa, only an unnamed form from the Eocene of Egypt ( McCartney and Seiffert 2016) could be more reliably assigned to tropidophiids, although certain European genera, such as the Eocene Szyndlaria Rage and Augé, 2010, might also be true members of that group (see Smith and Georgalis 2022). In any case, the extant genus Tropidophis is also known by few Pleistocene remains from the Caribbean Islands ( Aranda et al. 2017; Syromyatnikova et al. 2021).

Vertebral morphology further corroborates such distinction between Tropidophiidae and Ungaliophiidae , though admittedly it does not provide any support on the suggested sister group relationship of Tropidophiidae with Aniliidae . The most distinctive feature of the vertebral morphology of Tropidophiidae is the presence of a broad hypapophysis in lateral view throughout their trunk vertebrae, which in all mid-trunk vertebrae has a distinct anteroventral corner forming approximately a right angle. In other portions of the vertebral column, this anteroventral corner of the hypapophysis can either be either of a right angle as well or occasionally show a different degree of inclination that protrudes much anteriorly (see examples in Dowling and Duellman 1978: fig. 104.2; Lee and Scanlon 2002: fig. 10G; McCartney and Seiffert 2016: fig. 6A). Besides, it is also the morphology in the cloacal and caudal series, possessing a longitudinal sequence of subcentral structures, that seems unique among snakes: cloacal and anterior caudal vertebrae of Tropidophiidae are provided with hypapophyses (differently shaped than the preceding trunk vertebrae), followed by distally forked (grooved) hypapophyses or small haemapophyses (or haemapophyses-like structures) in middle caudal vertebrae, and ultimately followed by keels retaining a medially located groove or a pit in posteriormost caudal vertebrae (for more details see Description and figures of Trachyboa and Tropidophis below).

Previous figures of vertebrae of extant Tropidophiidae have been so far presented by Holman (1967), Bogert (1968a, 1968b), Dowling and Duellman (1978), Lee and Scanlon (2002), Szyndlar and Rage (2003), Gauthier et al. (2012), Head (2021), Syromyatnikova et al. (2021), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series had never been figured so far, with the exception of a caudal vertebra (of unknown exact position in the tail) figured in Alfonso-Rojas et al. (2023) and a single μCT image of an articulated skeleton in Frýdlová et al. (2023); nevertheless, Szyndlar and Böhme (1996), Szyndlar and Rage (2003), and Szyndlar et al. (2008) emphasized considerably on the pattern of subcentral structures in the cloacal and caudal series of tropidophiids. Other authors (e.g., Underwood 1976; Dowling and Duellman 1978; Rage 1984; McDowell 1987; Holman 2000; Smith and Georgalis 2022) observed and highlighted the presence and/or shape of hypapophyses throughout the trunk portion of the column in tropidophiids. Smith and Georgalis (2022) took it a step further and suggested that the presence of the broad hypapophysis with a strong anteroventral corner in all mid-trunk vertebrae of all tropidophiids represents a diagnostic trait for the group. The presence of paracotylar foramina was recorded in Tropidophis haetianus (Cope, 1879) by Underwood (1976); based on the latter observation, McDowell (1987) erroneously mentioned the occurrence of paracotylar foramina in all tropidophiids, whereas Kluge (1991) erroneously mentioned the absence of these foramina in all of them.