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. 2019 Aug 7;6(8):182228. doi: 10.1098/rsos.182228

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© 2019 The Authors.

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

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Figure 8. — Phylogeny of Serpentes, highlighting the distribution of presence of the CCF and the hypothesized evolution of macrostomy (modified from the evolutionary pathways proposed by Palci et al. [6]). Groups marked with an asterisk (*) contain taxa lacking a CCF; note that Thamnophis is the only colubroid recognized as lacking this feature. The shaded boxes indicate taxa exhibiting macrostomy. These taxa do not form a monophyletic group, suggesting independent evolution of this condition; this interpretation of homoplasy is supported by the fact that different groups develop macrostomy via different developmental pathways. Each pathway is represented by a different colour of box and is depicted schematically in the corresponding diagram below the phylogeny. The grey box for Tropidophiidae indicates a lack of data regarding the development of macrostomy in this clade. Macrostomatan family and subfamily names in green represent groups for which representatives have been directly observed, either by us or in other studies [6]. (a) Booids (boas, pythons and relatives) achieve macrostomy via elongation of the supratemporal and quadrate, with the quadrate becoming ventrolaterally deflected but remaining in the same transverse plane. (b) Caenophidians (acrochordids and colubroids) increase gape size via elongation and rotation of the quadrate, while the supratemporal is proportionally unchanged. (c) Thamnophis and Homalopsis—both caenophidians—achieve macrostomy via both rotation of the quadrate and elongation of the supratemporal and quadrate. This pathway may be more widespread than previously recognized, but has currently only been observed in these two genera. br, braincase; co, compound bone; q, quadrate; st, supratemporal. Phylogenetic relationships mainly from [11], with phylogeny of Colubridae from [27] and placement of Dinilysia from [21] and [28]. Distribution of the CCF is as presented by Palci & Caldwell [21], with incorporation of new data regarding T. radix. Hypothesized pathways of macrostomy development are as discussed by Palci et al. [6].