Abstract:Accurate knowledge of skeletal ontogeny in extant organisms is crucial in understanding important morpho-functional systems and in enabling inferences of the ontogenetic stage of fossil specimens. However, detailed knowledge of skeletal ontogeny is lacking for most squamates, including snakes. Very few studies have discussed postnatal development in snakes, with none incorporating data from all three major ontogenetic stages—embryonic, juvenile and adult. Here, we provide the first analysis encompassing these … Show more
“…Atractaspis compared to adults of other colubroids (e.g., Thamnophis), a feature again resembling the condition among juvenile snakes (Palci et al, 2016;Strong et al, 2019). The lack of a well-developed dorsum sellae overhanging the sella turcica in Atractaspis, Anomochilus, and scolecophidians may represent another paedomorphic feature, related to the lack of crests and ridges on the external surface of the braincase.…”
Section: Heterochronic Modification Of the Jaws Palate Suspensorimentioning
confidence: 82%
“…Abbreviations: a.Smx, articulatory surface for the septomaxilla; dl.N, dorsal lamina of the nasal; fv, fenestra vomeronasalis; lal, lateral ascending lamina; mal, medial ascending lamina; mnf, medial nasal flange; n, nasal; np.Pmx, nasal process of the premaxilla; pmx, premaxilla; plp, palatal process; pmp, premaxillary process; pmxf, premaxillary foramen; pp.Smx, posterior process of the septomaxilla; smx, septomaxilla; tp, transverse process; v, vomer; vc.Smx, vomeronasal cupola of the septomaxilla; vc.V, vomeronasal cupola of the vomer; vp, vomerine process Rieppel (2007), Rieppel (1979), Maisano (2007), Rieppel et al (2009), and Underwood and Kochva (1993). Anatomical terminology in our descriptions follows these papers, as well as Strong et al (2019). In the figures, abbreviations of elements are from Strong et al (2019), in turn modified from Rieppel et al (2009) and Rieppel and Maisano (2007); abbreviations of features are original, though follow the format of Chretien et al (2019).…”
Section: Imagingmentioning
confidence: 93%
“…The dentary of Atractaspis is quite unique compared to other, non- Scanferla, 2016;Strong et al, 2019;Racca et al, 2020).…”
Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins. However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro-computed tomography (micro-CT) scans of Atractaspis irregularis. The skull of Atractaspis presents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large-gaped feeding in snakes). Comparison to scolecophidians-a group of blind, fossorial, miniaturized snakes-in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso-frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as "regressed alethinophidians," in contrast to their traditional placement as the earliest-diverging snake lineage. We propose that Atractaspis and scolecophidians fall along a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorph Atractaspis.
“…Atractaspis compared to adults of other colubroids (e.g., Thamnophis), a feature again resembling the condition among juvenile snakes (Palci et al, 2016;Strong et al, 2019). The lack of a well-developed dorsum sellae overhanging the sella turcica in Atractaspis, Anomochilus, and scolecophidians may represent another paedomorphic feature, related to the lack of crests and ridges on the external surface of the braincase.…”
Section: Heterochronic Modification Of the Jaws Palate Suspensorimentioning
confidence: 82%
“…Abbreviations: a.Smx, articulatory surface for the septomaxilla; dl.N, dorsal lamina of the nasal; fv, fenestra vomeronasalis; lal, lateral ascending lamina; mal, medial ascending lamina; mnf, medial nasal flange; n, nasal; np.Pmx, nasal process of the premaxilla; pmx, premaxilla; plp, palatal process; pmp, premaxillary process; pmxf, premaxillary foramen; pp.Smx, posterior process of the septomaxilla; smx, septomaxilla; tp, transverse process; v, vomer; vc.Smx, vomeronasal cupola of the septomaxilla; vc.V, vomeronasal cupola of the vomer; vp, vomerine process Rieppel (2007), Rieppel (1979), Maisano (2007), Rieppel et al (2009), and Underwood and Kochva (1993). Anatomical terminology in our descriptions follows these papers, as well as Strong et al (2019). In the figures, abbreviations of elements are from Strong et al (2019), in turn modified from Rieppel et al (2009) and Rieppel and Maisano (2007); abbreviations of features are original, though follow the format of Chretien et al (2019).…”
Section: Imagingmentioning
confidence: 93%
“…The dentary of Atractaspis is quite unique compared to other, non- Scanferla, 2016;Strong et al, 2019;Racca et al, 2020).…”
Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins. However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro-computed tomography (micro-CT) scans of Atractaspis irregularis. The skull of Atractaspis presents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large-gaped feeding in snakes). Comparison to scolecophidians-a group of blind, fossorial, miniaturized snakes-in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso-frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as "regressed alethinophidians," in contrast to their traditional placement as the earliest-diverging snake lineage. We propose that Atractaspis and scolecophidians fall along a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorph Atractaspis.
“…4). The size constraint in their heads is so severe that the canals become dorso-ventrally compressed, possibly because their heads are more dorso-ventrally flattened than those of mammals 47 . A flat head may enable squamates to hide from predators in small crevices 48 , but it may at the same time reduce bite force capacity in lizards 49 and (see further) the semi-circular canal sensitivity.Figure 4Semi-circular canal eccentricity in function of the radius of curvature (R) in squamates.…”
Section: Resultsmentioning
confidence: 99%
“…Only in squamates, a negative correlation is found between canal size and canal ellipticity. This may well be related to a more flattened head shape in squamates 47,48 , enabling hiding from predators in small crevices. Computer models show that the canal ellipticity decreases the canal sensitivity the strongest in small, squamate-sized canals.…”
the semi-circular canals in the inner ear sense head rotations. it is widely recognised that the anatomy of the semi-circular canals is often adapted to the species-specific agility, in order to provide the necessary sensitivity. Based on research on mammals, the ellipticity of the semi-circular canal was so far considered as a non-important factor herein. A dataset of 125 squamate species and 156 mammalian species, now shows that the posterior semi-circular canal of squamates is much more elliptical (eccentricities ranging between 0.76 and 0.94) than that of mammals (eccentricities ranging between 0 and 0.71). Fluid-Structure Interaction computer models show that the effect of the ellipticity on sensitivity is strongest in small semi-circular canals. this new insight indicates that the high ellipticity in squamates leads to a severe reduction in sensitivity of up to 45%. In mammals, on the other hand, the reduction in sensitivity is limited to 13%, which is consistent with previous literature that found a limited effect of semi-circular canal ellipticity in mammals. Further, there is a strongly negative correlation between semi-circular canal size and eccentricity in squamates, which is absent in mammals. Hence, the smallest squamates have the most elliptical semi-circular canals. In general, the smaller the semi-circular canal, the less sensitive it is. Therefore, the highly elliptical squamate canals are probably the result of fitting the largest possible canal in small and flat head. Miniaturising the canals while maintaining a circular shape would reduce the sensitivity by another 73% compared to the highly elliptical canals.
Snakes-a subset of lizards-have traditionally been divided into two major groups based on feeding mechanics: "macrostomy," involving the ingestion of proportionally large prey items; and "microstomy," the lack of this ability. "Microstomy"-considered present in scolecophidian and early-diverging alethinophidian snakes-is generally viewed as a symplesiomorphy shared with non-snake lizards. However, this perspective of "microstomy" as plesiomorphic and morphologically homogenous fails to recognize the complexity of this condition and its evolution across "microstomatan" squamates. To challenge this problematic paradigm, we formalize a new framework for conceptualizing and testing the homology of overall character complexes, or "morphotypes," which underlies our re-assessment of "microstomy." Using micro-computed tomography (micro-CT) scans, we analyze the morphology of the jaws and suspensorium across purported "microstomatan" squamates (scolecophidians, early-diverging alethinophidians, and non-snake lizards) and demonstrate that key components of the jaw complex are not homologous at the level of primary character state identity across these taxa. Therefore, rather than treating "microstomy" as a uniform condition, we instead propose that non-snake lizards, early-diverging alethinophidians, anomalepidids, leptotyphlopids, and typhlopoids each exhibit a unique and nonhomologous jaw morphotype: "minimal-kinesis microstomy," "snout-shifting," "axle-brace maxillary raking," "mandibular raking," and "single-axle maxillary raking," respectively. The lack of synapomorphy among scolecophidians is inconsistent with the notion of scolecophidians representing an ancestral snake condition, and instead reflects a hypothesis of the independent evolution of fossoriality, miniaturization, and "microstomy" in each scolecophidian lineage. We ultimately emphasize that a rigorous approach to comparative anatomy is necessary in constructing evolutionary hypotheses that accurately reflect biological reality.
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