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glands, or anterior-positioned canaliculated fangs associated with high-pressure
venom glands. A study of SHG expression showed that front fangs develop from the
posterior end of the upper jaw and are strikingly similar in morphogenesis to rear
fangs, consistent with their proposed homology (Vonk et al., 2008). These research-
ers found that the anterior part of the upper jaw of front-fanged snakes lacked SHG
expression. Ontogenetic allometry displaces the fang from its posterior developmen-
tal origin to its adult front position, consistent with an ancestral posterior position of
the front fang (Vonk et al., 2008). The authors reported that the posterior maxillary
teeth of “rear-fanged” snakes develop from an independent posterior dental lamina
and retain their posterior position. Based on their data, they hypothesized a model
for the evolution of snake fangs in which a posterior subregion of the tooth-forming
epithelium became developmentally uncoupled from the remaining dentition. This
allowed the posterior teeth to evolve independently and in close association with the
venom gland (Vonk et al., 2008). As this could lead to notable modification in dif-
ferent lineages, the authors proposed that their model partly accounted for radiation
of advanced snakes in the Cenozoic era, with the resulting marked diversification in
extant colubroids (Vonk et al., 2008).
Some investigators have noted the presence of paired grooves and compound ser-
rations on the posterior teeth of Uatchitodon schneideri , an archosauriform 3 species
from the Late Epoch of the Triassic Period (also known as the Upper Triassic and pre-
viously as the Keuper Period, approximately 230-200 mya) that is known only from
dental specimens (Mitchell et al., 2010). These authors advanced the hypothesis that
the described dentitional modifications were markers of the evolutionary pathway/
trajectory for oral secretion/venom delivery in amniotes as well as evidence of venom
conduction in early diapsid 4 reptiles (Mitchell et al., 2010). Therefore, the aforemen-
tioned dental modifications on the fossilized teeth of U. schneideri were hypothetically
assigned a venom-delivery function (Mitchell et al., 2010). In this, another relevant
hypothesis is based on the presence of toxins (of shared classes) in oral secretions/
venoms of some lizards (e.g., the known venomous helodermatids, but also including
some varanoids, iguanids, and others; see later) as well as snakes that (in this hypoth-
esis) constitute a venom-secreting lineage of squamate reptiles belonging to a single
clade (Fry et al., 2006). Thus, as mentioned previously, some researchers have sug-
gested early evolution and subsequent derivation of venom-delivery systems (Fry et al.,
3 The archosaurs, or “ruling reptiles”, probably originated in the late Permian Period (approximately
250 mya), and their sauropod descendants (e.g., dinosaurs) thoroughly dominated the Mesozoic Era
[approximately 250-65 mya; it was composed of the Cretaceous (about 146-65 mya), Jurassic (about 208-
146 mya), and Triassic (about 245-208 mya) Periods]. The only surviving archosaurian descendants are
birds and crocodilians. Relevant to the hypotheses about the evolution of venom-delivery systems is the
thecodontal (teeth implanted into dentitional compartments or sockets in the jaws) condition as well as
other cranial properties present in archosaurians (see Romer, 1956).
4 Aside from the order Testudinata [turtles, terrapins, and tortoises, with only extant species recognized in
the synonymous order Chelonia, are anapsids (named as most lack temporal openings in their skulls)] and
a few extinct lineages, all reptiles are or were diapsids (including all of the dinosaurs), meaning that they
possess(ed) two temporal openings (fenestrae) in their skulls.
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