A new view on snake origins: snake ancestors were gecko sisters

Updated February 15, 2015 with the addition of several basal snake taxa to the large reptile tree which united terrestrial and burrowing snakes in one clade, all derived from basal gekkotans.

Earlier we looked at the role of tiny Jucaraseps (Figs. 1, 4) and larger Adriosaurus (Fig. 1) in the origin of terrestrial snakes (remember burrowing snakes had a separate convergent origin with Lanthanotus according to the large reptile tree). Today, larger Aphanizocnemus  (Dal Sasso and Pinna 1997, Fig. 5) and much larger Pontosaurus (Kornhuber 1873, Fig. 2) and are added to the matrix (Fig. 2).

Figure 1. Pontosaurus in the evolution of snakes. Here tiny Jucaraseps leads this lineage, followed by Adriosaurus, Pontosaurus all leading to the basal snake, Pachyrhachis.

Figure 1. Pontosaurus in the evolution of snakes. Here tiny Jucaraseps leads this lineage, followed by Adriosaurus, Pontosaurus all leading to the basal snake, Pachyrhachis.

Pontosaurus has an extraordinarily long tail. The tail (= everything posterior to the cloaca) is shorter in snakes, and the torso is longer.

Pontosaurus has nine cervicals. We don’t know how many cervicals snakes have, owing to a lack of a pectoral girdle, but nine to twelve is a good number to start with because Pontosaurus has nine cervicals and Aphanizocnemus has twelve.

Figure 2. Pontosaurus and its parts. Data from Caldwell 2006. This is one of the last taxa we know in the snake lineage that still had a pectoral girdle.

Figure 2. Pontosaurus and its parts. Data from Caldwell 2006. This is one of the last taxa we know in the snake lineage that still had a pectoral girdle.

The origin of snakes is not firmly understood or agreed upon
by established snake experts, but they’re pretty sure snakes arise from dolichosaurids.  These experts include Adriosaurus and Pontosaurus among these dolichosaurid varanids. Currently no snake expert considers the burrowing snakes as distinct and separate from the terrestrial and aquatic snakes and other phylogenetic analyses do not recover this topology (Fig. 3). To its credit, in this tree all sister taxa look very much alike and share a large number of traits without any large obvious gaps.

Figure 1. A subset of the large reptile tree focusing on squamates. Note snake ancestors are still sisters to basal geckos.

Figure 1. A subset of the large reptile tree focusing on squamates. Note snake ancestors are still sisters to basal geckos.

Living snakes are gekko sisters
Within the Squamata the legless trait occurred several times by convergence, including one within the geckos, Lialis. The latest data indicates that snake ancestors and gecko ancestors had a common ancestor separate from the varanids and helodermatids. That does not mean that snakes are geckos, or vice versa. Rather they shared a common ancestor that was not yet either. That ancestor was a sister to Tchingisaurus.

Figure 3. Snake ancestor skulls, Ardeosaurus, Eichstaettisaurus and Jucaraseps. Note the reduction of the premaxilla and the enlargement of the parietals.

Figure 4. Snake ancestor skulls, Ardeosaurus, Eichstaettisaurus and Jucaraseps. Note the reduction of the premaxilla and the enlargement of the parietals. Click to enlarge.

Taking a look at
Ardeosaurus, Eichstaettisaurus and Jucaraseps (Fig. 4) illustrates how several features of the snake skull evolved, first through miniaturization as Jucaraseps is very tiny. The premaxilla gets smaller, the cranium gets longer, the posterior skull bones become smaller.

Aphanizocnemus (sometimes misspelled Afanizocnemus, Fig. 5) nests in the large reptile tree between Jucaraseps and Adriosaurus. Wikipedia reports, “It is a marine lizard that lived during the Late Cretaceous. It is often classified in the family Dolichosauridae as a close relative of snakes, although some studies have placed it as an even closer relative to snakes than dolichosaurids.”

Figure 5. Aphanizocnemus libanensis, previously considered a dolichosaur, here recovered between Jucaraseps and Adriosaurus. Prior snake studies did not include Jucaraseps, a key taxon.

Figure 5. Aphanizocnemus libanensis, previously considered a dolichosaur, here recovered between Jucaraseps and Adriosaurus. Prior snake studies did not include Jucaraseps, a key taxon. Click to enlarge. This taxon is about 12 inches or 30 cm long.

Pythonomorpha
Paleontologists are still not in agreement as to the origin of snakes. According to Wikipedia, Pythonomorpha was originally proposed by paleontologist Edward Drinker Cope (1869) as a reptilian order comprising snakes and mosasaurs... and refers to the generally serpentine body plan of members of the group. The validity of Pythonomorpha remains debated however, indeed there is no consensus about the relationships of snakes or mosasaurs to each other, or to the rest of the lizards. An analysis by Conrad (2008) placed mosasaurs with varanoid lizards, and snakes with skinks. The most recent analysis, by Gauthier et al. (2012) suggests that mosasaurs are more primitive than either snakes or varanoids.”

Gauthier et al. 2012 nested Adriosaurus and Pontosaurus as sister to the mosasauroids Aigialosaurus and Tylosaurus. They also nested Scleroglossa and Tchingisaurus as the proximal outgroups to that combined clade and the Iguania as the proximal outgroup to that combined clade. So that study shares with this study an origin near the base of the Scleroglossa. Unfortunately Gauthier et al. nested snakes with Dibamus and other legless amphisbaenids, extremely derived taxa that the large reptile tree nests with skinks.

These new tree topologies arise by the addition of taxa and the reception of better data.

References
Caldwell MW 2006. A new species of Pontosaurus (Squamata, Pythonomorpha) from the Upper Cretaceous of Lebanon and a phylogenetic analysis of Pythonomorpha. Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano 34:1–42.
Caldwell MW and Palci A 2010. A new species of marine ophidiomorph lizard, Adriosaurus skrbinensis, from the Upper Cretaceous of Slovenia. Journal of Vertebrate Paleontology 30:747-755. doi:10.1080/02724631003762963.
Dal Sasso C and Pinna G 1997. Aphanizocnemus libanensis n. gen. n. sp., a new dolichosaur (Reptilia, Varanoidea) from the Upper Cretaceous of Lebanon. Paleontologia Lombarda 7:1–31.
Haas G 1979. On a new snakelike reptile from the Lower Cenomanian of Ein Jabrud, near Jerusalem. Bulletin du Muséum national d’Histoire Naturelle de Paris Ser. 4 (1):51-64.
Kornhuber A 1901. Opetisaurus bucchichi, eine neue fossile Eidechse aus der unteren Kreide von Lesina in Dalmatien. Abhandlungen der geologischen Reichsanstalt Wien 17:1-24.
Lee MSY and Caldwell MW 1998. Anatomy and relationships of Pachyrhachis problematicus, a primitive snake with hindlimbs. Philosophical Transactions of the Royal Society London B 353:1521-1552.
Palci A and Caldwell MW 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27:1-7. doi:10.1671/0272-4634(2007)27[1:VFAAEI]2.0.CO;2.
Pierce SE and Caldwell MW 2004. Redescription and phylogenetic position of the Adriatic (Upper Cretaceous; Cenomanian) dolichosaur Pontosaurus lesinensis (Kornhuber, 1873). Journal of Vertebrate Paleontology 2:373-386.
Seeley HG 1881. On remains of a small lizard from Neocomian rocks of Comen, near Trieste, preserved in the Geological Museum of the University of Vienna. Quarterly Journal of the Geological Society of London 37:52-56.

wiki/Adriosaurus
wiki/Pachyrhachis
wiki/Aphanizocnemus
wiki/Pontosaurus

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One thought on “A new view on snake origins: snake ancestors were gecko sisters

  1. On the topic of how many of the snake vertebrae are cervical vertebrae, there’s been some interesting work on hox gene expression and muscle patterns in snakes.

    Hox genes are the genes which control the development of the anterior-posterior axis of an embryo. In effect the tell each segment in an animal what segment it is meant to be e.g. in arthropods, which segment should produce legs, which antenae, which mandibles, which spinnerets etc. In vertebrates they control the identity of the vertebrae. Anyway there are two interesting (and unfortunately contradictory) studies on snakes. Cohn & Tickle (1999) looked at python Embryos, and found that HoxC6 and HoxC8 (the genes associated with thoracic identity) were expressed over all vertebrae anterior to the hindlimbs, which would imply that the neck region is completely absent.

    However, Wolthering et al. (2009), working on corn snakes, found that, although the hoxC6 expression shows gradual reduction starting at the 11th vertebrae, fading towards the head. This suggests that, instead of the thoracic region expanding to cover the cervical region, it is the cervico-thoracic boundary that has expanded. This would imply that the ancestral boundary was at the 11th vertebrae. However Wolthering et al did note that in snakes, many hox genes did not correlate with the smae Transition of morphology as the same genes in lizards, indicating a Change in their expression at some point in snake Evolution.

    Tsuihiji et al. (2006) suggested that the cervicoquadratus muscle in snakes (which runs between the head and 11th vertebra) was homologous to the episternocleidomastoideus muscle in lizards, which runs between the skull and the pectoral girdle, implying that the ancestral snake neck covered the first 11 vertebrae.

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