Dolichosaurus, Coniasaurus and the origin of snakes

Yesterday we looked at several ancestral taxa recovered by the large reptile tree in the origin of snakes as gecko sisters via Tchingisaurus,  Ardeosaurus, Eichstaettisaurus and Jucaraseps. Today we’ll add Dolichosaurus (Owen 1850, Caldwell 2000; Figs. 1, 2) a snake-like reptile with 19 cervicals, tiny limbs, an elongate torso of 40 vertebrae and a likely much shorter tail than its sister, Pontosaurus.

Figure 1. Dolichosaurus added to the lineage of snake origins.

Figure 1. Dolichosaurus added to the lineage of snake origins. Click to enlarge. Note Dolichosaurus does not appear to have had such a deep or long swimming tail as found ion Pontosaurus and the limbs were also smaller and less ossified.

Cervical number
Yesterday we noted that Pontosaurus had 9 cervicals (8-9 is typical of most squamates) and Aphanizocnemus had 12. Dolichosaurus raises this number to 19, the largest number of cervicals among any non-snake squamate. In other limbless and reduced limb squamates the dorsal count increases, but the cervical count is reduced.

Figure 2. Dolichosaurus is known from several specimens and the skull is poorly known. Click to enlarge.

Figure 2. Dolichosaurus is known from several specimens and the skull is poorly known. Click to enlarge. Note the reduction of the limbs, the lack of deep chevrons in the tail, the apparent reduction in the number of tail vertebrae based on the posterior taper. The ghosted skull of Pontosaurus provides some clues as to the identity of the few skull bones preserved in the holotype of Dolichosaurus.

Chronology
Dolichosaurus lived during the Late Cretaceous, long after the origin of snakes in the middle Jurassic (Caldwell et al. 2014). So this pre-snake morphology was a good design, able to survive for tens of millions of years during the heyday of mesozoic reptile predators on land, sea and air.

Coniasaurus
Caldwell (2000) reported frustration that Dolichosaurus lacked good skull material and Coniasaurus lacked good post-crania. He wrote, “The possibility remains that these taxa are congeneric.” 

That doesn’t appear to be correct.
From all appearances (Fig. 3), Coniasaurus is a tiny, yet classic mosasauroid like Aigialosaurus.

Figure 3. Aigialosaurus, a small mosasauroid, compared to Coniasaurus, an even smaller mosasauroid, not related to Dolichosaurus and snakes.

Figure 3. Aigialosaurus, a small mosasauroid, compared to Coniasaurus, an even smaller mosasauroid, not related to Dolichosaurus and snakes. Note that Coniasaurus teeth are closely set. Jurassic snake teeth, (Fig. 4) are not. like Click to enlarge.

Teeth and rostrum
Coniasaurus teeth are conical and closely set. Jurassic snake teeth (Fig. 4) and Eichstaettisaurus teeth are widely spaced. The rostra of Coniasaurus and Aigialosaurus were elongated with an orbit in the posterior half of the skull. In basal snakes and their Eichstaettisaurus-like ancestors the rostrum was short with an orbit that entered the anterior half of the skull.

Figure 1. Maxillae from several Mid-to-Late Jurassic snakes discovered by Caldwell et al. 2015, including Parviraptor, Diabolosphis and Portualophis.. These are compared to Lanthanotus, an extant limbed precursor to burrowing snakes, like Cylindrophis. Also shown are Eichstaettisaurus, a limbed varanid precursor to terrestrial snakes along with the skulls of Adriosaurus, an elongate limbed pre-snake and Pachyrhachis, a Late Cretaceous snake with vestigial hind limbs and no forelimbs or pectoral girdle.

Figure 1. Maxillae from several Mid-to-Late Jurassic snakes discovered by Caldwell et al. 2015, including Parviraptor, Diabolosphis and Portualophis.. These are compared to Lanthanotus, an extant limbed precursor to burrowing snakes, like Cylindrophis. Also shown are Eichstaettisaurus, a limbed varanid precursor to terrestrial snakes along with the skulls of Adriosaurus, an elongate limbed pre-snake and Pachyrhachis, a Late Cretaceous snake with vestigial hind limbs and no forelimbs or pectoral girdle.

Mosasauroids,
despite their sea-going ways, don’t appear to have as many snake-like traits as Jucaraseps, Pontosaurus and their Eichstaettisaurus-like kin. Coniasaurus, despite its small size, does appear to be a mosasauroid and therefore unrelated to Dolichosaurus. Shifting snakes and their ancestors back to Adriosaurus to the base of the varanids and mosasauroids adds 26 steps to the large reptile tree.

I wrote to Michael Caldwell
and sent him the squamate subset of the large reptile tree. He replied, “Hi David, just had a quick look…first thoughts….seems unlikely to me that snakes are polyphyletic, and mosasauroids as well.  Just my first impressions.” 

References
Caldwell MW 1999. Description and phylogenetic relationships of a new species of Coniasaurus Owen, 1850 (Squamata). Journal of Vertebrate Paleontology 19:438-455.
Caldwell MW 2000. On the aquatic squamate Dolichosaurus longicollis Owen, 1850 (Cenomian, Upper Cretaceous) and the evolution of elongate necks in Squamates. Journal of Vertebrate Paleontology 20(4):720-735.
Caldwell MW and Cooper JA 1999. Redescription, palaeobiogeography and palaeoecology of Coniasaurus crassidens Owen, 1850 (Squamata) from the Lower Chalk (Cretaceous; Cenomanian) of SE England. Zoological Journal of the Linnean Society 127:423-452.
Owen R 1850. Description of the fossil reptiles of the Chalk Formation: pp. 378-404 in F. Dixon (ed) The geology and fossils of the Tertiary and Cretaceous Formations of Sussex. Longman, Brown, Green and Longman, London.

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