The Origin of the Amphisbaenia

Updated August 12, 2014. Moving Cryptolacerta to a closer relationship to Heloderma, elevating Sineoamphisbaena to its place between skinks and amphisbaenids.

Amphisbaena literally means, “goes both ways.” The name is that of the mythological “Mother of Ants,” an ant-eating serpent with a head on both ends. Amphisbaena is also a genus within the Amphisbaenia. Like its mythological namesake, Amphisbaena can back up as easily as it moves forward, despite having no legs.

Amphisbaenia are the worm lizards, typically (with one exception, Figure 1) legless squamates that burrow and have a superficial resemblance to earth worms, including having their scales arranged in rings (annuli). Their right lung is reduced to make more room for the left lung. (In snakes the left lung is reduced.) The eyes are reduced and deeply recessed and the tail resembles the head. Amphisbaenians are so different from other squamates that they have been considered a third suborder, after lizards and snakes.

The primitive Amphisbaenian, Bipes.

Figure 1. The primitive amphisbaenian, Bipes.

Bipes, a Primitive Extant Amphisbaenian
Bipes (Figure 1) is a living amphisbaenian with strong front legs. The hand is stout, like that of a mole, with digits 2 and 3 the longest, digit 1 absent and digit 5 vestigial. The vestigial hind limbs do extend beyond the body wall. By contrast, in typical lizards digit 4 is the longest.

Extinct Burrowers
Several fossil taxa have been linked to amphisbaenians. Tamaulipasaurus lived during the Early Jurassic. Sineoamphisbaena and Crythiosaurus lived during the Late Cretaceous.  Spathorhynchus lived during the early Oligocene. Most of these are known from skulls and partial skulls. No hands yet known in any of these taxa.

Amphisbaenian Origins – part 1 – Sineoamphisbaena
Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that a round-skulled Late Cretaceous squamate, Sineoamphisbaenea (Figure 2), was the oldest known amphisbaenian. Unfortunately, it didn’t look very much like most amphisbaenians (Figure 2) which made accurate nesting something of a problem in the eyes of many.

Amphisbaenian Origins – part 2 – Not Sineoamphisbaena
Kearney (2003) argued that Sineoamphisbaena nested closer to Macrocephalosaurus and that Amphisbaena nested with Dibamus and snakes, not far from Gekko and the legless geckos, the Pygopodidiae.

 

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Amphisbaenian Origins – part 3 – Cryptolacerta
Müller et al. (2011) argued that a new Eocene lizard, Cryptolacerta  (Figure 2), was the sister to the Amphisbaenia and both were sisters to Sineoamphisbaena and the Teiioidea, a lizard taxon that includes the skinks, Gymnophthalmus and Chalcides. 

Where Do Amphisbaenians Nest in the Large Study?
Here Cryptolacerta does indeed nest close to skinks and amphisbaenians, but it nests closer to Heloderma, another burrowing lizard. Müller et al. (2011) reported that Cryptolacerta had sealed up its upper temporal fenestrae with expansion of the very large postfrontal bone. I was unable to duplicate that reconstruction. Instead I found upper temporal fenestrae in the specimen. A GIF movie and comparative reconstructions can be found here. In any case, amphisbaenians do not add bone to their skulls, they lose bone.

Amphisbaenians nest close to skinks with Sineoamphisbaena nesting close to the base of the other amphisbaenians. In consideration of Kearney (2003), I deleted all amphisbaenians, then all skinks and amphisbaenians from the large study, but those tests failed to dislodge Sineoamphisabaenia form its node, which kept it far from Macrocephalosaurus.

Summary
While amphisbaenians are distinct from most other lizards, they are closer to skinks and legless skinks than to any other lizard taxa. More legless taxa will be added to the large tree as time goes by and I will report on each one in turn.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References:
Cope ED 1894. On the genera and species of Euchirotidae. American Naturalist 28: 436-437.
Gao K 1997.
 
Sineoamphisbaena phylogenetic relationships discussed. Canadian Journal of Earth Sciences. 34: 886-889. online article
Kearney M 2003. The Phylogenetic Position of Sineoamphibaena hextabularis reexamined. Journal of Vertebrate Paleontology 23 (2), 394-403.
Müller J, Hipsley CA, Head JJ, Kardjilov N, Hilger A, Wuttke M and Reisz RR 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature 473 (7347): 364–367. doi:10.1038/nature09919
Wu XC., Brinkman DB, Russell AP, Dong Z, Currie PJ, Hou L, and Cui G 1993. Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia. Nature (London), 366: 57 – 59.
Wu X-C Brinkman DB and Russell AP 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences, 33: 541-577.
Papenfuss TJ 1982. The Ecology and Systematics of the Amphisbaenian Genus Bipes. Occasional papers of the California Academy of Science 136: 1-42.

wiki/Cryptolacerta
wiki/Amphisbaenia
wiki/Bipes
wiki/Sineoamphisbaena

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s