Sunday, August 5, 2012

Do Snakes Have Necks?

Erpeton tentaculus. Is this animals mostly an elongated neck?
Garth Underwood, has been reported to have said that "....snakes are not necks..." in response to research that suggested that snakes evolved from lizards by  elongating  their cervical regions at the expense of their bodies.

The absence of a pectoral girdle in snakes makes it difficult to determine whether or not snakes have a distinct neck, and it clouds an understanding how the neck and trunk of snakes evolved  from their limbed lizard ancestor.  Tsuihiji and colleagues (2012) found three alternative hypotheses that had been proposed over the years about snake necks: (1) The neck in snakes is extremely elongated. (2) The cervical region is completely lacking in snakes. (3) The various anatomical structures usually associated with the neck–trunk boundary in squamates with limbs are displaced relative to one another in snakes.

Hypothesis 1 was based on the fact that snakes have numerous precloacal vertebrae bearing hypapophyses as well as have the heart and lung positioned rather posteriorly. Similarly, some authors regarded snakes as having a very long neck based on the posterior extent of hypapophysis-bearing vertebrae and then proposed the presence of such  a long neck as a potential synapomorphy that unites snakes and the Cretaceous dolichosaurids as sister clades, an idea first proposed by Nopcsa  in  1908. The possible presence of a long neck in ancestral snakes has led to the hypothesis that the ancestral snake lived in an aquatic environment.

Hypothesis 2 was proposed by Cohn and Tickle (1999), who observed that the anterior expression boundaries of two HOX proteins (HOXC8 and C6) that are located in the trunk region or close to the neck–trunk boundary in other tetrapods extend up to the craniovertebral boundary in the somites and lateral plate mesoderm in Python sp. These expression patterns were interpreted as indicating that the entire precloacal vertebral column has a ‘‘dorsal’’ or trunk identity with no actual cervical region present in this species. Cundall and Greene (2000) also suggested that the cervical and anterior trunk regions are absent in snakes based on observations that ribs, hypaxial muscles, and the pleuroperitoneal cavity all extend to the craniovertebral boundary in snakes.

 Hypothesis 3 was proposed by Hoffstetter and Gasc (1969), who observed that the anatomical structures usually associated with the neck–trunk boundary in limbed  lizards such as  the anterior end of the body wall musculature, posterior extents of the hypapophyses and subvertebral musculature, and position of the heart, tend to be displaced relative to one another in snakes. Hoffstetter and Gasc proposed that elongation of the precloacal region in snakes cannot be attributed solely to elongation of the cervical region.

Thus, various anatomical structures have been proposed as landmarks delimiting the neck–trunk boundary in snakes in the past. However, the axial myology has not been extensively examined for  defining the neck–trunk boundary of snakes despite the fact that the morphology of the axial muscles itself has been studied rather intensively in snakes.

But, Pregill (1977) examined changes in the axial myology in the anterior precloacal region toward the head in several colubrid snakes and demonstrated differentiation and emergence of craniovertebral muscles inserting on the skull as seen in the cervical as well as the anterior trunk regions of lizards with limbs. He concluded snakes have a distinct neck based upon the musculature. Similarly, Al Hassawi (2007) showed the presence of craniovertebral muscles in the viperid Trimeresurus (=Tropidolaemus) wagleri. These results agree with previous observations by Nishi (1916) that Python molurus has modifications of the axial musculature producing craniovertebral muscles homologous with those in four-legged lizards, but he did not specifically address the issue of the snake neck.

Tsuihiji and colleagues (2012) examined the cervical and trunk regionalization of the precloacal musculoskeletal system in snakes based on observations of the axial myology, using an approach similar to the one used by Pregill (1977). In particular, they  focused on the positions where craniovertebral muscles arise in various noncolubroid snakes to shed light on the plesiomorphic location of the neck–trunk boundary in snakes. In addition, several other limb-reduced squamates such as amphisbaenians that still retain vestigial pectoral girdle elements were compared to snakes. Based these observations, they attempted to test the three hypotheses concerning the neck–trunk boundary of snakes outlined above.

They found that the four craniovertebral muscles in lizards and reduced-limb squamates are also present in all  of the snakes they examined, confirming observations by Nishi (1916), Pregill (1977), and Al Hassawi (2007) that the anterior-most  region in snakes shows the same modification of vertebral muscles as observed in the cervical region of limbed lizards and has musculature  distinct from the more posterior region.

Tsuihiji and colleagues (2012)  confirmed the musculature features characterizing the neck in  squamates with well-developed limbs are retained in all examined snakes, contradicting the idea that snakes completely lack a neck  However, the posterior-most origins of the craniovertebral muscles and the anterior-most bony attachments of the body wall muscles that are located at  the neck–trunk boundary in limbed lizards are dissociated anteroposteriorly in snakes. This combined with results of a recent study that shows the anterior expression boundaries of Hox genes coinciding with the neck–trunk boundary in amniotic tetrapods  were dissociated anteroposteriorly in a colubrid snake, support the hypothesis that structures usually associated with the neck–trunk boundary in limbed lizards are displaced relative to one another in snakes.  Therefore suggesting that the trunk of the body, not the neck contributed most to the elongation of the snake.

Tsuihiji, T., Kearney, M. and Rieppel, O. (2012) Finding the neck–trunk boundary in snakes: Anteroposterior dissociation of myological characteristics in snakes and its implications for their neck and trunk body regionalization. Journal of  Morphology. doi: 10.1002/jmor.20037

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