Allometry and Isometry in Shinisaurus Ontogeny

There are those who insist that pterosaur juveniles and hatchlings had a short rostrum and large orbit (Bennett 1995, 1996), citing similar allometric changes during ontogeny in mammals and archosaurs. The fact that pterosaurs are not mammals or archosaurs does not appear to matter. The large reptile tree nests pterosaurs firmly within the Fenestrasauria, within the Tritosauria, within the Lepidosauria (outside the Squamates) and within the Lepidosauriformes.

Earlier we looked at isometry (relative lack of change) during ontogeny (maturation) in several pterosaurs for which we have juveniles associated with adults. These observations don’t seem to matter much to pterosaur experts who want to believe that hatchling pterosaurs had cute features. Isometry during ontogeny is generally found trait among lepidosaurs and especially so among tritosaur lepidosaurs, as evidenced by Reynoso (1989) who noticed little to no change between a juvenile and an adult Huehuecuetzpalli.

Today we’ll take a look at allometry AND isometry during ontogeny in a rare living lizard (Squamata, Autarchoglossa, Anguimorpha), Shinisaurus crocodilurus (Figs. 1, 2, the Chinese crocodile lizard).

Figure 1. Lateral views of Shinisaurus adult and juvenile, to scale and to the same skull length. While the skull proportions are roughly the same (isometry) changes that can be noted are noted (allometry).

Figure 1. Lateral views of Shinisaurus adult and juvenile, to scale and to the same skull length. While the skull proportions are roughly the same (isometry) changes that can be noted are noted (allometry). Note there is no rostral elongation during maturation in this taxon. Images from Digimorph.org

The skulls of the juvenile and adult
show very little rostral elongation during maturity. The orbit is only slightly reduced in the adult. Larger changes are noted on the figures. Surprisingly, the teeth are relatively smaller in the adult. The expanded braincase in the juvenile is reduced in the adult, but an expanded (inflated) occiput is retained and further expanded in several burrowing lizards, retained in a process called neotony.

Figure 2. Dorsal views of Shinisaurus juvenile and adult with notes on isometric and allometric changes.

Figure 2. Dorsal views of Shinisaurus juvenile and adult with notes on isometric and allometric changes. Left image from Digimorph.org.

Wikipedia reports: Shinisaurus, the Chinese crocodile lizard, was once also regarded as a member of Xenosauridae, but most recent studies of the evolutionary relationships of anguimorphs consider Shinisaurus to be more closely related to monitor lizardsand helodermatids than to Xenosaurus. It is now placed in its own family Shinisauridae. The large reptile tree agrees with this nesting.

References
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species. Journal of Paleontology 69: 569–580.
Bennett SC 1996. Year-classes of pterosaurs from the Solnhofen limestones of Germany: taxonomic and systematic implications. Journal of Vertebrate Paleontology 16:432–444.

http://digimorph.org/specimens/Shinisaurus_crocodilurus/juvenile/
wiki/Chinese_crocodile_lizard

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