Allometry during ontogeny in the basal tritosaur, Huehuecuetzpalli

Huehuecuetzpalli (Reynoso 1998) is a basal tritosaur according to the large reptile tree, a lepidosaur nesting outside of the Squamata, and ancestral to Tanystropheus, Macrocnemus, drepanosaurids, fenestrasaurs and ultimately, pterosaurs. The lineage of pterosaurs is shown here.

Huehuecuetzpalli specimens are only known from the Early Cretaceous, with ghost lineage origins going back to the Late Permian. Long species survival is not uncommon among lepidosaurs, as in the extant Sphenodon with relatives in the Triassic.

Figure 1. Two specimens of Huehuecuetzpalli were found, one adult and one juvenile. Here they are shown together to scale along with manus and pes comparisons scale to a common length for metacarpal 4 and metatarsal 4.

Figure 1. Two specimens of Huehuecuetzpalli were found, one adult and one juvenile. Here they are shown together to scale along with manus and pes comparisons scale to a common length for metacarpal 4 and metatarsal 4.

Reynoso 1998 reported,
“The relative length of the snout, and the proportions of the skull and limbs relative to the presacral vertebral column, do not show signifcant differences between the juvenile and adult specimens, although these features usually change in ontogeny. This suggests that adult proportions were already acquired at the ontogenetic stage of the younger specimen in spite of its relatively smaller size.”

I have been repeating this observation
with regard to pterosaurs, which likewise do not show any significant differences (apart from the enlargement of any skull crests) in their morphological proportions. For examples click here, here and here and other references therein.

But is it true for Huehuecuetzpalli?
That’s why side-by-side comparisons are so useful. Sadly, I have not done so until just yesterday (Figs. 1, 2).

Figure 2. Huehuecuetzpalli, adult and juvenile skulls to scale. note the relatively shorter rostrum in the juvenile, which also had smaller teeth and a shorter set of parietals (with a smaller braincase and smaller jaw adductor chamber). In the juvenile the ascending process of the premaxilla was more robust.

Figure 2. Huehuecuetzpalli, adult and juvenile skulls to scale. note the relatively shorter rostrum in the juvenile, which also had smaller teeth and a shorter set of parietals (with a smaller braincase and smaller jaw adductor chamber). In the juvenile the ascending process of the premaxilla was more robust and the tooth-bearing portion was shorter with fewer teeth.

Reynoso 1998 reported,
“The complete fusion of the cranial elements suggests that the larger specimen is of post-juvenile age, and probably an adult condition was already acquired. The olecranon process of the ulna, however, is not completely ossified and attached to the ulna, and only a ball of hard tissue (calcified cartilage or bone) is preserved. It was impossible to find information in the literature about the time when the precursor of the olecranon process become fused to the ulna.

“The age of the smaller specimen is more difficult to establish. The complete ossification of the fourth distal tarsal and the still separated astragalus and calcaneum undoubtedly suggest a post-hatchling stage. The complete fusion of the frontal, however, shows that it is older than Rieppel’s specimen number 18 and the hatchling of Cyrtodactylus pubisulcus (Gekkonidae) illustrated by Rieppel (1992a: ¢g. 1). The high degree of ossification indicates that it is close to the latest stages of development preceding complete ossification. Juvenile skull characters are the presence of a broader parietal table with short lateral processes. Compared with the adult skull, the juvenile parietal table is more than 15% broader on the narrower section excluding the ventrolateral flanges for the dorsal attachment of the jaw adductor musculature.”

We looked at olecranon ossification in tritosaurs earlier here.

As a rule, lepidosaurs don’t change much during ontogeny
as we’ve seen earlier here with Shinisaurus. But they do change… a little.

References
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.

 

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