Was Vellbergia really a juvenile basal lepidosaur? Let’s check…

Earlier we looked at tiny Vellbergia
(Sobral, Simoes and Schoch 2020; Middle Triassic) represented by a disarticulated tiny skull (Fig. 1). The large reptile tree (LRT) nested this hatchling with the much larger adult Prolacerta (Fig. 1). The MPT was 20263 steps for 1654 taxa.

The LRT nesting ran counter to the SuppData cladogram
of Sobral, Simoes and Schoch 2020, who nested Vellbergia among basal lepidosaurs, the closest of which are shown here (Fig. 1). Earlier I did not show the competing lepidosaur candidates. That was an oversight rectified today.

Figure 1. Vellbegia compared to the lepidosaurs it would nest with if Prolacerta and all Archosauromorpha were deleted.

Figure 1. Vellbegia compared to the lepidosaurs it would nest with if Prolacerta and all Archosauromorpha were deleted. Gray areas on Vellbergia indicate restored bone that is lost in the fossil.

To test the lepidosaur hypothesis of relationships,
I deleted all Archosauromorph taxa, including Prolacerta, from the LRT to see where among the Lepidosauromorpha Vellbergia would nest. With no loss of resolution, Vellbergia nested between Palaegama and Tjubina + Huehuecuetzpalli at the base of the Tritosauria plus Fraxinisaura + Lacertulus (Fig. 1) at the base of the Protosquamata. The resulting MPT was 20276 steps, only 13 more than the Prolacerta hypothesis of interrelationships.

That is a remarkably small number considering the great phylogenetic distance between these taxa in the LRT.

Rampant convergence
is readily visible among the competing taxa (Fig. 1). No wonder Prolacerta was named “before Lacerta“, the extant squamate. According to Wikipedia, “Due to its small size and lizard-like appearance, Parrington (1935) subsequently placed Prolacerta between basal younginids and modern lizards. In the 1970s (Gow 1975) the close link between Prolacerta and crown archosaurs was first hypothesized.” That was prior to cladistic software and suffered from massive taxon exclusion.

Allometry vs. Isometry
One of the lepidosaurs shown above, Huehuecuetzpalli (Fig. 1), is known from both an adult and juvenile. The older and younger specimens were originally (Reynoso 1998) considered identical in proportion. Such isometry is an ontogenetic trait shared with other tritosaur lepidosaur clade members, including pterosaurs. On the other hand, if Vellbergia was a hatchling of Prolacerta, some measure of typical archosauromorph allometry should be readily apparent… and it is… including incomplete ossification of the nasals, frontals and parietals along with a relatively larger orbit and shorter rostrum, giving Vellbergia a traditional ‘cute’ appearance appropriate for its clade.

Size
Sobral, Simoes and Schoch considered Vellbergia a juvenile, but it is similar in size to the adult lepidosaurs shown here (Fig. 1). On the other hand, Vellbergia is appropriately smaller than Prolacerta, in line with its hatchling status.

Time
Remember also that Vellbergia is from the Middle Triassic. Prolacerta is from the Early Triassic. They were not found together and some differences are to be expected just from the millions of years separating them.

For comparison: another juvenile Prolacerta,
this time from Early Triassic Antarctica (Spiekman 2018; AMNH 9520), is much larger than Vellbergia from Middle Triassic Germany (Fig. 2), but just as cute. Note the relatively larger orbit and shorter rostrum compared to the adult Prolacerta (Fig. 1), traits likewise found in Vellbergia.

Figure 2. Small Prolacerta specimen AMNH 9520 from Spiekman 2018 compared to scale with Vellbergia.

Figure 2. Small Prolacerta specimen AMNH 9520 from Spiekman 2018 compared to scale with Vellbergia. Sclerotic rings (SCL) identified by Spiekman 2018 are re-identified as pterygoids here.

Generally
crushed, disarticulated and incomplete juvenile specimens of allometric taxa are difficult to compare with adults. Even so, what is left of hatchling Vellbergia tends to resemble the larger juvenile and adult specimens of Prolacerta more than hatchling Vellbergia resembles the similarly-sized adult lepidosaurs it nests with in the absence of Prolacerta from the taxon list.

Phylogenetic analysis is an inexact science.
Nevertheless no other known method breaks down and rebuilds thousands of taxa more precisely. Only taxon exclusion appears to trip up workers at present.


References
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Parrington FR 1935. On Prolacerta broomi gen. et sp. nov. and the origin of lizards. Annals and Magazine of Natural History 16, 197–205.
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.
Sobral G, Simoes TR and Schoch RR 2020. A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications fro the early evolution of lepidosauromorphs and the Vellberg fauna. Nature.com Scientific Reports 10, Article number: 2273.
Spiekman SNF 2018. A new specimen of Prolacerta broomi from the lower Fremouw Formation (Early Triassic) of Antarctica, its biogeographical implications and a taxonomic revision. Nature.com/scientificreports (2018)8:17996

wiki/Prolacerta

Pterodaustro isometric growth series

Tradtional paleontologists think pterosaur babies had a cute short rostrum that became longer with maturity and a large orbit that became smaller with maturity (Fig. 1). This is a growth pattern seen in the more familiar birds, crocs and mammals.

Pterodaustro embryo as falsely imagined in Witton 2013. The actual embryo had a small cranium, small eyes and a very long rostrum.

Figure 1. Pterodaustro embryo as falsely imagined in Witton 2013. The actual embryo had a small cranium, small eyes and a very long rostrum.

Unfortunately
these paleontologists ignore the fossil evidence (Figs 2, 3). These are the data deniers. They see things their own way, no matter what the evidence is. The data from several pterosaur growth series indicates that hatchlings had adult proportions in the skull and post-crania. We’ve seen that earlier with Zhejiangopterus (Fig. 2), Tapejara, Pteranodon, Rhamphorhynchus and others. Still traditional paleontologists ignore this evidence as they continue to insist that small short rostrum pterosaurs are babies of larger long rostrum pterosaurs.

Figure 1. Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.

Figure 2 Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.

As readers know,
several pterosaur clades went through a phase of phylogenetic miniaturization, then these small pterosaurs became ancestors for larger clades. Pterosaurs are lepidosaurs and they grow like lepidosaurs do, not like archosaurs do.

Today we’ll look at
the growth series of Pterodaustro (Fig. 1), previously known to yours truly only from adults and embryos. Today we can fill the gaps with some juveniles.

This blog post is meant to help traditional paleontologists get out of their funk.

A recent paper
on the braincase of odd South American Early Cretaceous pterosaur Pterodaustro (Codorniú et al. 2015) pictured three relatively complete skulls from a nesting site (Fig. 1). I scaled the images according to the scale bars then added other available specimens.

Figure 1. Pterodaustro skulls demonstrating an isometric growth series. One juvenile is scaled to the adult length. One adult is scaled to the embryo skull length. There is no short rostrum and large orbit in the younger specimens.

Figure 1. Pterodaustro skulls demonstrating an isometric growth series. One juvenile is scaled to the adult length. One adult is scaled to the embryo skull length. There is no short rostrum and large orbit in the younger specimens. If you can see differences in juvenile skulls vs. adult skulls, please let me know. All these specimens come from the same bone bed.

You can’t tell which skulls are adults or juveniles
without scale bars and/or comparable specimens. As we established earlier, embryos are generally one-eighth (12.5%) the size of the adult. Pterodaustro follows this pattern precisely.  We have adults and 1/8 size embryos and several juveniles of intermediate size.

No DGS was employed in this study.

If you know any traditional paleontologists, 
remind them that the data indicates that pterosaurs matured isometrically, like other  lepidosaurs. Those small, short rostrum specimens, principally from the Late Jurassic Solnhofen Formation, are small adults, transitional from larger ancestors to larger descendants. Tiny pterosaurs experiencing phylogenetic miniaturization(as in birds, mammals, crocs, turtles, basal reptiles, and many other clades) that helped their lineage survive while larger forms perished, Sadly, no tiny pterosaurs are known from the Late Cretaceous when they all became extinct.

References
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Codorniú L, Paulina-Carabajal A and Gianechini FA 2015.
 Braincase anatomy of Pterodaustro guinazui, pterodactyloid pterosaur from the Lower Cretaceous of Argentina. Journal of Vertebrate Paleontology, DOI:10.1080/02724634.2015.1031340

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.

 

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