Development and evolution of the notarium in Pterosauria: notes and review

Aires et al. 2020
plotted the evolution of the notarium (= fused dorsal vertebrae anchoring the scapulae; Fig. 1) in pterosaurs. “The notarium is the structure formed by fusion of the dorsal vertebrae which occurred independently in pterosaurs and birds. This ankylosis usually involves two to six elements and in many cases, also includes the last cervical vertebra.

I have not seen incorporation of the most proximal of eight cervical vertebra into the notarium of pterosaurs yet. The authors note, “Vertebral fusion can be observed among contiguous centra, neural spines, and transverse processes, sometimes forming a ventral plate.” Not mentioned by the authors, dorsal vertebral fusion can occur independent of scapulocoracoid fusion and/or sacral fusion.

“Fusion can occur in different degrees, uniting the vertebral centra, the neural spines, the transverse processes, the ventral processes, or a combination of these sites. A detailed assessment of the fusion process of pterosaur dorsal vertebrae is still lacking. Here we identify the fusion sequence of pterosaur notarial elements, demonstrating the order of ossification in vertebral bodies and neural spines based on fossils and extant birds. In both Pterosauria and Aves, the notarium generally develops in a antero‐posterior direction, but the actual order of each fusion locus may present slight variations. Based on our data, we were able to identify seven developmental stages in the notarium formation, with broad implications for the prediction of ontogenetic stages for the Pterosauria.”

These ontogenetic implications do not pan out. They are based on an incorrect archosaur model of ontogeny, not the lepidosaur model of pterosaur ontogeny.

“In addition, we report the occurrence of a notarium in Ardeadactylus longicollum (Kimmeridgian, Southern Germany), the oldest occurrence of this structure in pterosaurs.”

That notarium was first published here (Fig. 1) in 2013 when that specimen was known as Pterodactylus longicollum, a taxon that nests in the Pterodactylidae far from the holotype of Ardeodactylus (Fig. 2) nesting with pre-azhdarchids.

Figure 1. Cladogram from Aires et al. 2020 suffering greatly from taxon exclusion to such an extent that unrelated pterosaurs are nested with one another.

Figure 1. Cladogram from Aires et al. 2020 suffering greatly from taxon exclusion to such an extent that unrelated pterosaurs are nested with one another. Black dot = notarium. White dot = no notarium. N = origin of notarium?

A collection of notarium data is good to have.
However the Aires et al. cladogram (21 taxa, Fig. 1) suffers greatly from taxon exclusion when compared to the large pterosaur tree (LPT, 251 taxa, Fig. 2 click to enlarge) where taxa with a notarium are highlighted in yellow.

As the outgroup Aires et al. mistakenly used 
a bipedal crocodylomorph with tiny fingers, Scleromochlus when the validated outgroups have been known for 20 years. This is an ongoing embarrassment for traditional pterosaur workers blindly supporting academic textbooks and lecturers.

Problems in the above illustration
(Fig. 1) begin with a sharp snouted Pteranodon nested with toothy, broad-snouted ornithocheirids (Scaphognathus descendants), rather than the sharp-snouted taxa with crests like tapejarids and dsungaripterids (Germanodactylus descendants). Excluding pertinent taxa in pterosaur cladograms is another ongoing embarrassment, where PhDs have let amateurs take the lead by simply adding taxa irregardless of bias, tradition and academic pressure.

Figure x. LPT. Taxa with notarium in yellow.

Figure 2. LPT. Taxa with notarium in yellow. Click to enlarge.

Oddly the Aires et al. conclusions do not place
a minimum size on notarium development. Principally large pterosaurs (and their juveniles) fuse vertebral spines (Figs. 1, 2) in the LRT.

Germanodactylus
Aires include a specimen of duck-sized  Germanodactylus (BSP 1892) among taxa with a notarium, citing the observations of others as evidence. The dorsal vertebrae in that specimen are exposed ventrally. No sister taxa in the LPT have a notarium, making that second-hand observation questionable for several reasons.

Nyctosaurus
The same holds true for the only Nyctosaurus listed (FHSM VP 2148) by Aires et al. 2020. The potential notarium is buried in the matrix, ventral surface exposed, each centrum distinct from the others. No other Nyctosaurus specimens have a notarium, including the UNSM 93000 specimen, which is exposed dorsally and the vertebrae are scattered. That’s why neither is marked with a notarium in the LPT (Fig. 2).

Diopecephalus = P. longicollum = Ardeadactylus. Normannognathus is in the box in the lower left.

Figure 1. Goose-sized Diopecephalus = P. longicollum = Ardeadactylus. Normannognathus is in the box in the lower left. Santanadactylus GIUA M 4895 at upper right. Note the unfused scapulocoracoid.

I found 16 pterosaur taxa with a notarium
out of 251 taxa in the LPT. One can add UNSM 50036, a postcranial Pteranodon of great size, which is not listed among the skull-only taxa. Several other post-cranial Pteranodon specimens (e.g FHSM VP 2062, UUPI R197) likely had a notarium given the morphology of the proximal scapulae. All are large specimens as adults. Taxa with a notarium nest in five separate clades in the LPT.

Figure 2. Forfexopterus compared to sisters Huanhepterus and Ardeadactylus and the BYU specimen of Mesadactylus.

Figure 2. Forfexopterus compared to sisters Huanhepterus and Ardeadactylus and the BYU specimen of Mesadactylus.

By contrast
some equally large pterosaurs (e.g. Arthurdactylus, Anhanguera) do not have a distinct notarium bar (fused and ossified parasagittal ligaments), but are reported to fuse the complete dorsal series. In the former, the sacrum was likewise fused to the dorsal series. In the later, 3 cervicals were fused to the dorsal series. Neither had a notarial bar (fused and ossified parasagittal ligaments).

Figure 2. FHSM 17956 compared to Ptweety. They are virtually identical, though Ptweety looks like a juvenile of a more robust variety of Pteranodon, thus a younger specimen because adults would be larger.

Figure 3. FHSM 17956 compared to Ptweety. They are virtually identical, though Ptweety looks like a juvenile of a more robust variety of Pteranodon, thus a younger specimen because adults would be larger.

Some inappropriate name changes are present in Aires et al.
Coloborhynchus spielbergi has been renamed Anhanguera spielbergi. Coloborhynchus robustus has been renamed Anhanguera robustus. Not sure why Aires et al. are promoting  this inappropriate lumping.

Figure 1. The new small Pteranodon wing, FHSM 17956, compared to Ptweety and the adult NMC41-358 specimen.

Figure 4. The new small Pteranodon wing, FHSM 17956, compared to Ptweety and the adult NMC41-358 specimen.

The juvenile Pteranodon
unnumbered and privately owned ‘Ptweety‘ also has a notarium, as expected. Pterosaurs, like all tritosaur lepidosaurs, developed isormetrically during ontogeny with phylogenetic ossification patterns traditionally misunderstood by pterosaur workers and their professors.

Figure 2. Gallus, the chicken, nests as a sister to the Early Cretaceous, Eogranivora, also a seed-eater.

Figure 5. Gallus, the chicken, fuses the entire backbone and sacrum, but note the scapula is not involved.

Bottom line
Colleagues, let’s all add taxa to your in-groups and out-groups so conclusions will not be undermined by an improper phylogenetic context. Let’s not cite the questionable observations of others unless confirmable by phylogenetic bracketing. There have been so many false positives published by pterosaur workers unchecked by other pterosaur referees and professors that interested amateurs are able to stand up and point the finger noting errors and omissions. Let’s fix this, together.


References
Aires AS, Reichert LM, Müller RT, Pinheiro FL and Andrade MB 2020. Development and evolution of the notarium in Pterosauria. Jounal of Anatomy. https://doi.org/10.1111/joa.13319