Pterosaur tooth scratches and diets

Bestwick et al. 2020 wrote:
“Pterosaurs, the first vertebrates to evolve active flight, lived between 210 and 66 million years ago. They were important components of Mesozoic ecosystems, and reconstructing pterosaur diets is vital for understanding their origins, their roles within Mesozoic food webs and the impact of other flying vertebrates (i.e. birds) on their evolution.

Vital? Is that what they call ‘hyperbole?’ For their outgroup, the authors employ the basal bipedal crocodylomorph with tiny hands and no toe 5, Scleromochlus, so… so far tooth scratches are not proving ‘vital for understanding their origins.‘ They ignored citations, scratches and common sense. Not a good start.

“However, pterosaur dietary hypotheses are poorly constrained as most rely on morphological-functional analogies. Here we constrain the diets of 17 pterosaur genera by applying dental micro wear texture analysis to the three-dimensional sub-micrometre scale tooth textures that formed during food consumption. We reveal broad patterns of dietary diversity (e.g. Dimorphodon as a vertebrate consumer; Austriadactylus as a consumer of ‘hard’ invertebrates) and direct evidence of sympatric niche partitioning (Rhamphorhynchus as a piscivore; Pterodactylus as a generalist invertebrate consumer).

That’s refreshing! Delivering results in an abstract. Unfortunately, there’s nothing new here and Nature papers usually break new ground.

“We propose that the ancestral pterosaur diet was dominated by invertebrates and later pterosaurs evolved into piscivores and carnivores, shifts that might reflect ecological displacements due to pterosaur-bird competition.”

Again, nothing new here.

The authors downplay fossilized stomach contents 
due to their limited preservation, so they put greater emphasis on the scratched enamel of pterosaur teeth. Comparisons are made to extant reptile tooth scratches from crocs and monitor lizards. Iguanids are not mentioned. The word ‘arboreal’ is likewise not found in the text.

Those tooth scratches are rather indistinct.
Odd that the authors downplay stomach contents in pterosaurs (based on their rarity) given the headline of their paper. So-called toothless pterosaurs are ignored despite the fact that the tips of beaks are teeth (Fig. 1). Oddly, so are dsungaripterids (Fig. 1) and ctenochasmatids, both of which have marginal teeth.

Juvenile diets are not mentioned appropriately
Rather phylogenetically miniaturized adult basal Rhamphorhynchus specimens are considered juveniles, forgetting the fact that all pterosaurs mature isometrically, as demonstrated by Pterodaustro (Fig. 2) and Zhejiangopterus ontogenetic series. We also have one juvenile Rhamphorhynchus, identical to larger adult.

References
Bestwick J, Unwin DM, Butler RJ and Purnell MA 2020. Dietary diversity and evolution of the earliest flying vertebrates revealed by dental micro wear texture analysis. Nature Communications https://doi.org/10.1038/s41467-020-19022-2

Bestwick J, Unwin DM, Butler RJ, Henderson DM and Purnell MA 2018. Pterosaur dietary hypotheses: a review of ideas and approaches. Biological Reviews https://doi.org/10.1111/brv.12431