Derived from the basal lizard, Huehuecuetzpalli (Fig. 1), tritosaurs branched off in four distinct directions.
Among these four clades, Cosesaurus and kin appear to have had the larger cranium (red line in Fig. 1), both in length and depth. Why?
The answer is not neotony.
As demonstrated by Huehuecuetzpalli (Fig. 1) and pterosaurs, these taxa grew isometrically. Hatchlings and juveniles did not have larger eyes and a shorter rostrum. Now, with that said, what happened early on inside the egg, likely carried nearly to full term by the mother, is probably a different matter.
Clues to the answer for the bigger cranium and brain
may lie in the coracoids and pelvis of Cosesaurus.
Each of these taxa (Fig. 3) were quadrupeds, but Langobardisaurus and Cosesaurus were facultative bipeds in the manner of living lizards capable of bipedal locomotion. Narrow gauge, digitigrade and occasionally bipedal tracks with pedal digit 5 far behind the others are identified as Rotodactylus (Peabody 1948) tracks and they match the pedes of these two taxa (Peters 2000, Reneto et al. 2002).
The ilium in all four taxa (Fig. 1) are anteroposteriorly long, but more so in Cosesaurus. Such a morphology is associated with bipedal locomotion in various reptiles, like theropod dinosaurs. Bipedal capabilities free the forelimbs to do something other than support the body on the substrate.
In Langobardisaurus the manus remains small without much change from Macrocnemus.
However in Cosesaurus and Jesairosaurus the hand is relatively larger with longer medial metacarpals and longer medial digits. In Cosesaurus the anterior coracoid is eroded away by enlargement of the fenestrations until just the immobile quadrant-shaped posterior rim remains. This is an indicator of flapping, as we discussed earlier. In Cosesaurus the ulna is trailed by filaments (Ellenberger 1993, Peters 2009), the precursors of aktinofibrils in pterosaur wings. In Cosesaurus, such filaments would have only added to its retinue of extradermal decorations, but these could be animated by virtue of flapping. There was also a pteroid on Cosesaurus (Peters 2009, a former centralia, now migrated to the pre-axis of the radius), which in pterosaurs anchors and partially frames a propatagium, which is a flight membrane that also keeps the elbow from overextending.
Flapping, it would appear, was a social, territorial and secondary sexual trait and if so, Cosesaurus likely competed with other cosesaurs. The need for added coordination while a biped, while flapping excitedly to woo a mate, while watching out for competitors, while shrinking in overall size all may have served to increase the relative size of the cranium in Cosesaurus. The higher needs for coordination and social display seem to have supported the enlargement of the brain. At least that’s how it appears from here.
The relatively large size of the cranium is not continued in pterosaurs, which often, but not always (think: anurognathids) have an elongated rostrum to hyper-elongated rostrum (think ctenochasmatids, ornithocheirids and pteranodontids).
In Archaeopteryx and its closest kin there is a similar and convergent expansion of the cranium that is otherwise not expressed in other nonvolant Meszoic bird-like taxa, like oviraptorids and veloceraptorids, but is present in Ichthyornis and living birds.
So, even in this regard, Cosesaurus can be considered “the Archaeopteryx of pterosaurs,” no matter the persistent rumors that such a creature remains unknown to science.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Peabody FE 1948. Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah. University of California Publications, Bulletin of the Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Renesto S, Dalla Vecchia FM and Peters D 2002. Morphological evidence for bipedalism in the Late Triassic Prolacertiform reptile Langobardisaurus. Senckembergiana Lethaea 82(1): 95-106.