The impact of allometry on vomer shape and its implications for the taxonomy and cranial kinesis of crown-group birds

Olivia Plateau,Christian Foth

doi:10.24072/pcjournal.19

Abstract

Crown birds are subdivided into two main groups, Palaeognathae and Neognathae, that can be distinguished, among others, by the organization of the bones in their pterygoid-palatine complex (PPC). Shape variation to the vomer, which is the most anterior part of the PPC, was recently analysed by Hu et al. (2019) with help of geometric morphometrics to discover morphological differences between palaeognath and neognath birds. Based on this study, the vomer was identified as sufficient to distinguish the two main groups (and even more inclusive neognath groups) and their cranial kinetic system. As there are notable size differences between the skulls of palaeognaths and neognaths, we here investigate the impact of allometry on vomeral shape and its implication for taxonomic classification by re-analysing the data of the previous study. Different types of multivariate statistical analyses reveal that taxonomic identification based on vomeral shape is strongly impaired by allometry, as the error of correct identification is high when shape data is corrected for size. This finding is evident by a great overlap between palaeognath and neognath subclades in morphospace. The correct identification is further influenced by the convergent presence of a flattened vomeral morphotype in multiple neognath subclades. As the evolution of cranial kinesis has been linked to vomeral shape in the original study, the existing correlation between shape and size of the vomer across different bird groups found in the present study questions this conclusion. In fact, cranial kinesis in crown birds results from the loss of the jugal-postorbital bar in the temporal region and ectopterygoid in the PPC and the combination of a mobilized quadrate-zygomatic arch complex and a flexible PPC. Therefore, we can conclude that the vomer itself is not a suitable proxy for exploring the evolution of cranial kinesis in crown birds and their ancestors.

Highlights

The pterygoid-palatine complex (PPC) of crown birds is mainly formed by five bones: the unpaired vomer that results from the fusion of the originally paired vomer elements, and the paired pterygoids and palatines
According to Zusi (1984), the kinetic system can be differentiated into three main types: (1) prokinesis describes the rotation of the whole beak around the nasal-frontal hinge; (2) amphikinesis is derived from prokinesis, including the rotation of the beak around the nasalfrontal hinge plus an additional flexion of the anterior portion of the beak; and (3) rhyn-chokinesis, which in contrast includes a simple flexion of the beak around one or several bending zones rostral to the nasal-frontal suture, lacking a true hinge
Depending on the position of the bending zones, rhynchokinesis can be further differentiated into five subtypes (Zusi, 1984)

Summary

Introduction

The pterygoid-palatine complex (PPC) of crown birds is mainly formed by five bones: the unpaired vomer that results from the fusion of the originally paired vomer elements, and the paired pterygoids and palatines. The pterygoids and palatines are highly connected, forming a rigid unit that articulates with the braincase via well-developed basipterygoid processes, while a contact with the parasphenoid is not present (see Bellairs and Jenkin, 1960; Zusi, 1993; Gussekloo et al, 2001; Mayr, 2017; Fig. 1A). The vomer is highly variable in size and shape and often has no connection with the upper jaw beyond an association with the nasal septum and the palatine. The vomer is greatly reduced or even absent (see Bellairs and Jenkin, 1960; Bock, 1964; Zusi, 1993; Mayr, 2017; Fig. 1A)

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Results

Conclusion