Extreme neck elongation evolved despite strong developmental constraints in bizarre Triassic reptiles – implication for neck modularity in archosaurs

The Triassic radiation of vertebrates saw the emergence of the modern vertebrate groups, as well as numerous extinct animals exhibiting conspicuous, unique anatomical characteristics. Among these, members of Tanystropheidae (Reptilia: Archosauromorpha) displayed cervical vertebral elongation to an extent unparalleled in any other vertebrate. Tanystropheids were exceptionally ecologically diverse and had a wide spatial and temporal distribution. This may have been related to their neck anatomy, yet its evolution and functional properties remain poorly understood. We used geometric morphometrics to capture the intraspecific variation between the vertebrae comprising the cervical column among early archosauromorphs, to trace the evolutionary history of neck elongation in these animals. Our results show that the cervical series of these reptiles can be divided into modules corresponding to those of extant animals. Tanystropheids achieved neck elongation through somite elongation and a shift between cervical and thoracic regions, without presacral vertebrae count increase - contrary to crown archosaurs. This suggests a peculiar developmental constraint that strongly affected the evolution of tanystropheids. The data obtained just at the base of the archosauromorph phylogenetic tree is crucial for further studies on the modularity of vertebral columns of not only Triassic reptile groups but extant and other extinct animals as well.

Methods
This dataset contains all of the untransformed morphometric data used in the study Rytel et al. 2024.

Landmarks (LMs) used in the main (two-dimensional) analysis were digitised onto specimen photographs in the tpsDig v2.31 software in accordance with a landmark set encompassing the outline of the vertebrae in lateral view. The landmarks cover the maximal extent of characteristic osteological features – vertebral centrum, neural spine, pre- and postzygapophysis. These points are considered to be homologous for all of the studied taxa. The chosen landmarks (together with semilandmarks) cover most of the shape of the vertebrae. Moreover, they are (aside from LM5 in some cases) extremely easy to locate, providing high repeatability potential.

Exact definitions of the true landmarks contained within the landmark set are listed below:

1.          Ventral-posterior edge of the centrum

2.          Ventral-anterior edge of the centrum

3.          Point of maximum curvature between prezygapophysis and the anterior edge of the centrum

4.          Dorsal-anterior edge of the prezygapophysis

5.          Point of maximum curvature between prezygapophysis and neural spine

6.          Dorsal-anterior edge of the neural spine

7.          Dorsal-posterior edge of the neural spine

8.          Dorsal-posterior edge of the postzygapophysis

Semilandmark lines encompassed the following number of semilandmarks: 16 ( between LM1 and LM2), 7 (LM2-LM3), 8 (LM3-LM4), 9 (LM4-L5), 6 (LM5-LM6), 14 (LM6-LM7), 13 (LM7-LM8).

Most of the other data are contained within the associated manuscript and the supplementary material.