The origin of sensitive dental structures in the past

Synchrotron Observations of the Coeval Aglaspidid Trigonodal Cuticles: A Comparison with Anatolepis

The lack of comparative data from diverse taxa made it impossible to distinguish between hypotheses on the origin of the odontodes.

High-resolution phase-contrast synchrotron scans of coeval aglaspidid arthropod cuticles—extracted from complete specimens—were compared to Anatolepis. Similar to animal skeletons, the aglaspidids’s exoskeleton is also phosphatic. Aglaspidid cuticles have a homogeneous lamellar structure that is interrupted by vertical pore canals like those described in Anatolepis3 (Fig. 1 and Extended Data Figs. 1–3). Some of these pore canals flare internally into teardrop-shaped cuticular organs, which vary in size through the specimen and are similar to the Anatolepis specimens (Fig. 1c,d and Extended Data Figs. 1e,f and 2h). What’s the location of Aglaspis? In addition to the vertical canals and cuticular organs, we resolve horizontal canals. This feature was not found in all of the scans, but it is not found in thicker cuticular regions such as parts of the tail spine. 3a,d). Virtual sections show partially infilled central cavities similar to the one in Anatolepis. The central tubules flare dorsally with a distinctive arrow-shaped cavity, as seen in Anatolepis, and are capped by a mineralized cone that sits within a pore (Fig. 2g). The tube-in-tube formation that is not shown in vertebrate dentine is surrounded by a hypermineralized layer. The honeycomb structure is hollow because of the diffuse tubules and the merger of the hollow cavities. There are a set of tubula that are peripheral to the central tubuale; they are simpler in morphology than the flared end and mineralized coating. Externally, aglaspidid cuticles have diverse tubercles that vary in size and morphology but are generally rounded, with a corresponding dimple on the underside, the same characters that define Anatolepis (Fig. 1a,e). Microanatomy was revealed when the external, scans and subsequent analysis of aglaspidid cuticle fragments revealed the hallmarks of Anatolepis in having a lamellarbastion with cuticular organs and vertical and horizontal canals. We found that Anatolepis is not a mammal but is an aglaspidid arthropod. As this taxon was the only putative stem-gnathostome with dentine from the Cambrian, this identification pushes the earliest definitive occurrence of the clade 40 million years into the Middle Ordovician6.