Segmentation in early Xiphosura and the evolution of the thoracetron

Xiphosuran chelicerates, also known as horseshoe crabs, are a long-lived
clade characterized by a highly distinctive morphology and are a classic
example of supposed evolutionary stasis. One key feature of horseshoe
crabs is the fusion of the opisthosomal segments into a single sclerite
referred to as a thoracetron. There has been historical uncertainty as to
whether the thoracetron originated once or multiple times within the
clade. Here, we review criteria for determining whether segments are fused
and apply them to a broad census of taxa for which their ontogeny is known
or the adult status of specimens can be reasonably asserted to explore the
evolution of the thoracetron within a developmental framework. Our
findings indicate that the thoracetron evolved once in the common ancestor
to Xiphosura. However, subsequent independent loss of the thoracetron
segment boundaries is identified and shown to be the result of
heterochronic processes acting upon a shared developmental pathway. The
multiple cases of effacement of the thoracetron within Limuloidea are
cases of peramorphically-driven parallelism, while the effacement of the
thoracetron in the paedomorphic Belinurina is a case of convergence.
Xiphosurids therefore represent an interesting case study for recognizing
parallelism and convergence on the same structure within closely related
lineages. We also demonstrate that somite VII has been incorporated into
the prosoma multiple times within the chelicerate lineage, which has
implications for interpreting the ground pattern of the group.