Literature

The term "bradytely" has been coined to describe the very slow morphological changes associated with an evolving lineage. One familiar example is the brachiopod Lingula, with living specimens closely matching fossil forms. Another is Limulus, the horseshoe crab. Although the living animals are significantly larger than the fossil forms, they all have the same distinctive body plan.

The North American horseshoe crab, Limulus polyphemus
Image credit: Florida Fish and Wildlife Conservation Commission

It will not be a surprise to be told that there have been numerous research publications on the evolutionary origins of the horseshoe crabs (Family: Limulidae, Class: Xiphosura). It is customary for researchers to identify stem group species and crown group species as they seek to find patterns in the fossil material. This approach has been followed within the class Xiphosura, with a group known as the synziphosurines considered as stem lineage animals living during the Silurian-Carboniferous Periods.
All this work is thrown into the melting-pot by the discovery of Lunataspis aurora, "the oldest horseshoe crab", from Late Ordovician deposits in Canada. This animal is essentially modern in appearance and it predates the synziphosurines. The authors write:

"In most discernible characters, Lunataspis appears to be much closer to certain Late Palaeozoic Paleolimulidae (Xiphosurida) than to any described synziphosurine, and is in fact more limuline in external morphology than many of the bizarre and clearly specialized Carboniferous xiphosurids, such as the Euproopidae. The presence of a presumed horseshoe crab of such modern appearance in Late Ordovician shallow subtidal to intertidal depositional settings has broader potential for constraining other critical stages in euchelicerate phylogeny [. . .]"

The implications of the find are apparent in the short quote above. Organisms thought to be stem group species are either unrelated to the lineage or are specialised, not primitive. This rethinking of the Xiphosura is said by the authors to be "a task well beyond the scope of this brief account". Indeed, but this discovery will fuel much discussion in the future. One wonders whether it will stimulate a rethink about the stem/crown conceptual model of lineage evolution? As an alternative, consider this approach based on stasis rather than darwinian transformation. Organisms can be considered as reservoirs of type-specific biological information at the outset, and their subsequent history can result in various permutations of that biological information (e.g. niche specialisation) or to loss (degenerate forms). This appears to be just as testable as other approaches and, in the case of the limulids, a very nice fit to the data.

The oldest horseshoe crab: a new xiphosurid from Late Ordovician Konservat-Lagerstatten deposits, Manitoba, Canada.
Rudkin, D.M., G.A. Young, and G.S. Nowlan.
Palaeontology, 51(1), 1-9, Jan 2008 | doi: 10.1111/j.1475-4983.2007.00746.x

Abstract: A remarkable new fossil horseshoe crab, Lunataspis aurora gen. et sp. nov., from recently discovered Upper Ordovician (c. 445 Ma) shallow marine Konservat-Lagerstatten deposits in Manitoba (Canada), is characterized by fusion of opisthosomal tergites into two sclerites. A broad mesosoma of six or seven fused segments, followed by a narrow metasoma of three reduced segments, represents an advanced transitional condition in the development of the xiphosurid thoracetron. Lunataspis further possesses a large crescentic prosomal shield bearing lateral compound eyes on weak ophthalmic ridges that flank a low cardiac lobe, and a keeled lanceolate telson. Lunataspis is much older than the proposed 'synziphosurine' stem lineage of Carboniferous and post-Palaeozoic Xiphosurida, yet is strikingly similar to crown group limuline horseshoe crabs, indicating that major features of the distinctive and highly conserved xiphosurid Bauplan evolved considerably earlier in the Palaeozoic than was previously suspected. In addition to establishing a new temporal benchmark for assessing hypotheses of early chelicerate relationships, the discovery of horseshoe crabs in a Late Ordovician marginal marine setting marks the earliest definitive record of this persistent ecological association.