A reappraisal of Paleozoic horseshoe crabs from Russia and Ukraine

Xiphosurida is an ingroup of marine Euchelicerata often referred to as “living fossils”. However, this oxymoronic term is inapplicable for Paleozoic and early Mesozoic forms, as during these periods the group experienced notable evolutionary radiations; particularly the diverse late Palaeozoic clade Belinurina. Despite the iconic nature of the group, select species in this clade have been left undescribed in the light of recent geometric morphometric and phylogenetic considerations and methodologies. To this end, we re-describe Prolimulus woodwardi Fritsch, 1899 using new and type specimens to reveal more details on appendage anatomy and possible ecology. Furthermore, we present geometric morphometric and phylogenetic analyses that uncover relationships between P. woodwardi and other belinurids without genal spines. Both approaches highlight that a clade containing Prolimulus Fritsch, 1899, Liomesaspis Raymond, 1944, Alanops Racheboeuf, Vannier & Anderson, 2002 and Stilpnocephalus Selden, Simonetto & Marsiglio, 2019 may exist. While we do not erect a new group to contain these genera, we note that these genera exemplify the extreme limits of the Belinurina radiation and a peak in horseshoe crab diversity and disparity. This evidence also illustrates how changes in heterochronic timing are a key evolutionary phenomenon that can drive radiations among animals.

Section: Phylogenetic Resultsmentioning

confidence: 68%

Section: Introductionmentioning

confidence: 99%

A revision ofProlimulus woodwardiFritsch, 1899 with comparison to other highly paedomorphic belinurids

Lustri

Laibl

Bicknell

2021

“…Included genera . Batracholimulus gen. nov.; Boeotiaspis gen. nov.; Dubbolimulus Pickett, 1984 ; Limulitella Størmer, 1952 [= Limulites Schimper, 1853 ]; Panduralimulus Allen & Feldmann, 2005 ; Psammolimulus Lange, 1922 ; Shpineviolimulus Bicknell, Naugolnykh & Brougham, 2020 ; Tasmaniolimulus Bicknell, 2019 ; Vaderlimulus Lerner, Lucas & Lockley, 2017 .…”

Section: Resultsmentioning

confidence: 99%

The phylogeny and systematics of Xiphosura

Lamsdell

2020

Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed ‘living fossils’, recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura—set out in the Treatise of Invertebrate Paleontology in 1955—to be outdated and in need of revision, with several common genera such as Paleolimulus Dunbar, 1923 and Limulitella Størmer, 1952 acting as wastebasket taxa. Here, an expanded xiphosuran phylogeny is presented, comprising 58 xiphosuran species as part of a 158 taxon chelicerate matrix coded for 259 characters. Analysing the matrix under both Bayesian inference and parsimony optimisation criteria retrieves a concordant tree topology that forms the basis of a genus-level systematic revision of xiphosuran taxonomy. The genera Euproops Meek, 1867, Belinurus König, 1820, Paleolimulus, Limulitella, and Limulus are demonstrated to be non-monophyletic and the previously synonymized genera Koenigiella Raymond, 1944 and Prestwichianella Cockerell, 1905 are shown to be valid. In addition, nine new genera (Andersoniella gen. nov., Macrobelinurus gen. nov., and Parabelinurus gen. nov. in Belinurina; Norilimulus gen. nov. in Paleolimulidae; Batracholimulus gen. nov. and Boeotiaspis gen. nov. in Austrolimulidae; and Allolimulus gen. nov., Keuperlimulus gen. nov., and Volanalimulus gen. nov. in Limulidae) are erected to accommodate xiphosuran species not encompassed by existing genera. One new species, Volanalimulus madagascarensis gen. et sp. nov., is also described. Three putative xiphosuran genera—Elleria Raymond, 1944, Archeolimulus Chlupáč, 1963, and Drabovaspis Chlupáč, 1963—are determined to be non-xiphosuran arthropods and as such are removed from Xiphosura. The priority of Belinurus König, 1820 over Bellinurus Pictet, 1846 is also confirmed. This work is critical for facilitating the study of the xiphosuran fossil record and is the first step in resolving longstanding questions regarding the geographic distribution of the modern horseshoe crab species and whether they truly represent ‘living fossils’. Understanding the long evolutionary history of Xiphosura is vital for interpreting how the modern species may respond to environmental change and in guiding conservation efforts.

“…When describing the material, we followed the systematic taxonomy of Bicknell & Pates (2020) and Bicknell et al (2021) and used anatomical terms presented in Lerner, Lucas & Lockley (2017) , Bicknell (2019) , Bicknell, Naugolnykh & Brougham (2020) , and Bicknell et al (2021) .…”

Section: Methodsmentioning

confidence: 99%

“… Note: Taxa are order by family and then alphabetically by genus and species. Temporal data taken from Bicknell & Pates (2020) , Bicknell, Naugolnykh & Brougham (2020) , Bicknell et al (2021) and Bicknell, Hecker & Heyng (2021) ). Note the uncertain placement of Limulitella in Austrolimulidae, and Yunnanolimulus henkeli .…”

Section: Introductionmentioning

confidence: 99%

New austrolimulid from Russia supports role of Early Triassic horseshoe crabs as opportunistic taxa

Bicknell

Shcherbakov

2021

Horseshoe crabs are extant marine euchelicerates that have a fossil record extending well into the Palaeozoic. Extreme xiphosurid morphologies arose during this evolutionary history. These forms often reflected the occupation of freshwater or marginal conditions. This is particularly the case for Austrolimulidae—a xiphosurid family that has recently been subject to thorough taxonomic examination. Expanding the austrolimulid record, we present new material from the Olenekian-aged Petropavlovka Formation in European Russia and assign this material to Attenborolimulus superspinosus gen. et sp. nov. A geometric morphometric analysis of 23 horseshoe crab genera illustrates that the new taxon is distinct from limulid and paleolimulid morphologies, supporting the assignment within Austrolimulidae. In considering Triassic austrolimulids, we suggest that the hypertrophy or reduction in exoskeletal sections illustrate how species within the family evolved as opportunistic taxa after the end-Permian extinction.