Gigantism among Late Jurassic limulids: New ichnological evidence from the Causses Basin (Lozère,France) and comments on body-size evolution among horseshoe crabs

An abundant ichnological material composed of xiphosuran trackways and isolated traces was discovered in Upper Jurassic limestones from the Causses Basin (Causse Méjean, Lozère, France). The morphology of the imprints supports their identification as Kouphichnium isp. In contrast to the most frequent case, the trackways are composed of omnipresent pusher imprints sometime associated with leg traces, but with no telson mark. We argue that this pattern reflects actual surface traces rather than an incomplete set of undertracks. The size distribution of the sampled ichnites is broadly bimodal. This is best explained by sexual dimorphism, a phenomenon frequently observed in modern xiphosurans. Analysis of the trace fossils further suggests that several growth stages are recorded and that the horseshoe crabs were walking in a protected and flat environment like a lagoon. This area, certainly close to a mating ground, was occasionally affected by a continental influence. The biometric study of the tracks suggests a gigantic size for the trackmakers whose body may have reached 84 cm in length. This discovery complements the few reports on other gigantic horseshoe crabs in the Jurassic of Western Europe, thus casting doubt on the postulated increase in body size from the Palaeozoic to the Recent. Furthermore, a literature review shows that there are still major gaps in the record of limulid body-fossils and tracks. Thus, neither of these archives can be taken at face value for quantifying the body-size evolution of horseshoe crabs.     

Complete Mitochondrial Genomes of Carcinoscorpius rotundicauda and Tachypleus tridentatus (Xiphosura,Arthropoda) and Implications for Chelicerate Phylogenetic Studies

Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine chelicerates and have been considered as keystone taxa for the understanding of chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for chelicerates than earlier analyses with a single Atlantic species.     

Behavior of the yolk nuclei during embryogenesis, and development of the midgut diverticulum in the horseshoe crab Limulus polyphemus

Abstract. Classical studies of horseshoe crab development have provided relatively little information about the earliest stages, and the contribution of yolk cells and yolk nuclei—a deficiency due in large part to the difficulty of preparing the eggs and embryos for sectioning. Using newly developed histological resins, we show that the yolk nuclei undergo a series of changes during embryogenesis, before cellularizing and forming the midgut epithelium during the first larval stage. The digestive diverticulum forms in a 2-step process. A mesodermally derived lamina divides the yolk mass into distinct lobes, defining the boundaries of the digestive caeca. The yolk nuclei then cellularize to form the midgut epithelium.     

A New Three-Dimensionally Preserved Xiphosuran Chelicerate from the Montceau-Les-Mines Lagerstätte (Carboniferous, France)

Excellent three dimensional preservation of 142 specimens of Alanops magnificus gen. et sp. nov. (Chelicerata: Xiphosura) from the Stephanian Konservat-Lagerstätte of Montceau-les-Mines (Saône-et-Loire, France), exposes the carapace design and hitherto unrecorded details of fossil xiphosuran ventral anatomy, and makes possible an interpretation of appendicular functional morphology. All legs are long, slender and chelate. The chelate condition of the fifth leg and the shape of the prosoma (highly vaulted with a subvertical frontal area) indicate that the animal was neither a burrower like Recent xiphosurans (e.g. Limulus ) nor an active swimmer as suggested for other extinct forms, but was more likely to have been a benthic crawler. The ability to fold up is attested by both partly enrolled specimens showing appendages withdrawn into the prosomal cavity, and by coaptive devices on the external and internal margins of both prosoma and opisthosoma. The low-level articulation system at the boundary between the two shields allowed both the carapace closure and the complete outstretching of the animal. The unusually small adult size of Alanops magnificus gen. et sp. nov. combined with the loss of ophthalmic ridges and spines is interpreted as indicating a paedomorphic derivation from a bellinurine stock. The depositional environment (limnic intramontane basin) and faunal association (dominated by syncarids and bivalves) of Alanops indicate that the animal probably lived in fresh water.     

Xiphosuran Digging Traces at the Late Carboniferous Joggins Fossil Cliffs UNESCO World Heritage Site,Nova Scotia,Canada

One hundred Selenichnites rossendalensis (excavations), sometimes with associated trackways, have been found at Joggins Fossil Cliffs (Nova Scotia, Canada; Pennsylvanian). The majority of the Selenichnites found in this study contain only one lunate cast and some are distinguished by linear ridges that project behind a single lunate cast. A detailed investigation of the morphology and orientation of these Selenichnites—attributable to xiphosurans, relatives of modern horseshoe crabs (Limulus polyphemus)—yields an understanding of xiphosuran digging behavior. Statistical analysis of 96 lunate casts from two populations of Selenichnites separated by several hundred thousand years shows, with 99% confidence, that the prosomas (heads) of the younger population are smaller. Further study of the wealth of Selenichnites at Joggins will be useful to determine which factors control xiphosuran body-size changes and the timescale on which those changes can be seen manifest in a population. The smallest known Selenichnites (7 mm wide) is also reported.     

Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura

The monophyly of the class Xiphosura is critically re‐examined. For the first time a phylogenetic analysis of a number of synziphosurine and xiphosurid taxa is performed together with representatives of the other chelicerate orders also included as ingroup taxa. Xiphosura as currently defined is shown to be paraphyletic, and a revised classification is presented. Previous characteristics used to unite the xiphosurids (possessing a fused thoracetron) and a paraphyletic grade of synziphosurines (retaining freely articulating opisthosomal tergites) include the presence of a cardiac lobe, ophthalmic ridges, an axial region of the opisthosoma, and a reduced first opisthosomal segment. All of these characteristics are, however, here shown to be present in other chelicerate groups, leaving Xiphosura without any defining synapomorphies. A number of other characters, including the form of the chelicerae and appendage VII, indicate that xiphosurans may be paraphyletic with respect to a clade consisting of chasmataspidids, eurypterids, and arachnids. What ramifications this has for the evolution of basal chelicerates is briefly discussed, and it is recognized that most of the currently known ‘synziphosurine’ taxa represent offshoots from the main chelicerate lineage with ghost ranges extending into at least the Middle Ordovician.