Silurian horseshoe crab illuminates the evolution of arthropod limbs

Abstract

The basic arrangement of limbs in euarthropods consists of a uniramous head appendage followed by a series of biramous appendages. The body is divided into functional units or tagmata which are usually distinguished by further differentiation of the limbs. The living horseshoe crabs are remnants of a much larger diversity of aquatic chelicerates. The limbs of the anterior and posterior divisions of the body of living horseshoe crabs differ in the loss of the outer and inner ramus, respectively, of an ancestral biramous limb. Here we report a new fossil horseshoe crab from the midSilurian Lagerstätte in Herefordshire, United Kingdom (approximately 425 Myr B.P.), a site that has yielded a remarkably preserved assemblage of soft-bodied fossils. The limbs of the new form can be homologized with those of living Limulus, but retain an ancestral biramous morphology. Remarkably, however, the two limb branches originate separately, providing fossil evidence to suggest that repression or loss of gene expression might have given rise to the appendage morphology of Limulus. Both branches of the prosomal limbs of this new fossil are robust and segmented in contrast to their morphology in Cambrian arthropods, revealing that a true biramous limb was once present in chelicerates as well as in the mandibulates.Xiphosurida | origin of limb morphology | Herefordshire Lagerstätte

Ancestral whole-genome duplication in the marine chelicerate horseshoe crabs

“…With the exception of details in abdominal segment and biramous limb morphology (Briggs et al, 2012) living horseshoe crabs, such as Carcinoscorpius rotundicauda (Figure 2a), are remarkably similar in appearance to fossil species 400 million years in age, and almost indistinguishable to some found in the Jurassic (Briggs et al, 2005). In addition, very limited diversity has been catalogued in the Xiphosura across geological time (Störmer 1952;Störmer 1955;Anderson and Selden, 1997).…”

Chelicerata

“…5.2 ;Snodgrass 1938 ). This putative homology is supported by the discovery of fossil synziphosurines with bona fi de exopods on the pedipalps and all walking leg segments, suggesting that the fl abellum is a vestige of the posterior-most exopod pair (Briggs et al 2012 ). Other sensory organs include two ocelli on the carapace and two lateral compound eyes.…”

“…Indeed, Damen et al ( 2002 ) observed stronger expression of pdm / nub throughout the developing legs (endopods) of Cupiennius salei than had been observed in insect or crustacean legs, which questions the utility of this marker for discerning appendage rami in arachnids based on strength of expression level alone. While expression of one of the two spider ap paralogs seems to be consistent with the position of vestigial epipods ( ap -1 is expressed dorsally to the walking legs in later stages of Cupiennius salei ), the fossil record of chelicerates reveals that biramous chelicerates bore exopods in this part of the body, not epipods (Boxshall 2004 ;Briggs et al 2012 ). Together with documented homoplasy of certain genes' expression patterns (Janssen et al 2011 ;Sharma et al 2014c ), these results indicate that the exact serial homology between the respiratory organs and prosomal appendages of chelicerates is not suffi ciently clear at present.…”

Segmentation and tagmosis in Chelicerata

“…23 mm long fossil looks like a small horseshoe crab and was formally described within Xiphosura. According to Briggs et al (2012) the prosomal dorsal shield of D. durgae lacks any evidence for eyes and appears to be associated with somites IeVI. The appendages of somite I are a uniramous pair of chelicerae.…”