Chelicerata

Introduction

The Chelicerata are a diverse group of arthropods—though not as diverse as the Mandibulata—named for the presence of chelicerae. Chelicerae, meaning “claw horn” in Greek, are a pair of appendages anterior to (i.e., in front of) the mouth. In different chelicerates, which include spiders, ticks, mites, scorpions (Scorpiones), horseshoe crabs (Xiphosura), sea scorpions (Eurypterida), and sea spiders (Pycnogonida; following Figure 3 in Giribet and Edgecombe, 2019), the chelicerae take different forms. For example, spider chelicerae are often modified into fangs and are attached to venom glands used to subdue prey, whereas the chelicerae of the Xiphosura (i.e., horseshoe crabs) and the extinct eurypterids (i.e., sea scorpions) often take the form of three-segmented pincers, used in feeding (see image below).

Within the Chelicerata are two highly diverse groups, Araneae (i.e., spiders, ~45,000 species) and Acari (i.e., ticks and mites; ~50,000 species), and a variety of other less diverse groups (e.g., Eurypterida, Pycnogonida, Scorpiones, and Xiphosura). The general chelicerate body plan includes two major regions, the prosoma and opisthosoma. The prosoma, which is similar to the cephalothorax region in other arthropod groups, is forwardmost and holds the chelicerae, a pair of appendages called pedipalps, and four pairs of walking legs (see image above). Pedipalps, like chelicerae, can take a variety of forms and fill different functions. In spiders, these structures are often used in reproduction; horseshoe crabs use them for locomotion; and, they form the intimidating pincers of scorpions. The opisthosoma, similar to the abdomen of other arthropod groups, typically contains vital organs, such as the heart and respiratory structures. As with the arthropod group in general, there is a high degree of between and within group variation in the body plans exhibited by the chelicerates.

As described by Giribet and Edgecombe (2019), the relationship between living groups of chelicerates remains uncertain. As seen in the figure above, many chelicerate groups (e.g., Opiliones, Ricinulei, Palpigradi) have undefined branching relationships. Conventionally, pycnogonids (i.e., sea spiders) are considered basal to all other chelicerates (i.e., Euchelicerata), with Xiphosura (i.e., horseshoe crabs) as the next most basal group, splitting from the clade “Arachnida.” This arrangement parsimoniously allows for a single marine to terrestrial transition in the chelicerates. Recent work by Ballesteros and Sharma (2019) has demonstrated, however, that the Xiphosura may in fact be most closely related to the Ricinulei (i.e., hooded tick spiders). This newer grouping presents an interesting scenario, in which the obligatorily marine xiphosurans secondarily evolved their marine lifestyle from a terrestrial ancestry. Though this is not the most parsimonious (i.e., simplest) evolutionary scenario, it does help explain the quasi-terrestrial phase in horseshoe crab reproduction (learn more about horseshoe crabs below). Though there is more confidence in some groupings within the Chelicerata—for example, the tetrapulmonates (i.e., Araneae, Amblypygi, and Thelyphonida) that possess two sets of book lungs—much work remains to resolve the relationships between these clades.

In the fossil record, the first known chelicerate is from the early Cambrian, approximately 514 million years ago. Found in the Emu Bay Shale of Australia and described by Jago et al. (2016), Wisangocaris barbarahardyae (see figure above) is similar to other early chelicerates, including Burgess Shale taxa like Sarotrocercus, Sanctacaris, and Sidneyia (view specimens in The Burgess Shale: Fossil Gallery). As discussed by Jago et al. (2016), these taxa are often placed together in a clade based on their morphological similarities and are considered by some to be in the chelicerate crown-group. In the sections below, a variety of other chelicerate fossils—some from extinct groups and others from extant groups—are introduced.

Acari - Mites and Ticks

Acari, which includes what are commonly called ticks and mites, is an extremely diverse subclass of chelicerates with more than 50,000 living species described. Whether this group is monophyletic remains the subject of debate. As shown in the phylogeny below, Acari is often thought to include the orders Acariformes and Parasitiformes. This recent analysis by Lozano-Fernandez et al. (2019) found strong support for this relationship; however, as evidenced by the more conservative phylogeny above from Giribet and Edgecomb (2019), this viewpoint has not yet been commonly accepted. Colloquially, at least, the grouping is supported by similarities in life modes and morphologies. Species in the Acari are often quite small, with even the largest remaining under an inch in length. Like other chelicerates, ticks and mites have a prosoma and an opisthosoma but these two regions are fused together, giving the appearance that these animals lack segmentation in their body plans. Acari are found in most habitats—including terrestrial, aquatic, and marine habitats—and often parasitize other animals, though there are predatory and detritivorous species.

As reviewed by Sidorchuk (2018), the fossil record of Acari is sparse and is limited to Acariformes during the Paleozoic and most of the Mesozoic. The first Acari in the fossil record is from the Lower Devonian, approximately 410 million years ago. Several other species have been described from the Devonian, including three from an early forest ecosystem of Gilboa, New York (USA). Scattered occurrences have also been reported from the Carboniferous. The record of Acari improves during the Mesozoic, thanks in large part to deposits of amber—fossilized tree resin that often captures small organisms (see an example in the image below). In some cases, the amber fossils are extraordinary, such as the one below showing ticks on a dinosaur feather. During the Mesozoic, extant forms first appeared. More specifically, specimens of the genus Hydrozetes have been reported from 195 million years ago, during the Jurassic. It was not until the Cretaceous, however, that specimens of Parasitiformes first appear, again in amber deposits. With few exceptions, the record of Acari during the Cenozoic is also restricted to amber inclusions.

Araneae - Spiders

Spiders are one of the most diverse orders on the planet, with more than 48,000 described living species. Found in almost all terrestrial habitats, and in many aquatic habitats, spiders are numerically the most common predators on land. Spiders exhibit the two-part body plan common to chelicerates, with a clearly distinguished opisthosoma and prosoma. These regions are connected via a structure called a pedicel. As mentioned above, the chelicerae of spiders are modified into fangs, facilitating the predatory lifestyle common in this order. Differentiating them from other arachnids, spiders possess spinnerets, which are used to spin the silk produced by the silk glands.

The oldest spider known in the fossil record, belonging to the genus Arthrolycosa, is from the Late Carboniferous, approximately 315 million years ago. As discussed by Garwood et al. (2016), this fossil spider is distinguished from other early arachnids by the presence of a spinneret, as other early arachnids (e.g., Idmonarachne brasieri) were very similar but lacked this key feature of the Araneae. By the end of the Paleozoic, one of two extant suborders, the Mesothelae, evolved. Often considered primitive compared to other living spiders, the Mesothelae are represented by only a single family, dwelling exclusively in southeast Asia. The second suborder, Opisthothelae, did not appear until the Triassic. This suborder includes the infraorders Mygalomorphae (e.g., tarantulas) and Araneomorphae (e.g., most familiar spiders like orb, wolf, and crab spiders).

As discussed by Selden and Penney (2017), the fossil record of spiders is relatively sparse, due in large part to the fragile, soft bodies of most spiders. What we know about fossil spiders is derived almost exclusively from Lagerstätten, or deposits with exceptionally well-preserved fossils—the Burgess Shale is a famous example of a Lagerstätten. In the Paleozoic and most of the Mesozoic, these exceptional fossils tend to be encased in sediments. Starting approximately 130 million years ago during the Cretaceous, and from that point onward, most fossil spiders are found in amber, fossilized resin from trees (check out the video above). Many modern spider families are represented in the record from the Eocene onward, suggesting that the Cenozoic spider fauna has remained relatively stable. These families are distinct from those of the Mesozoic and may have evolved earlier than the Eocene; however, amber deposits only become abundant during the Eocene, restricting the potential for study of fossil spiders earlier in the Cenozoic.

Eurypterida - Sea Scorpions

Eurypterids—commonly called “sea scorpions” based on their vaguely similar appearance—were one of the most diverse chelicerate groups during the Paleozoic, with more than 250 described species. The oldest of the eurypterids, recently described by Lamsdell et al. (2015), dates to the Darriwilian (467.3 – 458.4 million years ago) of the Middle Ordovician. Even at this first appearance, the eurypterid, Pentecopterus decorahensis, exhibited derived features, as do other eurypterids from the Middle Ordovician. As discussed by Lamsdell et al. (2015), this evidence suggests that eurypterids either have a longer evolutionary history than is preserved in the fossil record—potentially extending to the Cambrian—or, the clade underwent a rapid diversification after origination during the Ordovician.

Eurypterids reached their peak diversity in the Silurian (see figure below) and, for a time, some species are considered to have been apex predators. These species—belonging to the genera Jaekelopterus and Pterygotus—reached sizes of 2.5 meters and were likely the largest animals in the oceans during the Silurian and beginning of the Devonian. Indeed, Jaelkelopterus, though known from only incomplete specimens, is considered to be the largest arthropod to ever live. Learn more about this giant sea scorpion in the video below.

Large species like Jaekelopterus and stereotypical eurypterids, like Eurypterus (see reconstruction below), belong to the clade Eurypterina, which is one of two major eurypterid clades, along with Stylonurina. The Eurypterina, account for the majority of eurypterid fossils, potentially due to habitat preferences and the broad distribution of some eurypterine species. Indeed, it is the swimming appendage of eurypterines that most readily distinguishes them from the stylonurines, which bear a resemblance to horseshoe crabs in some cases (e.g., Hibbertopterus). In the Eurypterina, the sixth and posterior-most set of appendages form a paddle-like structure used to propel the sea scorpions through the water—compared by some to the swimming limbs of sea turtles. To the contrary, this last set of appendages form limbs for walking in the stylonurines (see reconstructions below).

Though the earliest known eurypterid (i.e., Pentecopterus) belongs to the Eurypterina, stylonurines from the Middle Ordovician have also been described. Both groups diversified during the Silurian before declining in the Devonian. This decline has been attributed to competition with newly evolving jawless fish during the Devonian; however, this explanation has lost support. For example, analysis of large predatory eurypterines by McCoy et al. (2015) showed a range of ecologies, including species specialized for direct pursuit, scavenging, and ambush predation, and also generalist species capable of multiple strategies. Given this ecological range, McCoy et al. (2015) dispelled the notion that competition with fish led to the demise of the eurypterids. Nonetheless, eurypterids declined during the Devonian, with only one eurypterine clade (Adelophthalmoidea) surviving the Late Devonian Mass Depletion (i.e., extinction). The stylonurines fared somewhat better and even underwent a small diversification in the Carboniferous. Ultimately, all eurypterids went extinct by the end of the Permian.

In the traditional view of chelicerate phylogenetics, with pycnogonids the first to branch off, eurypterids and xiphosurans (i.e., horseshoe crabs) are considered to be basal to the remaining groups. These remaining groups, referred to as Arachnida—the monophyly of this clade remains dubious—represent a hypothetical, single terrestrial radiation. As mentioned above, new evidence (i.e., a close relationship between Xiphosura and Ricinulei, with a secondary return to marine habitats by xiphosurans) has cast doubt on this traditional view. Even so, the presence of sclerophores (i.e., structures used for sperm-transfer in reproduction) in both eurypterids and arachnids suggests the basal position of eurypterids, with respect to Arachnida, may be warranted. Citing this structure as evidence, along with the potential that some eurypterids were capable of forays onto land, it has been proposed that land-dwelling chelicerates evolved from eurypterids, or at least a common ancestor of eurypterids and arachnids. Other researchers have suggested multiple independent ocean-to-land transitions, leaving the topic open to debate.

Explore the 3D models below to learn more about Eurypterids!