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Pathological Trilobites

Figure 14.14 Pathological trilobites: (a) Onnia superba - the fringe in the lower part of the photograph has an indentation and a smooth area, probably regeneration following an injury during molting (x4); (b) Autoloxolichas - the deformed segments on the left-hand side may be either genetic or the result of repair following injury (x3); and (c) Sphaerexochus - only two ribs are developed on the right-hand side, probably a genetic abnormality (x25). (Courtesy of Alan Owen.)

Figure 14.14 Pathological trilobites: (a) Onnia superba - the fringe in the lower part of the photograph has an indentation and a smooth area, probably regeneration following an injury during molting (x4); (b) Autoloxolichas - the deformed segments on the left-hand side may be either genetic or the result of repair following injury (x3); and (c) Sphaerexochus - only two ribs are developed on the right-hand side, probably a genetic abnormality (x25). (Courtesy of Alan Owen.)

Figure 14.15 Chelicerate morphology displaying features of (a) dorsal and (b) ventral surfaces. (Based on McKinney 1991.)

Limulus, together with the extinct sea scorpions, the eurypterids, are also members of the subphylum, defined in terms of a prosoma (head and thorax) with six segments (bearing appendages), an opisthosoma (abdomen) with at most 12 segments, and a pair of chelicerae (pincers) attached to the first segment of the prosoma (Fig. 14.15).

There have traditionally been two main chelicerate groups: (i) the merostomes, including the aquatic horseshoe crab Limulus and the giant sea scorpions or eurypterids; and (ii) the arachnids that mainly comprise the terrestrial spiders and scorpions. But this traditional split of the subphylum into marine meristomes and non-marine arachnids has been challenged; both groups probably had marine and non-marine representatives. The bizarre Sanctacaris from the Middle Cambrian Burgess Shale may be a basal outgroup to the clade Chelicerata, whereas the so-called "great appendage" arthropods such as Emer-aldella and Sidneyia together with the aglas-pids belong within the clade (Cotton & Braddy 2004).

The xiphosures, or horseshoe crabs, have a relatively large, convex prosoma, approximately equal in length to the opisthosoma, which usually contains less than 10 segments. The telson, or tail spine, is commonly long

Parastylonurus

Figure 14.16 Eurypterid functional morphology showing (a) swimming and (b, c) walking life modes. (From Clarkson 1998.)

and spiny, and the ophthalmic ridges and cardiac lobe are usually well preserved. Although some enigmatic xiphosure-like taxa, such as Eolimulus, have been described from the Lower Cambrian, the first are probably of Early Ordovician age. A trend towards larger size and a shorter fused abdomen is seen in most groups. Carboniferous taxa, for example Belinurus and Euproops, have well-developed cardiac lobes and ophthalmic ridges together with fused abdomen. Mesolimulus from the Upper Jurassic Solnhofen Limestone, however, is smaller than living taxa but it too was marine and left clear evidence of its appendages in a trackway in the lagoonal muds of Bavaria.

The eurypterids include the largest known and most scary of the arthropods, some approaching 2 m in length, that range in age from Ordovician to Permian. They occupied a variety of environments from marine to freshwater and some may have been amphibious. Much of our knowledge of eurypterid morphology has been derived from superbly preserved specimens from Silurian dolomites on the island of Saaremaa that were acid-etched from the rock by the Swedish paleontologist Gerhard Holm at the end of the 1800s. The exoskeleton is long and relatively narrow. The subrectangular prosoma bears a variety of appendages; the first pair of appendages were chelicerae adapted for grasping while others were modified for movement, with the last pair of large, paddle-like append ages probably adapted for swimming. The opisthosoma, comprising the pre- and postabdomen, consists of 12 visible segments. The telson was variably developed as a long spine or a flattened paddle.

With the exception of generalist feeders such as Baltoeurypterus, most eurypterids were predators, attacking fishes and other arthropods. Moreover, where relatively common, a number of eurypterid-dominated communities have been described, emphasizing the range of habitats occupied by these large, versatile animals. In the Silurian rocks of the Anglo-Welsh area, Pterygotus and its allies are associated with normal marine faunas, whereas Eurypterus itself preferred inshore environments. Hughmilleria and related forms dominated brackish to freshwater communities.

Many more than 50 genera of eurypterids have been described. The group was most abundant during the Silurian and Devonian, but only two families, the adeloopthalmids and the hibbertopterids, survived into the Permian. The varied styles of locomotion of the group suggest a diversity in lifestyles (Fig. 14.16).

The arachnids are a huge group of terrestrial carnivores containing mites, scorpions, spiders (Box 14.5) and ticks. There are probably over 100,000 known species of arachnids. The prosoma consists of six segments with a pair of chelicerae, a pair of sensory or feeding pedipalps and four pairs of walking

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