Page 390 - Introduction to Paleobiology and The Fossil Record

ECDYSOZOA: ARTHROPODS 377

(a) (b) (c)

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 (×4);
(b) Autoloxolichas – the deformed segments on the left-hand side may be either genetic or the result of
repair following injury (×3); and (c) Sphaerexochus – only two ribs are developed on the right-hand
side, probably a genetic abnormality (×25). (Courtesy of Alan Owen.)

chelicerae
pedipalp
ocelli walking legs mouth
prosoma compound walking leg metastoma
eye
for swimming
paddle
preabdomen
tergite genital
appendage
last gill
appendage
postabdomen sternite

pretelson
telson
telson

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

Limulus, together with the extinct sea scorpi- meristomes and non-marine arachnids has
ons, the eurypterids, are also members of the been challenged; both groups probably had
subphylum, defined in terms of a prosoma marine and non-marine representatives. The

(head and thorax) with six segments (bearing bizarre Sanctacaris from the Middle Cam-
appendages), an opisthosoma (abdomen) with brian Burgess Shale may be a basal outgroup
at most 12 segments, and a pair of chelicerae to the clade Chelicerata, whereas the so-called
(pincers) attached to the first segment of the “great appendage” arthropods such as Emer-

prosoma (Fig. 14.15). aldella and Sidneyia together with the aglas-
There have traditionally been two main pids belong within the clade (Cotton & Braddy
chelicerate groups: (i) the merostomes, includ- 2004).
ing the aquatic horseshoe crab Limulus and The xiphosures, or horseshoe crabs, have a
the giant sea scorpions or eurypterids; and relatively large, convex prosoma, approxi-
(ii) the arachnids that mainly comprise the mately equal in length to the opisthosoma,
terrestrial spiders and scorpions. But this which usually contains less than 10 segments.
traditional split of the subphylum into marine The telson, or tail spine, is commonly long