Evolution What the Fossils Say and Why it Matters

Spineless Wonders of Evolution 201

to their ancestry (Newell 1959; Fisher 1982, 1984) or related to both the horseshoe crabs and
trilobites (Briggs et al. 1979; Briggs and Fortey 1989), so both trilobites and horseshoe crabs
can be traced back to a fossil that looks like neither. Through the rest of the Paleozoic, there
are additional species of the horseshoe crab lineage (subclass Xiphosura, “sword tail” in
Greek), which develop progressively larger and larger head shields and fewer thoracic seg-
ments and, unique to the group, the long tail spine that gave them their name. Finally, by the
Jurassic we find specimens of Mesolimulus walchi, which are very similar to the living species
Limulus polyphemus, although their thorax is still not as heavily fused together and is much
more spiny than the modern species. Thus xiphosurans haven’t changed much in the past
100 million years, but they changed a lot before then. Plus, there are weird experiments such
as Austrolimulus fletcheri (fig. 8.14B), which has long boomerang-shaped spines on the corners
of its head shield (appropriately, it is from the Triassic of Australia), or Liomesaspis (fig. 8.14C),

FIGURE 8.14. Evolution of the horseshoe crabs. (A) Evolutionary trends within the lineage from the primitive
Cambrian aglaspids through more and more specialized and modern-looking horseshoe crabs. (B) The weird
“boomerang-shaped” Australian horseshoe crab known as Austrolimulus. (C) The peculiar double-button-
shaped horseshoe crab known as Liomesaspis. (Part (A) from Newell 1959; by permission of the American
Philosophical Society; (B and C) courtesy D. Fisher)

(A)

Cambrian

Silurian

Devonian

Carboniferous

Permian

Jurassic

Recent

Limulus

polyphemus

Mesolimulus

walshi

Paleolimulus

avitus

Prestwichinella

rotundala

Belinurus

alleganyensis

Pseudoniscus

roosewelti

Aglaspis

eatoni

(B)

(C)

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