Evolution, 4th Edition

444 CHAPTER 17

appearing in the late Cambrian, are the earliest fossils with cellular bone, a distinc-

tive feature of vertebrates (FIGURE 17.11B). The earliest definitive vertebrates, also

from the late Cambrian, are ostracoderms, jawless fishlike vertebrates that had bony

armor and lacked paired fins (FIGURE 17.11C) [41].

Especially at its end, the Cambrian was marked by considerable extinction.

For example, many of the more than 90 Cambrian families of trilobites became

extinct. Afterward, many of the animal phyla diversified greatly in the Ordovician

(485–443 Mya), giving rise to many new classes and orders that included new ways

of life. The major large predators were sea stars and nautiloids (shelled cephalo-

pods; that is, molluscs related to squids). The first reefs were built by two groups

of corals, with contributions from sponges, bryozoans, and cyanobacteria. The

Ordovician ended with a mass extinction, perhaps caused by a drop in tem-

perature and a drop in sea level, that in proportional terms may have been the

second largest of all time.

Among the groups that survived this extinction event were the nautiloids,

which gave rise to the ammonoids, shell-bearing cephalopods that are among the

most diverse groups of extinct animals (FIGURE 17.12). During the Silurian (443–

419 Mya), most vertebrates were armored agnathans (jawless vertebrates; FIGURE

17. 13A). The first known gnathostomes, vertebrates with jaws and two pairs of fins,
    Futuyma Kirkpatrick Evolution, 4e appeared at this time (FIGURE 17.13B,C). (How paired appendages first evolved is
    Sinauer Associates
    Troutt Visual Services
    Evolution4e_17.11.ai Date 12-31-2016

Eye

Gill pouches

Notochord and

vertebral elements

Dorsal n

Gut

Esophagus Ventral n Gonads Anus (?)

(A)

(B)

(C)

FIGURE 17.11 Cambrian vertebrates. (A) Photo and drawing of one of the earliest

known vertebrates, Haikouichthys, of the early Cambrian. The drawing calls attention

to features that are characteristic of vertebrates. (B) Bony, toothlike structures of Cam-

brian conodonts. Conodonts were slender, finless chordates believed to be related to

agnathans (jawless vertebrates such as lampreys). (C) Reconstruction of a jawless, limb-

less ostracoderm, Arandaspis, as it may have appeared in life. Note the heavy armor

on the front part of the body. Ostracoderm armor has been found in late Cambrian

rocks. (A courtesy of D.-G. Shu, from [84].)

17_EVOL4E_CH17.indd 444 3/22/17 1:37 PM