Evolution, 4th Edition

MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 519

Hadrocodium, and later mammals also show sequential steps in enlargement of the

brain, especially in the olfactory bulb and the neocortex [92].

This description touches on only a few highlights of a complex history. For exam-

ple, several of the changes in the lower jaw and middle ear occurred independently

in different cynodont lineages [64]. Nonetheless, the emergence of the class Mammalia

illustrates some themes that are common in the evolution of higher taxa. Most mamma-

lian characters (e.g., posture, tooth differentiation, skull changes associated with jaw

musculature, secondary palate, brain size, reduction of the elements that became the

small bones of the middle ear) evolved gradually. Evolution was mosaic, with different

characters evolving at different rates. No new bones evolved; in fact, many bones have

been lost in the transition to modern mammals [97], and all the bones that persist are

modified from those of the stem amniotes (and in turn, from those of early tetrapods

and even lobe-finned fishes). Some repeated elements, such as teeth, became indi-

vidualized: each molar in the mouth of a human has a distinct identity. Some major

changes in the form of structures are associated with changes in their function. The

most striking example is the articular and quadrate bones, which serve for jaw articu-

lation in all other tetrapods, but became the sound-transmitting middle-ear bones of

mammals [64]. Because the evolution of mammals from synapsids, over the course of

more than 130 My, has been gradual, there is no cutoff point for recognizing mam-

mals: the definition of “Mammalia” in a temporal context is arbitrary.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.02.ai Date 01-11-2017

Ectotympanic

(A) Thrinaxodon (Triassic cynodont)

(D) Homo (adult)

(B) Sinoconodon (Jurassic mammaliaform) (C) Didelphis and Monodelphis (extant mammals)

Stirrup

Hammer

Anvil

Anvil (from quadrate)

Hammer

(from articular)

Stirrup

d-sq joint

Anvil

Hammer

Anvil

d-sq joint

Articular

(hammer)

q-a joint

Anvil (quadrate)

Hammer

(articular)

Dentary

Articular

(hammer)

Squamosal

Stirrup Quadrate (anvil)

sq

Stirrup

FIGURE 20.2 Evolution of the middle-ear bones, the anvil (or

incus) and hammer (or malleus), from the jaw-joint bones (quad-

rate, articular) of cynodonts. The third middle-ear bone, the

stirrup (stapes), was present in ancestral tetrapods. (A) A Triassic

cynodont, Thrinaxodon. Three views, from top: skull, lower jaw,

and a slice through the left jaw joint, viewed from below. Note

the joint formed by the quadrate (q) and articular (a). The lower

jaw includes the dentary, as well as the angular (ectotympanic)

and surangular bones, which became reduced and then lost in

advanced mammals. (B) The same views of the mammal-like Ju-

rassic Sinoconodon, a relative of the late Triassic Morganucodon.

The dentary makes up most of the lower jaw, the other bones

having been reduced or lost. There is a double joint, between

the quadrate and articular, and also between the large dentary

(d) and the squamosal (sq) bone of the skull. (C) The skull and

lower jaw of living marsupials, here the opossums Didelphis

and Mondelphis. The dentary and squamosal form the single jaw

joint (d-sq). The quadrate and articular are now called the anvil

and hammer, two of the middle-ear bones (shown in close-up).

(D) The middle-ear bones in humans. The ectotympanic, or angu-

lar, persisted in marsupials but has been lost in placental mam-

mals. (From [64].)

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