Evolution, 4th Edition

MACROEvOLuTiON: EvOLuTiON AbOvE THE SPECiES LEvEL 517

The origin of mammals

The origin of mammals from earlier amniotes (see F i g u r e 17. 24) is one of the most

fully documented examples of the evolution of a major taxon [51, 64, 98]. Some

features of living mammals, such as hair and mammary glands, do not usually

become fossilized, but the evolution of the skeleton has been well documented,

including changes in the skull and jaw that we describe here.

Soon after the first amniotes originated, during the Carboniferous, they gave

rise to the Synapsida, which developed into diverse mammal-like lineages (FIG-

URE 20.1A). From among these arose the crown Mammalia, containing all the

lineages that descended from the common ancestor of the living mammals: the

Monotremata (egg-laying echidna and platypus), the Metatheria (marsupials),

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.01.ai Date 12-28-2016

Q: I set part label G as “Early mammal (Morganucodon)” to match formatting of other parts. OK?

Thrinaxodon

Procynosuchus

Biarmosuchus

Haptodus

Probainognathus

Sinoconodon

Morganucodon

Hadrocodium

Crown Mammalia

(A)

P Tr J K Pg Ng

Permian Mesozoic Cenozoic

299 252 200 145 66 23 Present

Time (Mya)

(B) Haptodus (early synapsid)

sq

q

(C) Biarmosuchus (therapsid)

sq

q

art

(D) Procynosuchus (early cynodont)

q

art

(E) Thrinaxodon (cynodont)

sq

sq

q

(F) Probainognathus

(advanced cynodont)

art

sq

q

(G) Morganucodon (early mammal)

sq

art

dc

f f f (partial

view)

Single-cusped

teeth Braincase

Multicusped

cheek teeth

d

j

art d d

d

d

d

FIGURE 20.1 A) A phylogeny of a few of the many mammalian (

and related lineages known from the fossil record. (B–G) Skulls

from some early synapsids and early mammals. (B) An early syn-

apsid, Haptodus. Note the temporal fenestra (f), multiple bones

in the lower jaw, single-cusped teeth, and articular/quadrate

(art/q) jaw joint. (C) An early therapsid, Biarmosuchus. Note the

enlarged temporal fenestra. (D) An early cynodont, Procynosu-

chus. The side of the braincase is now vertically oriented, sepa-

rated by a large temporal fenestra from a lateral arch formed by

the jugal (j) and squamosal (sq). Note the enlarged dentary (d).

(E) A later cynodont, Thrinaxodon. Note multiple cusps on the

rear teeth, the large upper and lower canine teeth, and the

greatly enlarged dentary with a vertical extension to which

powerful jaw muscles were attached. (F) An advanced cynodont,

Probainognathus. The cheek teeth had multiple cusps, and two

bones of the lower jaw articulated with the skull. (G) Morganu-

codon, often considered to be a mammal. Note the multicusped

cheek teeth (including inner cusps) and double articulation of the

lower jaw, including articulation of a dentary condyle (dc) with

the squamosal (sq). Abbreviations: art, articular; d, dentary; dc,

dentary condyle; f, fenestra; q, quadrate; sq, squamosal. (A after

[5]; B–G after [28], based on [10] and various sources.)

20_EVOL4E_CH20.indd 517 3/22/17 1:44 PM