Box The Replacement Of Perissodactyls By Artiodactyls

The history of hoofed terrestrial plant-eaters seems to show a replacement of the perissodactyls by the artiodactyls. The late Eocene woodlands of North America and Asia were dominated by early horses, rhinos and brontotheres, and then from the mid-Miocene onwards the camels, pigs and ruminants rose to prominence. Today, there are 79 genera of artiodactyls and only six of perissodactyls. The story of how this happened is often taken as a classic example of competitive ecological replacement on a large scale. The omnivorous suiforms and the ruminating selenodonts were able to sweep away all other plant-eaters in their path (see diagram (a)).

Diagram For Mesozoic Plants

little or no fermentation caecal fermentation

SS^tsome forestomach fermentation full rumination

little or no fermentation caecal fermentation

SS^tsome forestomach fermentation full rumination perissodactyls

2010

perissodactyls

EOCENE I OLIGOCENE I MIOCENE | P P

artiodactyls

EOCENE I OLIGOCENE I MIOCENE | P P

artiodactyls

2010

The supposed replacement of perissodactyls by artiodactyls: (a) phylogeny of the major perissodactyl and artiodactyl groups, showing their relative importance through time and their digestive mode: ruminants dominate today; (b) origination (•) and extinction (■) rates for perissodactyls and artiodactyls throughout the Tertiary show that both groups diversified and declined in tandem. Abbreviation: P-P, Pliocene, Pleistocene. [Figure (a) based on Janis, 1976; (b) after Cifelli, 1981.]

continued

But the statistics do not support this view. Cifelli (1981) found no evidence of a matching decline of one group and a rise in the other. In fact, the patterns of radiation and extinction of both perissodactyls and artiodactyls run more in parallel with each other than in opposition (diagram (b)), and it is likely that each group was evolving independently and responding similarly to a variety of environmental stimuli.

Artiodactyl success is said to have resulted from their superiority to perissodactyls, but such scenarios turn out to be seriously flawed (Janis, 1976). The hindgut digestion of perissodactyls is not inferior to the ruminating foregut digestion of the se-lenodont artiodactyls in all situations, as it is better adapted for coping with highly fibrous fodder. The timing of these events matches the spread of grasslands and the rise in the diversity of grazers, and the long-term reduction in numbers of browsers (Janis etal., 2002).

1999) now confirm the traditional view that the sperm whale is an odontocete.

In all modern whales, the bones of the top of the snout (premaxilla, maxilla, nasal) have moved right back over the top of the skull (Figure 10.34(e,f)), independently and convergently in each clade. This is associ ated with a backwards move of the nostrils to lie above the eyes (the blowhole), an adaptation for breathing at the surface, which has had the effect of telescoping the rest of the skull elements backwards.

The toothed whales radiated in the Miocene and dozens of fossil dolphin-like forms are known (Figure

Sivatherium

Fig. 10.33 The ruminant artiodactyls: (a) the Oligocene Hypertragulus; (b-e) restored heads and horns of (b) the Pliocene giraffe Sivatherium, (c) the modern pronghorn Antilocapra, (d,e) the Miocene pronghorns Ramocerosand Meryceros; (f) the giant Pleistocene sheep Pelorovis; (g) the giant Pleistocene deer Megaloceros. [Figure (a) after, Scott 1940; (b-e) based on various sources; (f, g) after Savage and Long, 1986.]

Fig. 10.33 The ruminant artiodactyls: (a) the Oligocene Hypertragulus; (b-e) restored heads and horns of (b) the Pliocene giraffe Sivatherium, (c) the modern pronghorn Antilocapra, (d,e) the Miocene pronghorns Ramocerosand Meryceros; (f) the giant Pleistocene sheep Pelorovis; (g) the giant Pleistocene deer Megaloceros. [Figure (a) after, Scott 1940; (b-e) based on various sources; (f, g) after Savage and Long, 1986.]

Kentriodon

Fig. 10.34 The whales: (a, b) one of the first whales, Pakicetus from the early Eocene, reconstructed skull in lateral view and tentative life restoration; (c) a whale with limbs, Ambulocetus from the middle Eocene, in walking (top) and swimming (bottom) postures; (d) the first giant whale, Basilosaurus,skeleton, detail of reduced hindlimb and typical triangular-crested tooth; (e) telescoping of the skull elements in a dorsal view of the skull of Kentriodon; (f) skeleton of Kentriodon,a Miocene dolphin. [Figure (a) altered from Gingerich and Russell, 1981; (b) after Savage and Long 1986; (c) modified from Thewissen etal., 1994; (d—f) after Kellogg, 1936; hindlimb from Gingerich etal., 1990.]

Fig. 10.34 The whales: (a, b) one of the first whales, Pakicetus from the early Eocene, reconstructed skull in lateral view and tentative life restoration; (c) a whale with limbs, Ambulocetus from the middle Eocene, in walking (top) and swimming (bottom) postures; (d) the first giant whale, Basilosaurus,skeleton, detail of reduced hindlimb and typical triangular-crested tooth; (e) telescoping of the skull elements in a dorsal view of the skull of Kentriodon; (f) skeleton of Kentriodon,a Miocene dolphin. [Figure (a) altered from Gingerich and Russell, 1981; (b) after Savage and Long 1986; (c) modified from Thewissen etal., 1994; (d—f) after Kellogg, 1936; hindlimb from Gingerich etal., 1990.]

10.34(e, f)), with up to 300 simple pointed, peg-like teeth. The toothed whales show a second advance in developing an echolocation system. The splayed bowl-like nasal region over the snout houses a fatty cushion-like mass that focuses whistles, clicks and squeaks produced in the nasal passages and sends them out as a directed beam of sound. The echoes are picked up in the narrow lower jaw and transmitted through bone to the ear. The mysticetes have lost their teeth and have instead baleen, or whalebone, a modified protein akin to horn, which is used for filtering planktonic organisms out of the sea-water.

0 0

Post a comment