Evolution of giraffes
http://www.talkorigins.org/faqs/faq-transitional/part2c.html
Artiodactyls (cloven-hoofed animals)
\\\"The early evolution of the artiodactyls is fairly well documented by both the dentition and the skeletal material and provides the basis for fairly detailed analysis of evolutionary patterns....the origin of nearly all the recognized families can be traced to the late Middle Eocene or the Upper Eocene...\\\" (Carroll, 1988)
* Chriacus (early Paleocene) -- A primitive oxyclaenid condylarth from the Lower Paleocene. Has many tooth features linking it to later Diacodexis; but in all other ways, including the legs, it was an unspecialized condylarth.
GAP: No artiodactyl fossils known from the late Paleocene. Similar late Paleocene gaps in rodents, lagomorphs, and perissodactyls are currently being filled with newly discovered Asian fossils, so apparently much late Paleocene herbivore evolution occurred in central Asia. Perhaps the new Asian expeditions will find Paleocene artiodactyl fossils too. At any rate, somewhere between Chriacus & Diacodexis, the hind leg changed, particularly the ankle, to allow smooth running.
* Diacodexis (early Eocene) -- A rabbit-sized with longer limbs than the condylarths. The fibula was reduced to a splint, and in some (but not all!) individuals, fused partially to the tibia. Artiodactyl-like \\\"double pulley\\\" ankle (because of this feature, Diacodexis is automatically classified as the first artiodactyl). The feet were very elongated, and the 3rd and 4th toes bore the most weight. Many primitive, non-artiodactyl features retained: collarbone, unfused ulna, primitive femur, unfused foot bones with all 5 toes, could still spread hind limb out to the side, very primitive skull & teeth (all teeth present, no gaps, simple cusps). In fact, in most ways, Diacodexis is just a leggy condylarth. Only the ankle shows that it was in fact the ancestor of all our modern cloven-hoofed animals (possible exception: the hippos & pigs may have split off earlier). There are abundant species-to- species transitions linking Diacodexis to various artiodactyl familes (see below).
Hippos & pigs:
* Helohyus or a similar helohyid (mid-Eocene) -- Primitive artiodactyl, larger than Diacodexis but with relatively shorter & stouter limbs, with bulbous cusps on the molars.
* Anthracotherium and later anthracotheriids (late Eocene) -- A group of heavy artiodactyls that started out dog-size and increased to be hippo-size. Later species became amphibious with hippo-like teeth. Led to the modern hippos in the early Miocene, 18 Ma.
* Propalaeochoerus or a similar cebochoerid/choeropotamid (late Eocene) -- Primitive piglike artiodactyls derived from the helohyids (see above).
* Perchoerus (early Oligocene) -- The first known peccary.
* Paleochoerus (early Oligocene, 38 Ma) -- First known true pig, apparently ancestral to all modern pigs. Pigs on the whole are still rather primitive artiodactyls; they lost the first toe on the forefoot and have long curving canines, but have very few other skeletal changes and still have low-cusped teeth. The main changes are a great lengthening of the skull & development of curving side tusks. These changes are seen Hyotherium (early Miocene), probably ancestral to the modern pig Sus and other genera.
Camels:
* Diacodexis (early Eocene, see above)
* Homacodon & other dichobunids (mid-Eocene) -- Similar to Diacodexis but with some advances; probably close to the ancestry of the rest of the artiodactyls.
* Poebrodon (late Eocene) -- First primitive camelid. Like other late Eocene artiodactyls, it had developed crescent-shaped grinding ridges on the cheek teeth. A small, short-necked, four-toed animal with little hooves on each toe.
* Poebrotherium (mid-Oligocene) -- A taller camelid with fused arm & leg bones, and missing toes 1, 4, and 5. Longer neck, though still much shorter than modern camels. Had hooves.
* From here the camel lineage developed pads in place of hooves on the feet, reverted to digitigrade posture, and began pacing instead of trotting, as shown by Miocene fossil footprints. This camel lineage goes through Protomeryx (early Miocene) and Procamelus (Miocene). The llamas split off here (Lama). The main camel lineage continued through Pliauchenia (Pliocene) and finally, in the late Pliocene, Camelus, the modern camels.
Ruminants: (see Scott & Janis, in Szalay et al., 1993, for details)
It\\\'s been very difficult to untangle the phylogeny of this fantastically huge, diverse, and successful group of herbivores. From the Eocene on, there are dozens of similar species, only some of them leading to modern lineages, with others in dozens of varied offshoot groups. Only recently have the main outlines become clear. The phylogeny listed below will probably change a bit as new information comes in.
* Diacodexis (early Eocene, see above)
* Homacodon & other dichobunids (mid-Eocene, see above)
* Mesomeryx (late Eocene) -- A more advanced dichobunid; probably close to the ancestry of the rest of the artiodactyls.
* Hypertragulus, Indomeryx or a similar hypertragulid (late Eocene) -- Primitive ruminants with a tendency toward crescent ridges on teeth, high-crowned teeth, and loss of one cusp on the upper molars. Long- legged runners and bounders, with many primitive features, but with telltale transitional signs: Still 5 toes on front and 4 behind, but the side toes are now smaller. Fibula still present (primitive), but now partially fused at the ends with the tibia. Upper incisors still present, but now smaller. Upper canine still pointed, but now the lower canine is like an incisor. Ulna and radius fused (new feature). Postorbital bar incomplete (primitive feature). Two ankle bones fused (new feature). Mastoid bone exposed on the surface of the skull (primitive feature).
* Hyemoschus or other tragulids (Oligocene) -- Slightly more advanced ruminants called \\\"tragulids\\\" that have the above features plus loss of part of the first toe, some more bones fused, fibula shaft no longer ossifies. Too late to be actual ancestors; probably \\\"cousins\\\". Some later tragulids are still alive and are considered the most primitive living ruminants.
* Archaeomeryx, Leptomeryx (mid-late Eocene) -- Rabbit-sized ruminants. Still had small upper incisors. The mastoid bone becomes less and less exposed in these \\\"leptomerycids\\\".
* Bachitherium (early Oligocene) -- A later, more advanced leptomerycid.
* Lophiomeryx, Gelocus (late Eocene, early Oligocene) -- The most advanced ruminants yet, called \\\"gelocids\\\", with a more compact and efficient ankle, still smaller side toes, more complex premolars and an almost completely covered mastoid bone. A slightly different lineage split off from this gelocid family in the late Eocene or early Oligocene, eventually giving rise to these four families:
1. Deer: Prodremotherium (late Eocene), a slightly deerlike ruminant, and Eumeryx (Oligocene), a more deer-like ruminant, Dicrocerus (early Miocene), with the first antlers (similar to living muntjacs), Acteocemas (Miocene), and then a shmoo of successful Miocene & Pliocene groups that survive today as modern deer -- cervines, white- tails, moose, reindeer, etc.
2. Giraffes: Branched off from the deer just after Eumeryx. The first giraffids were Climacoceras (very earliest Miocene) and then Canthumeryx (also very early Miocene), then Paleomeryx (early Miocene), then Palaeotragus (early Miocene) a short-necked giraffid complete with short skin-covered horns. From here the giraffe lineage goes through Samotherium (late Miocene), another short-necked giraffe, and then split into Okapia (one species is still alive, the okapi, essentially a living Miocene short-necked giraffe), and Giraffa (Pliocene), the modern long-necked giraffe.
3. Pronghorns: Paracosoryx prodromus (early Miocene, 21 Ma) a primitive antilocaprid, probably derived from a North American branch of the bovid lineage. Next came Merycodus (Miocene), with branched permanent horns. Led to numerous antilocaprids in the Pliocene. Only the pronghorn is still alive.
4. Bovids: known from isolated teeth in the late Oligocene, then from Eotragus, a primitive ancestral mid-Miocene bovid. Protragocerus (Miocene) soon followed. The first sheep (Oioceros) and gazelles (Gazella) are known from the mid-late Miocene (14 Ma), the first cattle (Leptobos, Parabos) from the early Pliocene (5 Ma).
