Free Republic
Browse · Search
News/Activism
Topics · Post Article

To: connectthedots
In the case of evolution, the lack of a fossil record of transitional forms is actual evidence that transitional forms did not and do not exist.

Oh yeah, no transitional forms whatsoever exist in the record:

http://www.talkorigins.org/faqs/faq-transitional.html

Summary of the known vertebrate fossil record

(We start off with primitive jawless fish.)

Transition from primitive jawless fish to sharks, skates, and rays

GAP: Note that these first, very very old traces of shark-like animals are so fragmentary that we can't get much detailed information. So, we don't know which jawless fish was the actual ancestor of early sharks.

A separate lineage leads from the ctenacanthids through Echinochimaera (late Mississippian) and Similihari (late Pennsylvanian) to the modern ratfish.

Transition from from primitive jawless fish to bony fish

GAP: Once again, the first traces are so fragmentary that the actual ancestor can't be identified.

Eels & sardines date from the late Jurassic, salmonids from the Paleocene & Eocene, carp from the Cretaceous, and the great group of spiny teleosts from the Eocene. The first members of many of these families are known and are in the leptolepid family (note the inherent classification problem!).

Transition from primitive bony fish to amphibians

Few people realize that the fish-amphibian transition was not a transition from water to land. It was a transition from fins to feet that took place in the water. The very first amphibians seem to have developed legs and feet to scud around on the bottom in the water, as some modern fish do, not to walk on land (see Edwards, 1989). This aquatic-feet stage meant the fins didn't have to change very quickly, the weight-bearing limb musculature didn't have to be very well developed, and the axial musculature didn't have to change at all. Recently found fragmented fossils from the middle Upper Devonian, and new discoveries of late Upper Devonian feet (see below), support this idea of an "aquatic feet" stage. Eventually, of course, amphibians did move onto the land. This involved attaching the pelvis more firmly to the spine, and separating the shoulder from the skull. Lungs were not a problem, since lungs are an ancient fish trait and were present already.

GAP: Ideally, of course, we want an entire skeleton from the middle Late Devonian, not just limb fragments. Nobody's found one yet.

More info on those first known Late Devonian amphibians: Acanthostega gunnari was very fish-like, and recently Coates & Clack (1991) found that it still had internal gills! They said: "Acanthostega seems to have retained fish-like internal gills and an open opercular chamber for use in aquatic respiration, implying that the earliest tetrapods were not fully terrestrial....Retention of fish-like internal gills by a Devonian tetrapod blurs the traditional distinction between tetrapods and fishes...this adds further support to the suggestion that unique tetrapod characters such as limbs with digits evolved first for use in water rather than for walking on land." Acanthostega also had a remarkably fish-like shoulder and forelimb. Ichthyostega was also very fishlike, retaining a fish-like finned tail, permanent lateral line system, and notochord. Neither of these two animals could have survived long on land.

Coates & Clack (1990) also recently found the first really well- preserved feet, from Acanthostega (front foot found) and Ichthyostega (hind foot found). (Hynerpeton's feet are unknown.) The feet were much more fin-like than anyone expected. It had been assumed that they had five toes on each foot, as do all modern tetrapods. This was a puzzle since the fins of lobe-finned fishes don't seem to be built on a five-toed plan. It turns out that Acanthostega's front foot had eight toes, and Ichthyostega's hind foot had seven toes, giving both feet the look of a short, stout flipper with many "toe rays" similar to fin rays. All you have to do to a lobe- fin to make it into a many-toed foot like this is curl it, wrapping the fin rays forward around the end of the limb. In fact, this is exactly how feet develop in larval amphibians, from a curled limb bud. (Also see Gould's essay on this subject, "Eight Little Piggies".) Said the discoverers (Coates & Clack, 1990): "The morphology of the limbs of Acanthostega and Ichthyostega suggest an aquatic mode of life, compatible with a recent assessment of the fish-tetrapod transition. The dorsoventrally compressed lower leg bones of Ichthyostega strongly resemble those of a cetacean [whale] pectoral flipper. A peculiar, poorly ossified mass lies anteriorly adjacent to the digits, and appears to be reinforcement for the leading edge of this paddle-like limb." Coates & Clack also found that Acanthostega's front foot couldn't bend forward at the elbow, and thus couldn't be brought into a weight-bearing position. In other words this "foot" still functioned as a horizontal fin. Ichthyostega's hind foot may have functioned this way too, though its front feet could take weight. Functionally, these two animals were not fully amphibian; they lived in an in-between fish/amphibian niche, with their feet still partly functioning as fins. Though they are probably not ancestral to later tetrapods, Acanthostega & Ichthyostega certainly show that the transition from fish to amphibian is feasible!

Hynerpeton, in contrast, probably did not have internal gills and already had a well-developed shoulder girdle; it could elevate and retract its forelimb strongly, and it had strong muscles that attached the shoulder to the rest of the body (Daeschler et al., 1994). Hynerpeton's discoverers think that since it had the strongest limbs earliest on, it may be the actual ancestor of all subsequent terrestrial tetrapods, while Acanthostega and Ichthyostega may have been a side branch that stayed happily in a mostly-aquatic niche.

In summary, the very first amphibians (presently known only from fragments) were probably almost totally aquatic, had both lungs and internal gills throughout life, and scudded around underwater with flipper-like, many-toed feet that didn't carry much weight. Different lineages of amphibians began to bend either the hind feet or front feet forward so that the feet carried weight. One line (Hynerpeton) bore weight on all four feet, developed strong limb girdles and muscles, and quickly became more terrestrial.

Transitions among amphibians

From there we jump to the Mesozoic:

Finally, here's a recently found fossil:

Transition from amphibians to amniotes (first reptiles)

The major functional difference between the ancient, large amphibians and the first little reptiles is the amniotic egg. Additional differences include stronger legs and girdles, different vertebrae, and stronger jaw muscles. For more info, see Carroll (1988) and Gauthier et al. (in Benton, 1988)

The ancestral amphibians had a rather weak skull and paired "aortas" (systemic arches). The first reptiles immediately split into two major lines which modified these traits in different ways. One line developed an aorta on the right side and strengthened the skull by swinging the quadrate bone down and forward, resulting in an enormous otic notch (and allowed the later development of good hearing without much further modification). This group further split into three major groups, easily recognizable by the number of holes or "fenestrae" in the side of the skull: the anapsids (no fenestrae), which produced the turtles; the diapsids (two fenestrae), which produced the dinosaurs and birds; and an offshoot group, the eurapsids (two fenestrae fused into one), which produced the ichthyosaurs.

The other major line of reptiles developed an aorta on left side only, and strengthened the skull by moving the quadrate bone up and back, obliterating the otic notch (making involvement of the jaw essential in the later development of good hearing). They developed a single fenestra per side. This group was the synapsid reptiles. They took a radically different path than the other reptiles, involving homeothermy, a larger brain, better hearing and more efficient teeth. One group of synapsids called the "therapsids" took these changes particularly far, and apparently produced the mammals.

Some transitions among reptiles

I will review just a couple of the reptile phylogenies, since there are so many.... Early reptiles to turtles: (Also see Gaffney & Meylan, in Benton 1988)

Here we come to a controversy; there are two related groups of early anapsids, both descended from the captorhinids, that could have been ancestral to turtles. Reisz & Laurin (1991, 1993) believe the turtles descended from procolophonids, late Permian anapsids that had various turtle-like skull features. Others, particularly Lee (1993) think the turtle ancestors are pareiasaurs:

Mid-Jurassic turtles had already divided into the two main groups of modern turtles, the side-necked turtles and the arch-necked turtles. Obviously these two groups developed neck retraction separately, and came up with totally different solutions. In fact the first known arch-necked turtles, from the Late Jurassic, could not retract their necks, and only later did their descendents develop the archable neck. Early reptiles to diapsids: (see Evans, in Benton 1988, for more info)

GAP: no diapsid fossils from the mid-Permian.

Some species-to-species transitions:

Transition from synapsid reptiles to mammals

This is the best-documented transition between vertebrate classes. So far this series is known only as a series of genera or families; the transitions from species to species are not known. But the family sequence is quite complete. Each group is clearly related to both the group that came before, and the group that came after, and yet the sequence is so long that the fossils at the end are astoundingly different from those at the beginning. As Rowe recently said about this transition (in Szalay et al., 1993), "When sampling artifact is removed and all available character data analyzed [with computer phylogeny programs that do not assume anything about evolution], a highly corroborated, stable phylogeny remains, which is largely consistent with the temporal distributions of taxa recorded in the fossil record." Similarly, Gingerich has stated (1977) "While living mammals are well separated from other groups of animals today, the fossil record clearly shows their origin from a reptilian stock and permits one to trace the origin and radiation of mammals in considerable detail." For more details, see Kermack's superb and readable little book (1984), Kemp's more detailed but older book (1982), and read Szalay et al.'s recent collection of review articles (1993, vol. 1).

This list starts with pelycosaurs (early synapsid reptiles) and continues with therapsids and cynodonts up to the first unarguable "mammal". Most of the changes in this transition involved elaborate repackaging of an expanded brain and special sense organs, remodeling of the jaws & teeth for more efficient eating, and changes in the limbs & vertebrae related to active, legs-under-the-body locomotion. Here are some differences to keep an eye on:


# Early Reptiles Mammals

1 No fenestrae in skull Massive fenestra exposes all of braincase
2 Braincase attached loosely Braincase attached firmly to skull
3 No secondary palate Complete bony secondary palate
4 Undifferentiated dentition Incisors, canines, premolars, molars
5 Cheek teeth uncrowned points Cheek teeth (PM & M) crowned & cusped
6 Teeth replaced continuously Teeth replaced once at most
7 Teeth with single root Molars double-rooted
8 Jaw joint quadrate-articular Jaw joint dentary-squamosal (*)
9 Lower jaw of several bones Lower jaw of dentary bone only
10 Single ear bone (stapes) Three ear bones (stapes, incus, malleus)
11 Joined external nares Separate external nares
12 Single occipital condyle Double occipital condyle
13 Long cervical ribs Cervical ribs tiny, fused to vertebrae
14 Lumbar region with ribs Lumbar region rib-free
15 No diaphragm Diaphragm
16 Limbs sprawled out from body Limbs under body
17 Scapula simple Scapula with big spine for muscles
18 Pelvic bones unfused Pelvis fused
19 Two sacral (hip) vertebrae Three or more sacral vertebrae
20 Toe bone #'s 2-3-4-5-4 Toe bones 2-3-3-3-3
21 Body temperature variable Body temperature constant

(*) The presence of a dentary-squamosal jaw joint has been arbitrarily selected as the defining trait of a mammal.

GAP of about 30 my in the late Triassic, from about 239-208 Ma. Only one early mammal fossil is known from this time. The next time fossils are found in any abundance, tritylodontids and trithelodontids had already appeared, leading to some very heated controversy about their relative placement in the chain to mammals. Recent discoveries seem to show trithelodontids to be more mammal- like, with tritylodontids possibly being an offshoot group (see Hopson 1991, Rowe 1988, Wible 1991, and Shubin et al. 1991). Bear in mind that both these groups were almost fully mammalian in every feature, lacking only the final changes in the jaw joint and middle ear.

So, by the late Cretaceous the three groups of modern mammals were in place: monotremes, marsupials, and placentals. Placentals appear to have arisen in East Asia and spread to the Americas by the end of the Cretaceous. In the latest Cretaceous, placentals and marsupials had started to diversify a bit, and after the dinosaurs died out, in the Paleocene, this diversification accelerated. For instance, in the mid- Paleocene the placental fossils include a very primitive primate-like animal (Purgatorius - known only from a tooth, though, and may actually be an early ungulate), a herbivore-like jaw with molars that have flatter tops for better grinding (Protungulatum, probably an early ungulate), and an insectivore (Paranyctoides).

The decision as to which was the first mammal is somewhat subjective. We are placing an inflexible classification system on a gradational series. What happened was that an intermediate group evolved from the 'true' reptiles, which gradually acquired mammalian characters until a point was reached where we have artificially drawn a line between reptiles and mammals. For instance, Pachygenulus and Kayentatherium are both far more mammal-like than reptile-like, but they are both called "reptiles".

Transition from diapsid reptiles to birds

In the mid-1800's, this was one of the most significant gaps in vertebrate fossil evolution. No transitional fossils at all were known, and the two groups seemed impossibly different. Then the exciting discovery of Archeopteryx in 1861 showed clearly that the two groups were in fact related. Since then, some other reptile-bird links have been found. On the whole, though, this is still a gappy transition, consisting of a very large-scale series of "cousin" fossils. I have not included Mononychus (as it appears to be a digger, not a flier, well off the line to modern birds). See Feduccia (1980) and Rayner (1989) for more discussion of the evolution of flight, and Chris Nedin's excellent Archeopteryx FAQ for more info on that critter.

GAP: The exact reptilian ancestor of Archeopteryx, and the first development of feathers, are unknown. Early bird evolution seems to have involved little forest climbers and then little forest fliers, both of which are guaranteed to leave very bad fossil records (little animal + acidic forest soil = no remains). Archeopteryx itself is really about the best we could ask for: several specimens has superb feather impressions, it is clearly related to both reptiles and birds, and it clearly shows that the transition is feasible.

[Note: a classic study of chicken embryos showed that chicken bills can be induced to develop teeth, indicating that chickens (and perhaps other modern birds) still retain the genes for making teeth. Also note that molecular data shows that crocodiles are birds' closest living relatives.]

Overview of the Cenozoic

The Cenozoic fossil record is much better than the older Mesozoic record, and much better than the very much older Paleozoic record. The most extensive Cenozoic gaps are early on, in the Paleocene and in the Oligocene. From the Miocene on it gets better and better, though it's still never perfect. Not surprisingly, the very recent Pleistocene has the best record of all, with the most precisely known lineages and most of the known species-to-species transitions. For instance, of the 111 modern mammal species that appeared in Europe during the Pleistocene, at least 25 can be linked to earlier European ancestors by species-to-species transitional morphologies (see Kurten, 1968, and Barnosky, 1987, for discussion).

Timescale

Pleistocene 2.5-0.01 Ma Excellent mammal record
Pliocene 5.3-2.5 Ma Very good mammal record
Miocene 24-5.3 Ma Pretty good mammal record
Oligocene 34-24 Ma Spotty mammal record. Many gaps in various lineages
Eocene 54-34 Ma Surprisingly good mammal record, due to uplift and exposure of fossil-bearing strata in the Rockies
Paleocene 67-54 Ma Fair record early on, but late Paleocene is lousy

For the rest of this FAQ, I'll walk through the known fossil records for the major orders of modern placental mammals. For each order, I'll describe the known lineages leading from early unspecialized placentals to the modern animals, point out some of the remaining gaps, and list several of the known species-to-species transitions. I left out some of the obscure orders (e.g. hyraxes, anteaters), groups that went completely extinct, and some of the families of particularly diverse orders.

Primates

I'll outline here the lineage that led to humans. Notice that there were many other large, successful branches (particularly the lemurs, New World monkeys, and Old World monkeys) that I will only mention in passing. Also see Jim Foley's fossil hominid FAQ for detailed information on hominid fossils.

GAP: "The modern assemblage can be traced with little question to the base of the Eocene" says Carroll (1988). But before that, the origins of the very earliest primates are fuzzy. There is a group of Paleocene primitive primate-like animals called "plesiadapids" that may be ancestral to primates, or may be "cousins" to primates. (see Beard, in Szalay et al., 1993.)

The tarsiers, lemurs, and New World monkeys split off in the Eocene. The Old World lineage continued as follows:

GAP: Here's that Oligocene gap mentioned above in the timescale. Very few primate fossils are known between the late Eocene and early Oligocene, when there was a sharp change in global climate. Several other mammal groups have a similar gap.

GAP: There are no known fossil hominids or apes from Africa between 14 and 4 Ma. Frustratingly, molecular data shows that this is when the African great apes (chimps, gorillas) diverged from hominids, probably 5-7 Ma. The gap may be another case of poor fossilization of forest animals. At the end of the gap we start finding some very ape-like bipedal hominids:

Known species-species transitions in primates:

Phillip Gingerich has done a lot of work on early primate transitions. Here are some of his major findings in plesiadapids, early lemurs, and early monkeys:

And here are some transitions found by other researchers:

Bats

GAP: One of the least understood groups of modern mammals -- there are no known bat fossils from the entire Paleocene. The first known fossil bat, Icaronycteris, is from the (later) Eocene, and it was already a fully flying animal very similar to modern bats. It did still have a few "primitive" features, though (unfused & unkeeled sternum, several teeth that modern bats have lost, etc.)

Carnivores

GAP: few miacoid skulls are known from the rest of the Eocene -- a real pity because for early carnivore relationships, skulls (particularly the skull floor and ear capsule) are more useful than teeth. There are some later skulls from the early Oligocene, which are already distinguishable as canids, viverrids, mustelids, & felids (a dog-like face, a cat-like face, and so on). Luckily some new well-preserved miacoid fossils have just been found in the last few years (mentioned in Szalay et al., 1993). They are still being studied and will probably clarify exactly which miacoids gave rise to which carnivores. Meanwhile, analysis of teeth has revealed at least one ancestor:

From the Oligocene onward, the main carnivore lineages continued to diverge. First, the dog/bear/weasel line.

Dogs:

Bears:

The transitions between each of these bear species are very well documented. For most of the transitions there are superb series of transitional specimens leading right across the species "boundaries". See Kurten (1976) for basic info on bear evolution.

Raccoons (procyonids):

Weasels (mustelids):

Pinniped relationships have been the subject of extensive discussion and analysis. They now appear to be a monophyletic group, probably derived from early bears (or possibly early weasels?).

Seals, sea lions & walruses:

Now, on to the second major group of carnivores, the cat/civet/hyena line. Civets (viverrids):

Cats:

Hyaenids:

Species-species transitions among carnivores:

Rodents

Lagomorphs and rodents are two modern orders that look superficially similar but have long been thought to be unrelated. Until recently, the origins of both groups were a mystery. They popped into the late Paleocene fossil record fully formed -- in North America & Europe, that is. New discoveries of earlier fossils from previously unstudied deposits in Asia have finally revealed the probable ancestors of both rodents and lagomorphs -- surprise, they're related after all. (see Chuankuei-Li et al., 1987)

Squirrels:

Beavers:

Rats/mice/voles:

Cavies:

GAP: No cavy fossils are known between Paramys and the late Oligocene, when cavies suddenly appear in modern form in both Africa and South America. However, there are possible cavy ancestors (franimorphs) in the early Oligocene of Texas, from which they could have rafted to South America and Africa. Known species-species transitions in rodents:

Lagomorphs

Known species-to-species transitions in lagomorphs:

Condylarths, the first hoofed animals

Within a few million years the condylarths split into several slightly different lineages with slightly different teeth, such as oxyclaenids (the most primitive), triisodontines, and phenacodonts (described in other sections). Those first differences amplified over time as the lineages drifted further and further apart, resulting ultimately in such different animals as whales, anteaters, and horses. It's interesting to see how similar the early condylarth lineages were to each other, in contrast to how different their descendants eventually, slowly, became. Paleontologists believe this is a classic example of how 'higher taxa" such as families and orders arise.

Says Carroll (1988, p.505): "In the case of the cetaceans [whales] and the perissodactyls [horses etc.], their origin among the condylarths has been clearly documented....If, as seems likely, it may eventually be possible to trace the ancestry of most of the placental mammals back to the early Paleocene, or even the latest Cretaceous, the differences between the earliest ancestral forms will be very small -- potentially no more than those that distinguish species or even populations within species. The origin of orders will become synonymous with the origin of species or geographical subspecies. In fact, this pattern is what one would expect from our understanding of evolution going back to Darwin. The selective forces related to the origin of major groups would be seen as no different than those leading to adaptation to very slightly differing enviromments and ways of life. On the basis of a better understanding of the anatomy and relationships of the earliest ungulates, we can see that the origin of the Cetacea and the perissodactyls resulted not from major differences in their anatomy and ways of life but from slight differences in their diet and mode of locomotion, as reflected in the pattern of the tooth cusps and details of the bones of the carpus and tarsus." (p. 505)

Species-to-species transitions among the condylarths:

Cetaceans (whales, dolphins)

Just several years ago, there was still a large gap in the fossil record of the cetaceans. It was thought that they arose from land-dwelling mesonychids that gradually lost their hind legs and became aquatic. Evolutionary theory predicted that they must have gone through a stage where they had were partially aquatic but still had hind legs, but there were no known intermediate fossils. A flurry of recent discoveries from India & Pakistan (the shores of the ancient Tethys Sea) has pretty much filled this gap. There are still no known species-species transitions, and the "chain of genera" is not complete, but we now have a partial lineage, and sure enough, the new whale fossils have legs, exactly as predicted. (for discussions see Berta, 1994; Gingerich et al. 1990; Thewissen et al. 1994; Discover magazine, Jan. 1995; Gould 1994)

In the Oligocene, whales split into two lineages:

  1. Toothed whales:
    • Agorophius (late Oligocene) -- Skull partly telescoped, but cheek teeth still rooted. Intermediate in many ways between archaeocetes and later toothed whales.
    • Prosqualodon (late Oligocene) -- Skull fully telescoped with nostrils on top (blowhole). Cheek teeth increased in number but still have old cusps. Probably ancestral to most later toothed whales (possibly excepting the sperm whales?)
    • Kentriodon (mid-Miocene) -- Skull telescoped, still symmetrical. Radiated in the late Miocene into the modern dolphins and small toothed whales with asymmetrical skulls.
  2. Baleen (toothless) whales:
    • Aetiocetus (late Oligocene) -- The most primitive known mysticete whale and probably the stem group of all later baleen whales. Had developed mysticete-style loose jaw hinge and air sinus, but still had all its teeth. Later,
    • Mesocetus (mid-Miocene) lost its teeth.
    • Modern baleen whales first appeared in the late Miocene.

Perissodactyls (horses, tapirs, rhinos)

Here we come to the most famous general lineage of all, the horse sequence. It was the first such lineage to be discovered, in the late 1800's, and thus became the most famous. There is an odd rumor circulating in creationist circles that the horse sequence is somehow suspect or outdated. Not so; it's a very good sequence that has grown only more detailed and complete over the years, changing mainly by the addition of large side-branches. As these various paleontologists have said recently: "The extensive fossil record of the family Equidae provides an excellent example of long-term, large-scale evolutionary change." (Colbert, 1988) "The fossil record [of horses] provides a lucid story of descent with change for nearly 50 million years, and we know much about the ancestors of modern horses."(Evander, in Prothero & Schoch 1989, p. 125) "All the morphological changes in the history of the Equidae can be accounted for by the neo-Darwinian theory of microevolution: genetic variation, natural selection, genetic drift, and speciation." (Futuyma, 1986, p.409) "...fossil horses do indeed provide compelling evidence in support of evolutionary theory." (MacFadden, 1988)

So here's the summary of the horse sequence. For more info, see the Horse Evolution FAQ.

GAP: There are almost no known perissodactyl fossils from the late Paleocene. This is actually a small gap; it's only noticeable because the perissodactyl record is otherwise very complete. Recent discoveries have made clear that the first perissodactyls arose in Asia (a poorly studied continent), so hopefully the ongoing new fossil hunts in Asia will fill this small but frustrating gap. The first clue has already come in:

SMALL GAP: It is not known which Merychippus species (stylodontus? carrizoensis?) gave rise to the first Dinohippus species (Evander, in Prothero & S 1988).

Compare Equus to Hyracotherium and see how much it has changed. If you think of animals as being divided into "kinds", do you think Equus and Hyracotherium can be considered the same "kind"? Tapirs and rhinos:

Species-species transitions:

Elephants

GAP: Here's that Oligocene gap again. No elephant fossils at all for several million years.

Meanwhile, the elephant lineage became still larger, adapting to a savannah/steppe grazer niche:

The Pleistocene record for elephants is very good. In general, after the earliest forms of the three modern genera appeared, they show very smooth, continuous evolution with almost half of the speciation events preserved in fossils. For instance, Carroll (1988) says: "Within the genus Elephas, species demonstrate continuous change over a period of 4.5 million years. ...the elephants provide excellent evidence of significant morphological change within species, through species within genera, and through genera within a family...."

Species-species transitions among the elephants:

Sirenians (dugongs & manatees)

GAP: The ancestors of sirenians are not known. No sirenian-like fossils are known from before the Eocene.

h3>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)

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.

Hippos & pigs:

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.

Species-species transitions in artiodactyls:

Species-species transitions known from other misc. mammal groups

This concludes our tour of the Cenozoic placental mammal record! However, please do not unfasten your seatbelts until the FAQ has come to a complete stop.

A quote from Gingerich (1985) about Eocene mammals also applies to the mammal record as a whole: "The fossil record of early Eocene mammals appears to be both gradual and punctuated. It is gradual in the sense that early and late representatives of all species, whether changing or not, are connected by intermediate forms. Some ancestor-descendant pairs of species are also connected by intermediates. The record is punctuated in the sense that new lineages appear abruptly at the Clarkforkian-Wasatchian boundary, and some possible ancestor-descendant pairs of species are not connected by intermediates."

In summary, as Carroll (1988) said, "There is considerable evidence from Tertiary mammals that significant change does occur during the duration of species, as they are typically recognized, and this change can account for the emergence of new species and genera."

138 posted on 10/07/2005 3:00:14 PM PDT by RogueIsland
[ Post Reply | Private Reply | To 77 | View Replies ]


To: RogueIsland

wow!


360 posted on 10/08/2005 11:12:12 PM PDT by King Prout (19sep05 - I want at least 2 Saiga-12 shotguns. If you have leads, let me know)
[ Post Reply | Private Reply | To 138 | View Replies ]

Free Republic
Browse · Search
News/Activism
Topics · Post Article


FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson