Only the lineage of the modern giraffe evolved a long neck after Prothero Most of these taxa are known from skulls and jaws and a few from skeletons, but the neck vertebrae are not often preserved. However, Nikos Solounias , personal communication is currently publishing a description of a new fossil of the giraffid Bohlinia that preserves a neck that is intermediate in length between Giraffa and the okapi Fig.
Thus, we do know how the giraffe got its long neck, and we have the transitional fossils to show how and when it occurred! Once again, the fossil record has provided a specimen whose very existence the creationists have long denied.
Neck vertebrae of a recently discovered fossil giraffid Bohlinia that is intermediate in length between those of primitive giraffids Okapia , bottom and the modern long-necked species Giraffa , top. Both molecular and paleontological evidence agree that artiodactyls and perissodactyls are a natural group of ungulates.
However, when it comes to a third major hoofed mammal clade, the tethytheres elephants, sirenians, and their kin , there is a conflict between molecular evidence which places them in the Afrotheria Springer et al. We will not discuss this issue further here, because numerous laboratories and paleontologists are working to resolve the conflict. Many of primitive tethytheres had hooves, so we will treat them as hoofed mammals in an ecological sense, even if it is not clear that they are part of the Ungulata.
The Proboscidea, or the order of elephants and their extinct relatives, have an outstanding fossil record Shoshani and Tassy , since they are large-bodied heavy-boned animals that fossilize well. The details of their systematics are still not fully worked out Lambert and Shoshani , but many of the broader trends are well documented. The earliest proboscidean in the fossil record is known as Phosphatherium ; it comes from the late Paleocene of Morocco Gheerbrant et al.
Although it consists of a partial skull, the teeth already have the classic mastodont pattern. By the early Eocene, we have Numidotherium from Algeria Mahboubi et al. By the late Eocene and Oligocene, we have the well-known Moeritherium , which looked more like a large tapir or pygmy hippo than an elephant Fig. Nevertheless, the skull shows evidence of a short proboscis, short tusks in the upper and lower jaws, teeth typical of primitive mastodonts, and many details in the rest of the skull that unquestionably link it with the Proboscidea.
Details of the evolution of the skull, tusks, and trunk of proboscideans, from the pygmy hippo-like Moeritherium through mastodonts with longer tusks and trunks to mammoths after Scheele From Moeritherium , there was a tremendous radiation of mastodonts and mammoths in the Oligocene and Miocene Fig.
All of these can be traced back to primitive gomphotheres of the Oligocene and Miocene, which had short trunks and tusks, but were otherwise unspecialized. Once again, the phylogeny is bushy and branching, although it can be summarized in terms of its general trends Fig.
Evolutionary history of the elephants and their kin Proboscidea , starting with pygmy hippo-like forms like Moeritherium with no trunk or tusks, through mastodonts with short trunks and tusks, and concluding with the huge mammoths and the two living species. Early in their history, the other tethytheres branched off from the Proboscidea. These include the manatees, order Sirenia, the extinct desmostylians, and the extinct horned arsinotheres from Prothero Today, these peaceful aquatic creatures float in shallow tropical waters and graze on sea grass.
They are fully aquatic, with two front flippers and no visible hind legs. For decades, their relationships to the rest of the mammals were unknown until McKenna showed that they are the sister taxon of the Proboscidea and part of a group he called the Tethytheria. Subsequent work has found a huge number of highly specialized features that confirm this hunch, so it is now a well-established notion.
In the past decade, the monophyly of the Tethytheria was also confirmed by later molecular analysis. However, most sirenian fossils are incomplete, usually consisting of the distinctive extremely dense bone of their ribs used for ballast or occasional skulls and teeth.
Yet this creature had four perfectly good legs complete with terrestrial hands and feet, not flippers as seen in the living sirenians Fig. The mounted skeleton of Pezosiren portelli , the sirenian with feet rather than flippers, next to Daryl Domning, who described and named it photo courtesy of Dr.
Raymond L. One could not ask for a better example of a transitional fossil! It closely parallels the intermediate pattern of locomotion seen in walking whales such as Ambulocetus Thewissen, this volume.
When creationists have addressed this discovery at all on their websites; none of their books mention it yet , they show their complete ignorance of the basics of anatomy and paleontology. In short, they do not understand the basic notion of homology and analogy. This kind of mental straitjacket and getting out of a dilemma by defining it away might make them feel better, but it is no excuse for knowing their anatomy or fossils or getting the facts straight.
In short, the fossil record of hoofed mammals is full of transitional fossils and even longer transitional sequences that demonstrate the origins of nearly all the living ungulates and tethytheres from ancestors that looked almost completely unlike their descendants.
We now have the fossils that show where the perissodactyls came from phenacodonts, Radinskya and that document the radiation of the earliest horses, tapirs, rhinos, and brontotheres when they were almost indistinguishable to the untrained eye Fig.
We have the fossils that demonstrate the evolution of the horse family, the rhinoceroses, the tapirs, and the brontotheres, along with other examples not covered in this article. Their phylogenies are now much more bushy and branching, but otherwise, the general trends are the same that were observed over a century ago. Likewise, we now have the fossils to document the early stages of the radiation of the artiodactyls and especially the bushy branching history of camels and giraffes, both of which lacked humps or long necks in their respective early histories.
One of the best transitional fossils of all is Pezosiren portelli , a perfect intermediate form that shows how the aquatic manatees evolved from walking ancestors. All of these examples are largely ignored by creationists, or when they do mention them, they use completely outdated arguments, quotes out of context, or simple lies and distortions that demonstrate the fact that creationists have no training in anatomy or paleontology and cannot tell one bone from another.
In the most extreme cases, the creationists resort to semantic gyrations that define the problem away, so that if a fossil has terrestrial legs and feet, it cannot be a sirenian or a whale, even if every other aspect of the anatomy clearly indicates its phylogenetic affinities. Arguments such as this reveal the dogmatism and complete intellectual and scientific bankruptcy of creationists.
Since they do not even bother to do this, their arguments are worthless. Domning DP. The earliest known fully quadrupedal sirenian. Erfurt J, Metais G. The evolution of artiodactyls. Google Scholar. Froehlich DJ.
Quo vadis Eohippus? The systematics and taxonomy of the early Eocene equids Perissodactyla. Zool J Linn Soc Lond. Article Google Scholar. Gazin CL. A review of the upper Eocene Artiodactyla of North America. Smithson Misc Contrib. A Palaeocene proboscidean from Morocco. Paenungulata Sirenia, Proboscidea, Hyracoidea, and relatives. The rise of placental mammals. Gish D. Evolution, the fossils still say NO!. Superfamily Suoidea. Evolution of tertiary mammals of North America.
Kemp TS. The origin and evolution of mammals. Oxford: Oxford University Press; Lambert WD, Shoshani J. MacFadden BJ. Fossil horses. Earliest known proboscidean from early Eocene of North Africa. Matthew WD. The evolution of the horse: a record and its interpretation. Q Rev Biol.
McKenna MC. Toward a phylogenetic classification of the Mammalia. Phylogeny of the primates: a multidisciplinary approach.
New York: Plenum; Chapter Google Scholar. Finally, anthropologist Brian Hare of Harvard University and his colleagues describe in this same issue the results of a study showing that domestic dogs are more skillful than wolves at using human signals to indicate the location of hidden food.
Yet "dogs and wolves do not perform differently in a nonsocial memory task, ruling out the possibility that dogs outperform wolves in all human-guided tasks," they write.
Therefore, "dogs' social-communicative skills with humans were acquired during the process of domestication. The tale of human evolution is divulged in a similar manner although here we do have an abundance of fossils , as it is for all concestors in the history of life. We know evolution happened because innumerable bits of data from myriad fields of science conjoin to paint a rich portrait of life's pilgrimage.
Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. We know that pakicetids were closely related to whales and dolphins based on a number of unique specializations of the ear. But pakicetids lived on land and had nostrils at the front of the skull, as modern cows and sheep do.
The ancestors of whales probably looked something like Pakicetus. How did evolution go from something like Pakicetus to modern whales below right , with nostrils aka, the blowhole at the top of the head? If a pakicetid-like ancestor gave rise to modern whales, we would expect the lineage to have passed through an intermediate form — one with nostrils in the middle of the skull. And indeed we do find evidence of this transition in the fossils of Aetiocetus above middle , which had nostrils in the middle of its skull.
The fossil record of horses provides other examples of transitional features. Fossil hominid skulls: Labeled with specimen name, species, age, and cranial capacity in milliliters cranial capacity is the volume of the space inside the skull, and correlates closely with brain size. That graph illustrates one particular aspect of human evolution, the growth of the brain over the last 3.
At no point could anyone credibly point to a discontinuity between our australopithecine ancestors and modern humans which is not filled by some ancestral fossil form. Even twenty-seven years ago, the record of species-level transitions was considered quite good, and at higher taxonomic levels, the situation was improving and quite strong in situations where preservation of fossils had been favorable.
Since that time, the state of transitional fossils has only improved. Explore Evolution uses a reference in an attempt to discredit these recent fossil discoveries without actually mentioning what those discoveries are. Staying up to date with research in science is critical for students and for textbook authors, and Explore Evolution 's reliance on an outdated, non-applicable, 25 year old reference is unacceptable. Tiktaalik is a transitional form in the evolution of vertebrates on four legs.
Ahlberg and Clack describe the importance of the discovery:. And when Shubin started investigating sedimentary rocks laid down in shallow water about million years old, he found Tiktaalik roseae.
Based on known fossils, scientists could estimate what time period the transitional form had to have existed in. Based on the known locations of fossil beds, they could select a bed known to be from the right time and to have possessed the right environment million years ago to contain a transitional form. They knew what sorts of fossils to look for at that site by considering the known fossils from before and after the era in question.
And after selecting that site, they found exactly the fossils they sought, a transitional form which allowed a detailed examination of the evolution of critical structures in the transition from aquatic fish to terrestrial tetrapods. This process is exactly how science works, and a textbook interested in encouraging students to explore the way evolutionary biology is practiced would do well to help students see how paleontologists actually deal with gaps in our knowledge of the fossil record.
The approach Explore Evolution takes does not present any such understanding of the inquiry-based process of science. Gaps in our current knowledge are treated as insurmountable barriers. If scientists truly took that approach, we would never have achieved the sorts of advances seen in paleontology over the last 20 years, let alone the last It would be very interesting to know who "some scientists" actually are.
The "they" Explore Evolution discusses have not published in the peer-reviewed literature, because there is in fact substantial agreement that all the major phyla lines do indeed come from a single, branching tree.
Look through any college-level biology textbook Campbell, p. A multiple-tree view is not shown simply because the evidence for it is so weak, and the evidence for a single-tree so strong, that the multiple-tree model can be discarded in the same way a flat earth model can be discarded from a geography textbook.
This straw man logic set up a false claim, knock it down easily, declare victory is itself a lesson on how not to teach logic and rhetoric to children.
These unnamed, uncited "advocates" strike again. While certainly paleontologists would enjoy having more "transitional" fossils whatever that vague terms actually means there are many examples of fossils that bridge the gap between species.
Thus, the phrase "absence of transitions" is wrong to imply that there are no transitions. Make a Donation Today. Give a Gift Membership.
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