| email - September 2003 |
| by Do-While Jones |
Our essays on whale evolution (August, 1999,, and November, 2001,) prompted Peter to send us several emails.
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I read your very nice article "Whale Tale Two" with great interest. Please can you assist me - what exactly is the definition of a cetacean, at least for the way the fossil record is currently interpreted? … The reason I ask is the wider question of what makes a fossil bone a whale bone. My understanding as I read the literature is that a skull with a tympanic bone with a sigmoid process and an involucrum is by definition a whale. … Thank you so much Peter |
The short answer to Peter’s question is,
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What causes scientists to declare the creature [Pakicetus] a whale? Subtle clues in combination--the arrangement of cusps on the molar teeth, a folding in a bone of the middle ear, and the positioning of the ear bones within the skull--are absent in other land mammals but a signature of later Eocene whales. 1 |
Peter has discovered that arbitrary criteria are used to classify animals. For example, consider the difference between a porpoise and a dolphin.
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Although the terms "dolphin" and "porpoise" are often used interchangeably, they describe two different groups of cetaceans. Dolphins belong to the Family Delphinidae, while porpoises belong to the Family Phocoenidae. Many external characteristics exist that are useful in distinguishing between these two families. For example, the extended beak present in most delphinids is absent in most phocoenids . Similarly, most phocoenids lack a melon, while the bulbous forehead is often observed in delphinids. In addition, the dorsal fin of delphinids is often hooked or curved, while the dorsal fin in most phocoenids is triangular in shape. Some members of both families lack dorsal fins altogether. Also, merely the first two cervical are fused in delphinids, while the first six cervical are fused in phocoenids. This increased number of fused cervical limits the range of mobility in the porpoise neck. Equally, the teeth of dolphins and porpoises are distinct from one another. Delphinids possess homodont conical teeth in both jaws, while phocoenids possess spatulate, or spade-shaped, teeth in both jaws. 2 |
In other words, classification depends upon the shape of the head, the shape of the back fin, some bones, and the shape of the teeth. But words like “most”, “often”, and “some”, let you break the rules if you like. Terms like “dolphin”, “porpoise”, and “whale” denote artificial divisions that biologists use to make it easier to study and discuss similar creatures.
This seems to trouble Peter, because he wrote again a few days later,
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Thank you. I see what you mean. Yet whale paleontologists have to know what they are speaking about, even if it is in reference to a single assemblage or an announcement of a single species. They may differ among themselves and from what common sense might declare but in my reading I still cannot find someone spell out exactly what it is that makes a set of fossils a cetacean and what does not. A case in point - the original announcement of Pakicetus to the world in Science, 1983 fails to spell this out clearly and would seem that the accompanying "news and views" equivalent drew attention to the matter, but abdicated an objective alternative. Am I missing something, or am I reading it correctly? Thank you for your time with me. |
Perhaps they didn’t want to state their criteria explicitly because that would just open up the door to arguments about the criteria. Since classification criteria are just a matter of opinion, and opinions differ, there is always controversy.
Ironically, the day we got Peter’s second email, there was a similar letter in Science News.
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To be human I have always been fascinated by the subject of “African Legacy: Fossils plug gap in human origins” (SN: 6/14/03, p.371). I have a simple question: Is there a definitive set of standards (physical or behavioral or both) that defines modern Homo sapiens? Lew Roberts, Franklin Square, N.Y, No. this is a topic that inspires much discussion and debate. - B. Bower 3 [italics in original] |
When you get down to basics, it turns out that a fossil is classified as one thing or another because someone who thinks he is much smarter than you says it is. The “missing links” all depend upon an ancestry that is based on the subjective classification of species, and assumed evolutionary relationships between them.
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Footnotes:
1
Chadwick, National Geographic, November 2001, “Evolution of Whales”, page 68
(Ev)
2
http://www.theporpoisepage.com/porpvdolph.php
(Ev)
3
Science News, August 16, 2003, Vol. 164, page 111
(Ev)