True - but then such intermediates are of no relevance to the question of transitions. If one were interested in a gradual transition in cranial capacity from Homo erectus to Homo sapiens, it would be of no interest that one species of Australapithecine had an intermediate cranial capacity.
In the case of the giraffe, we know that living okapis have short necks and living giraffes have long necks. We also know that fossil giraffes older than Samotherium major had necks as long as today’s. Some taxonomists even suggest that the earliest true giraffes may fall within the range of the present species. And that long and short necked forms coexisted for millions of years.
So the presence of a not-quite-so-short neck in one species of Samotherium gives no indication whatsoever that that trait existed at its divergence from the giraffes several million years before. Given the shorter necks of other, older, species in the Samotherium genus, in fact, it makes it vanishingly unlikely.
We don’t even know if a specific H. erectus skull is ancestral to any living humans. H. erectus also coexisted with later species within Homo. It would seem that H. erectus holds the same position as the intermediate giraffe species. I would agree that they need to find earlier examples of the transitional species, but the facts are that there were giraffes that had intermediate morphology.
So ‘intermediate’ then, has at least two broad meanings: 1. morphologically in between in terms of characteristics, and 2. genealogically in between by descent.
Most people have #2 in mind when they are casually speaking of “transitional fossils”, but this may betray the common misconception of evolution as a long linear path rather than a branching bush. Yet #1 can be as vague as noting that a bird is an intermediate (by size) between a gnat and a watermelon, which of course has nothing at all to do with evolution. But knowledgeable paleontologists aren’t being so vague as all that when they do speak of transitional forms. So would it be fair to say they are making use of an (transitional!) definition between #1 and 2?
By noting that there are vaguely intermediate forms of ‘giraffe-like’ animals with shorter or longer necks, they are noting the presence of many expected ‘twig-like’ branches of ancestry that might be expected to share some characteristics but not others. The fossils finds of actual direct lineage over long spans of time would have to be infinitesimally small – like the chances of one set of random bones being the great-grandparent (of some degree) of another randomly found set of bones. Is that a fair take-away?
I think the important point is that the intermediate nature of physical characteristics is independent of ancestry. Even if common ancestry were not true you could still find intermediate features. We conclude that common ancestry is probably true because intermediates fall into the predicted nested hierarchy which is why paleontologists link intermediate features with ancestral relationships.
Another good example is Tiktaalik roseae which was portrayed as a side branch of the tetrapod lineage in the first papers that described the species.
The biggest take-away is that the information contained in a fossil can not tell us who its parents or offspring are. Only DNA evidence can really tell us about direct ancestry. Paleontologists have been guilty in the past of describing a fossil as being a direct ancestor, so that also causes some confusion. However, even Darwin recognized that we can’t really determine if a fossil is a direct ancestor and that sister taxa can still provide important information about the past evolutionary history of the direct lineage.
It seems to me that it’s easy to take a reasonable suggestion of Darwin’s and stretch the logic to breaking point.
If, for example, there’s a trend amongst a number of genera of elephant-like creatures to develop trunks, then to find a medium length and full length trunk in different genera, both of which end up with long trunks, is a finding that can justly be generalised to the other genera.
Likewise, I suppose. a trend towards single toed horses need not deal specifically with ancestors.
But Samotherium, as a genus, is long separated from the giraffes, is of a completely different build,has a different geographical distribution, lived contemporary with long-necked true giraffes, and has no known long-necked descendants.
The platypus is long separated from point of divergence between monotremes and the marsupial/placental branch of the mammal tree. The platypus also lives concurrently with both marsupial and placental mammals. However, the platypus can still provide information about that common ancestor and the transition between earlier reptillian morphology and later mammalian morphology. A species with fur and rudimentary mammary glands that lays leathery eggs from a cloaca is evidence for the transition between earlier reptiles and later mammals. The same logic holds for Samotherium.
Thanks! I like Don Prothero. He’s a professor, a paleontologist, and he’s forever writing books! I read his book “Rhinoceros Giants: The Paleobiology of Indricotheres”
Inside Nature’s Giants: Giraffe is the ultimate giraffe video. It shows a necropsy on a young giraffe that died at a wildlife park, and discusses giraffe physiology and evolution. And we get to see the Recurrent Laryngeal Nerve in all its glory!