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> For example, we can't build a proper crown- or node-based definition of
> mammals because nobody knows when monotremes branched off from the basal
> mammal line, so we don't know the last common ancestor of modern
> mammals. We could probably put together a decent stem-based definition
> of mammals, but it would wind up including many animals that most people
> probably wouldn't think of as mammals. Also, it seems like cladograms
> and cladists have a logical contradiction in their thinking. They
> depend on cladograms to tell them relationships -- but cladograms never,
> ever say "X is descended from Y." If cladograms don't tell you what's
> descended from what, then how can you base a time-spanning, evolutionary
> definition of a taxon on a cladogram?
As I understand it (which might be oversimplified or wrong), taxon X
can only be ancestor of taxon Y if X only possesses primitive
(relative to Y) features and no specialisations on its own
(i.e. autapomorphies which are not shared by taxon Y). If X has
derived features which are not found in Y then it is barred from
direct ancestry, however a very near relative, which very resembles
X, is then considered as the most recent ancestor of both X and Y.
In the fossil record, no such 'real ancestors' are found, all known
intermediate taxa possess their own, unique derived features, how
close they even fit the fenotype of the 'ancestor'. Even if the
remains of such a creature would not possess autapomorphies, we
might not be sure of it, as only skeletal features can be considered,
since we would have no or very few data about behaviour, soft
tissue anatomy, colour pattern, which could have been quite unique.
Hence it would be more correct or safe to consider a hypothetical
ancestor not identical but very similar to the animal we found.
There has been a lot of thought about the observation that no real
ancestors are found in the fossil record; it gave inter alia rise to
the theory of punctuated equilibrium as proposed by Eldredge and
> I don't understand why people got away from the plain old-fashioned idea
> of basing definitions on physiology: if X and Y have features 1, 2, and
> 3 in common, then X and Y are in the same taxon. And taxa are defined
> as "the first organism to have features 1, 2, and 3, and all of its
> descendants." Tetrapoda then would be the first four-legged partially
> land-dwelling animal and all of its descendants. Amniota would be the
> first animal that laid hard-shelled eggs, and all of its descendants.
> Tyrannosauria would be the first theropod to have the shrunken forelimbs
> and whatever other autapomorphies distinguish tyrannosaurs, and all of
> its descendants. Aves would be the first animal to have flight
> feathers, and all of its descendants. Ceratopsia would be the first
> animal to have the parrot beak and neck frill, and all of its
> descendants. And so on. This is close to the "node-based" definition,
> but it's based on hard facts, not theorizing about "relationships."
Definitions as 'laying hard shelled eggs', 'presence of feathers'
which are very useful in classifying present-day taxa have little
value when trying to establish relationships of taxa of which only
some skeletal remains are known. Then you need to search for unique
skeletal features which can be quite subtle. Descendants also can be
quite different from ancestors as they are adapted to different
environments; for example pleurosaurs compared with their terrestrial
sphenodontian ancestors, whales compared with mesonychids. There is
also the big problem of convergent evolution; unrelated taxa which
adapt to similar environments can be very similar. For example
turtles and cyamodoid placodonts, hupehsuchians and ichthyosaurs,
gorgonopsians and pristerognathids... How would you define Crocodilia
for example, when there have been such lookalikes as proterosuchians,
proterochampsids, phytosaurs and champsosaurs?
> Can anyone on the list explain this to me in words of ten syllables or
Hope this gives some answer to your questions.