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Re: A Clutter of Duckbills
In a message dated 95-10-14 18:39:19 EDT, RaptorRKC@aol.com
>Just what keeps two species in the same genus? And just what
>keeps them separate?
The conflict between lumpers and splitters is what keeps any two
species in the same genus or in two different genera (heh heh).
The entire system of Linnaean taxonomy is an arbitrary means of
cutting apart the Tree of Life (the "big cladogram" of all life
on earth) into manageable pieces. All those phyla, classes,
orders, families, and so forth, are completely made up by life
scientists to make it easy for them to talk among themselves.
There is nothing but accepted usage that determines which levels
and groups go where on the "big cladogram."
The smallest major pieces are the species. Among extant animals,
Nature draws the lines for us: species are different if their
populations cannot freely interbreed. In this sense, species are
"real" entities. But because there is no way to interbreed
populations of fossils, we have found other ways to define
species of extinct animals. Fossil species are usually defined by
their physical descriptions and by comparison with the "type
specimen." The type specimen belongs in its species _by
definition_, and all other specimens that are candidates for
inclusion in the species are (supposed to be) compared against
the type specimen. If a competent paleontologist asserts (by
publication in a journal, for example) that a particular specimen
matches the type specimen closely enough, then it is placed into
the same species; otherwise it may be classified in a different
species. If it is significantly different, it may be classified
in a different genus, or a different family, and so forth.
The subjective and arbitary nature of this process disturbs many
biologists, who dislike the sometimes endless and unscientific
debates that ensue about whether certain specimens belong in
different species, different genera, and so forth. This is one
reason cladistic analysis and cladistic taxonomy evolved (these
are the subject of a whole slew of recent postings on
sci.bio.paleontology and I won't go into them now).
>I have looked at photos of fossils, skeletal drawings, and fine
>reconstructions of several duckbilled dinosaurs. Two groups of
>closely related genera and species. I have thought about it,
>and it seems odd that such closely related animals are kept
You are a lumper at heart.
>Hypacrosaurus, Corythosaurus, and Lambeosaurus, for example.
>Without the crests, it is rather hard distinguishing one animal
>from the other. Indeed, one juvenile AMNH specimen of a
>lambeosaurine duckbill could easily have either been a
>Corythosaurus or a Lambeosaurus (I think the final arguments
>placed it in Lambeosaurus). That's how indistinguishable those
>species are. What details keep this lump of animals in different
>genera? Actually, what kind of details keep one species in one
>genus or the other? The details on these three lambeosaurine
>genera certainly indicate how closely related they are.
>Hypacrosaurus and Corythosaurus have the same type of crest, the
>"helmet" crest, and Lambeosaurus has a similar crest except that
>the lambeosaurus crest has a thin spine sticking backwards on
>it. Without these crests, the rest of the skeleton indicate that
>these animals had probably evolved from a recent common ancestor
>a few million years before they came about.
>What keeps Lambeosaurus, Corythosaurus, and Hypacrosaurus in
>different genera? Stanley Friesen suggested to me that all of
>the species of these three genera may actually belong to only
>one genus -- the first one coined, Hypacrosaurus. Sounds like a
>decent lumping to me. (Much more probable than lumping
>Deinonychus into Velociraptor.)
Stanley Friesen's suggestion, which goes back at least to Greg
Paul's paper in DINOSAURS PAST AND PRESENT (a beautiful two-
volume set that I highly recommend you obtain for your dino
library), as well as some papers by William Morris in the 1970s,
is on as sound a footing as the idea to keep the species in separate
genera. If you think the situation is bad now, imagine what it must
have been like just before 1975, when Peter Dodson published his
study demonstrating the likelihood that something around a dozen
species that had been described in the aforementioned genera were
actually only four, that they had all been based on the kind of variation
usually found within any population of large vertebrates.
Lambeosaurus is distinguished from Corythosaurus by (among other
things) having more of the premaxillary bone incorporated into the
cranial crest; Corythosaurus has more of the nasal bone in the crest.
Also, the cranial crest of Lambeosaurus is more sharply angled to the
beak and emerges in a more forward direction when looked at from the
side. Don't go by the backward-pointing prong, because some
individuals of Corythosaurus (once known as the distinct species
Corythosaurus intermedius) had a small one. Hypacrosaurus is
distinguished from either of these genera by the much taller
neural spines on its dorsal vertebrae.
Lambeosaurus and Corythosaurus are Judith River Formation forms,
Hypacrosaurus is a Horseshoe Canyon Formation form, significantly
younger geologically. This supports generic distinction of Hypacrosaurus
from the others.
Horner & Currie recently (1994: DINOSAUR EGGS AND BABIES)
described a species almost exactly intermediate between
Lambeosaurus (but not Corythosaurus) and Hypacrosaurus,
suggesting that Lambeosaurus--and not Corythosaurus--was directly
ancestral to Hypacrosaurus. They called it Hypacrosaurus
stebingeri. It comes from the Two Medicine Formation, which lies
between the Judith River and the Horseshoe Canyon.
All these distinctions among the genera are fairly minor, and one
could easily argue that we have only one genus, for which the
name Hypacrosaurus would take priority. But which is easier: to
have to write the full names Hypacrosaurus casuarius,
Hypacrosaurus lambei, Hypacrosaurus altispinus, Hypacrosaurus
magnicristatus, Hypacrosaurus laticaudus, and Hypacrosaurus
stebingeri every time you want to talk about these dinos, or just
to write one generic name for each? I'd put them in six different
genera, if it were up to me.
>Another group of hadrosaurs that confuses me is Gryposaurus,
>Hadrosaurus, Kritosaurus, and Brachylophosaurus. I know little
>about these genera in particular, except for the fact that they
>are so closely related and similar.
>Any thoughts about which genus and species should go where?
>Which one was coined first? Hadrosaurus was the first good
>hadrosaur skeleton to be discovered in N. America. Just how
>good was it? Was it good enough to show that Gryposaurus,
Kritosaurus, and Brachylophosaurus were actually different
>species of Hadrosaurus?
Jack Horner (1992) clearly and definitely separated
Brachylophosaurus from the others in that group except
Hadrosaurus. Hadrosaurus, whose type specimen lacks a skull,
cannot presently be defined below the level of family and is a
doubtful genus: we don't know whether it had a skull resembling
that of (for example) Gryposaurus, Saurolophus, Anasazisaurus,
Naashoibitosaurus, or Maiasaura; and until we find a better
specimen with good skull material (from the same New Jersey
formation, preferably), it must remain a doubtful genus. Skulls
are critical to the classification of hadrosaurids, because they
are practically indistinguishable by their postcranial skeletons.
(Any generic or specific differences are swamped by individual
Jack considered Kritosaurus to be different from Gryposaurus
(which is the correct name for the familiar hook-nosed
hadrosaurid), and in his paper he figured it with a distinctly
different skull that was based on two nearly complete skulls from
the same formation in New Mexico where Kritosaurus was originally
discovered. The next year, Adrian Hunt and Spencer Lucas asserted
that the two skulls are actually different enough from one
another to belong in different genera, and they named the genera
Anasazisaurus and Naashoibitosaurus. Since the type specimen of
Kritosaurus (the back of a skull, which also resembles the back
of the skull of Gryposaurus--which is why two generations of
20th-century paleontologists called Gryposaurus Kritosaurus)
cannot be distinguished from either of their genera, it is a
doubtful genus. Horner disagrees and considers the two Hunt &
Lucas genera to be synonyms of the genus Kritosaurus. That's
where that debate currently stands.
There is a "systematics cycle" that happens whenever fossils are
classified. When only one specimen is available, it seems so
clearly different from anything else that's known that a new
species, a new genus, and even a new family seem unquestionably
warranted. Later a few more specimens are found, and they appear
to differ enough from the original specimen to warrant new
species or new genera themselves. But as more and more specimens
accumulate, they tend to blur and fill in the gaps between the species
already named, making it look as if we really had only one species
after all. And then another specimen is found, that is _just_ different
enough from all the others...