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Re: number of species of dinos



In a message dated 95-10-05 16:06:05 EDT, swf@ElSegundoCA.ATTGIS.COM (Stan
Friesen) writes:

>From: bk090@freenet.carleton.ca (David Brez Carlisle)
> >  If (as
> > I suggested) the real number is more like 100 species at any
> > one time, then the total number of speciess should be about
> > 3,700.
> > 
> > How many species were there in toto?  Anybody?
> > 
>
>Known or actual?
>
>My dino list used to have about 400 *genera* in it.  Most of these
>are nomina dubia, which leaves about 100-150 valid genera. At an
>average of about 1.5 species per genus (generous), this gives about
>150-200 described species.
>
>How many were there actually? Well, Dodson's paper gives estimates
>ranging over an order of magnitude on that.  I.e., we don't really
>know, the best we can do is guess.
>
>I would be surprised if there were fewer than 1500 species.
>
>

Here's my slant on the situation, excerpted from the forthcoming _Mesozoic
Meanderings_ #2:

HOW MANY dinosaurs were there? This question was recently addressed by Dodson
(1990a), who applied statistical methods and a few seemingly reasonable
assumptions about collection and preservation biases to the dinosaur fossil
record. The total number of dinosaur genera that ever lived, according to his
calculations, lies somewhere between 645 and 3285, with a most probable value
between 900 and 1200.
I was asked the same question in an interview for a television show on
dinosaurs, and I estimated the answer differently: Dinosaurs dominated the
world's terrestrial biota for approximately 150 million years. Extant
vertebrates that likewise dominate their domains are bony fish, birds, and
mammals, and there are several thousand species of each: over 20,000 species
of fish, 9672 species of birds, over 3000 species of mammals. The present
epoch is, in my opinion, one of average vertebrate diversity, neither
particularly depauperate nor particularly bounteous, and "several thousand
species" may be taken as characteristic of the diversity of any of the
high-level vertebrate taxa. If anything, the ice ages and the advent of man
have made the present epoch more depauperate than usual. In any case,
exactitude is not necessary in this kind of order-of-magnitude calculation
(as if we could ever know the exact number of vertebrate species that existed
during a particular epoch!), so I chose a figure of 6000 as a likely number
of dinosaur species extant at any one time. If, like Dodson, one imagines
dinosaurs to have occupied only megafaunal niches, then the number could well
be substantially smaller. But I believe that the fossil record is hyperbiased
against the preservation of the smaller, Compsognathus-size dinosaurs.
Dinosaurs of this size class, which are known from scattered and fragmentary
remains from all over the world, must have been much more numerous and
diverse than might be imagined solely from the minuscule number of specimens
in the fossil record.

Only two known specimens, one German and one French, have been referred to
the genus Compsognathus. Very few places in the world have the fine-grained
lithographic limestones wherein the fossils of small, delicate-boned
dinosaurs like Compsognathus might be preserved, so it is extremely unlikely
that the only compsognathids that ever lived just happened to live there. I
contend that scores of as-yet-unknown compsognathid species existed
worldwideenough species that two individuals were bound on death to drift
into those rare European limey lagoons to be preserved.
Dinosaur species appear to originate, flourish, and become extinctor evolve
into new speciesduring a span of time considerably less than 4.5 million
years. This figure is obtained simply by dividing the number (four) of
well-known successional dinosaurian faunas in the Campanian to Maastrichtian
of North America (Fox, 1978) into the span of time (18 million years) during
which those faunas endured. On this time scale, even the generic turnover
between the faunas was for all practical purposes complete, and I have not
considered the non-depositional, trangressive periods between the faunas,
when species of intermediate morphology (Horner, Varricchio & Goodwin, 1992)
must have existed, without being preserved.

One hundred fifty million years thus yields some 33 or 34 total species
turnovers among the dinosaurs. When multiplied by an average of 6000 species
per epoch, this gives a grand total of about 200,000 dinosaur species that
ever lived. Assuming a conservative average of 10 species per genussome
extant vertebrate genera have more than 30 species per genus, others only one
or twothis amounts to 20,000 genera: nearly an order of magnitude more than
the maximum figure calculated by Dodson.

What is the source of this huge discrepancy? I see nothing unreasonable about
the assumptions and back-of-the-envelope calculations outlined above. But as
I just explained, Dodson may have underestimated the effects of preservation
biases. There is not enough space here to question his assumptions in detail;
that is the stuff of a separate paper. Instead, I will offer my own thoughts
about such biases. Perhaps this will explain the discrepancy.

One factor apparently not considered in calculating preservational biases is
that all known dinosaur fossils necessarily occur in depositional
environments: coastal, fluviatile, lacustrine, or eolian. Such environments
(e.g., seashores, river deltas, lake bottoms, and deserts) presently comprise
about 10% of the earth's total land area (a crude estimate based on visual
examination of world maps). At any given time, it seems, some 90% of the
world's land area is subject to erosion and subsequent deposition on the
other 10%. Epicontinental seas, such as the one that covered North America
during the Cretaceous Period, foster the development of coastal, fluviatile,
and lacustrine depositional regions, and the fraction of earth's land area
that was depositional during periods of major epicontinental transgression
may have attained 25% (Haubold, 1990). Even so, this means that between
7590% of the dinosaur species extant in any one epoch may never be found as
fossils, simply because they did not frequent depositional environments.
Dinosaur fossils are often found in assemblages that include the remains of
freshwater vertebrates such as fish and crocodylians. We are quite certain
that the fish and crocodylians lived exclusively in or near depositional
environments. Why, then, should we imagine that most of the known dinosaur
speciesparticularly the smaller species, for which extensive migrations were
unlikelydid otherwise?

Weishampel (1990a) listed 416 dinosaur-bearing localities worldwide. This
seems a large number, but it is as nothinga few data pointswhen compared
with the great expanse of the earth's surface where dinosaurs could and did
once flourish. I believe we have already collected a reasonable fraction of
all the dinosaurs that ever lived in depositional environments. But we have
only meager evidencea few isolated, fragmentary remains of animals that
happened to fall into rivers when they diedof the other kinds of dinosaurs
that once inhabited the earth. Dinosaurs undoubtedly lived in the Mesozoic
equivalents of grasslands, prairies, rain forests, and highlands, but because
these are not places where sediment accumulates, dinosaurs that lived there
would have been entirely lost to the fossil record.

Another factor to consider is that typical depositional environments were
probably not very speciose. Today's tropical rain forests contain the most
diverse faunas by far, but such forests, in which the carcasses of even large
animals are quickly dismembered and decompose, are taphonomically inimical to
the preservation of fossils. Triassic "rain forests," not swamps, river
deltas, and lakeshores, would have been the most favorable environments for
the evolution of arboreal animals. This might well explain why the fossil
record of basitheropods remains highly impoverished.

If dinosaurs diversified into an average of 6000 species per 4.5-million-year
depositional epoch, we would expect by my previous area-ratio calculation
that only 6001500 species inhabited environments where fossilization might
occur. This, then, becomes a loose upper bound on the number of species per
epoch we could in principle discover. Quite a few such deposits have already
eroded away, taking their loads of species with them; as a rule, the farther
back we go in the Mesozoic, the fewer such deposits have remained to be
prospected. It is well known, for example, that the Upper Cretaceous provides
the largest number of dinosaur-bearing formations (126 out of 416;
Weishampel, 1990a) throughout the world and, correspondingly, exhibits the
greatest dinosaur diversity. This I ascribe in large part to the
formation-age fossilization bias.

Further supporting my assertion that dinosaur diversity was considerably
higher than Dodson's estimates is the observation that formations laid down
at different times in different places seldom yield the same dinosaur
species. The dinosaurs of China, Mongolia, Argentina, Africa, North America,
Europe, and Australia all belong to distinct species and almost always to
distinct genera. Whenever a fresh locality with good dinosaur material is
prospected (and there are never enough such sites), new dinosaur species are
soon added to my table. It does not seem to be the case that dinosaurs
wandered between unconnected depositional areas, for then we would find many
more instances of the same genera and species in widely separated localities.
Some depositional areas, however, seem to have extended for thousands of
miles without significant geographical barriersalong the shores of
epicontinental seas, or when all the continents were joined into a single
Pangaean land masswhich did permit certain genera and species considerable
ranges. But I feel on reasonably firm ground when I maintain that the
dinosaurs in between the regions of depositionthe dinosaurs unavailable in
the fossil recordbelonged to genera and species different from those already
known.

The diversity of the dinosaurs we already know practically demands the
existence of those numerous "transitional" genera and species so notoriously
absent from the fossil record. For example, Horner, Varricchio & Goodwin
(1992) reported no less than three pachyrhinosaurine taxa from the Two
Medicine Formation of Montana transitional between the closely related genera
Styracosaurus and Pachyrhinosaurus (as well as one transitional
pachycephalosaurid, lambeosaurid, and tyrannosaurid species), and there is
good reason to expect several more besides. (Two of the three transitional
pachyrhinosaurines represent new genera: Einiosaurus and Achelousaurus:
Sampson, 1994.) If three or morein most cases many moreunknown taxa stand
behind every known dinosaur taxon, it is easy to see how the total number of
dinosaur species might approach or even exceed 200,000.

Although I have not analyzed the situation with any rigor whatsoever, it
seems that the number of new dinosaurs described at any time correlates best
with the number of paleontologists at work and with the strength of
establishment support for paleontology during that time (see, e.g., Dodson &
Dawson, 1991). This certainly shows that we have not yet approached a true
upper limit to dinosaur diversity, when excavating new sites would just add
more specimens of dinosaur taxa already known, no matter how many persons are
at work.

Finally, it should be noted once more that the bulk of species diversity in
extant vertebrate groups generally lies among the smaller forms. If there
were only large dinosaurs, then Dodson's estimates could well be near the
mark, for as he asserts, his diversity estimates agree in order of magnitude
with those of extant large mammals. But I contend that there were many more
small dinosaursincluding but by no means limited to pigeon-size and
chicken-size arboreal, climbing, gliding, and flying formsthan anyone has
previously guessed. Their existence follows from the pattern of archosaurian
evolution as outlined in this work. Their physical traces include the
unidentifiable teeth and other skeletal fragments of small theropodomorphs
and ornithischians found in many museum collections, and rich footprint
faunas that document significantly higher local dinosaur diversities than do
skeletal remains (Lull, 1953; Ellenberger, 1970, 1972, 1974; Olsen & Galton,
1984; and others).

Dodson (pers. comm.) has suggested that juvenile and subadult large dinosaurs
occupied many niches that would otherwise have been taken by adult small
dinosaurs. This effect would have depressed the number of small-dinosaur
genera and species and kept the total number of species well below my
estimates. Unfortunately, juvenile large dinosaurs are often difficult to
distinguish from adult small dinosaurs (Callison & Quimby, 1984), especially
in view of the poor fossil record of small dinosaurs, so a definitive test of
this thesis is probably not yet possible. It bears pointing out, however,
that many diagnostic small-dinosaur remains do exhibit adult features (fusion
of the skull elements, fusion of neural arches to centra, and so forth), so
small adult dinosaurs probably did exist. On the other hand, Horner (in
Horner & Gorman, 1988) found patterns of rapid growth in Maiasaura hatchlings
and juveniles, suggesting that dinosaurs in general and large dinosaurs in
particular spent most of their lives at or near fully adult size. Taken
together, these factors suggest a relatively minor ecological role for the
small juveniles of large-adult dinosaurs.

After taking these kinds of biases into account, we are left with the handful
of species per epoch that are actually present in our museum collections. The
point to ponder here is that enough slack still exists in the statistics to
allow the fossil record to support either Dodson's "lowball" diversity
estimates or my "highball" estimates. Consequently, we may ultimately have to
look elsewhere than the fossil record to estimate dinosaur diversity more
accurately.