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Stratigraphy, biogeography & cladograms



Howdy,

To add fuel to the fire, or better yet to put things in perspective, some
weaknesses inherent in stratigraphic and biogeographic data that are absent
in morphological or molecular data.

Specifically, in "traditional" types of phylogenetic data (morphologies or
genes) it is possible to distinguish between the actual absence of a
particular character state and non-recovery of that state (i.e., "unknown").

You can go to a _Tyrannosaurus_ fossil and recognize that it has the derived
condition "arctometatarsus".  You can pick up the foot of a _Deinonychus_
and demonstrate that it lacks the derived state "arctometatarsus".  You can
go to the type and only specimen of _Microvenator_ and recognize that you
cannot recover this information (since it lacks a foot) and so code it "?".

You can go to the Cloverly Formation and find fossils of _Deinonychus_, and
record the known presence of dromaeosaurids for the Aptian-Albian of western
North America.  HOWEVER, you can not demonstrate positively whether the
absence of tyrannosaurid fossils from this unit documents the true absence
of tyrant dinos from that time and place OR if you simply haven't recovered
said fossils.

In other words: in stratigraphy and in biogeography, there is only "1" and
"?"; you cannot distinguish between "0" (demonstrable absence) and "?"
(unknown).

One could (and some do) make statistical arguments about when sufficient
fossils are sampled to be sufficient to code a "0", but I remain highly
skeptical that such numbers are possible for rare large terrestrial
vertebrates such as dinosaurs (see my oft-repeated example on how long it
was before Morrison ankylosaurs were documented).  These numbers might work
for marine invertebrates and protists, but even then I wonder about the
significance of very long range Lazarus taxa like _Neopilina_.*

[* For those who don't know, _Neopilina_ is sort of the "coelacanth" of the
mollusc world.  For many years a group called the Monoplacophora were known
only from the Cambrian (high diversity) through the Devonian (decreasing
diversity), and then they apparently went extinct.  Until 1952, when
specimens of a modern genus of monoplach was pulled up from off the seashore
of Costa Rica...  Monoplachs thus have a 350 million year gap of
non-recovery in the fossil record, making coelacanth's 65 million gap pale
in comparison.  Of course, both are for similar reasons (only deep sea
lineages seem to have survived, and the fossil record of the deep sea is
very poor).]

Thus, operationally there is a difference between a data type in which "0"
(absence of the derived state) can be distinguished from "?" (uncertainty)
and data types where these two condition are indistinguishable.

Or, for a more positive take on matters, if you include biogeography or
stratigraphy into the analysis, you can't use them to choose between trees
equally parsimonious of molecular or morphological data.  Doing so would
leave you open to charges of circularity.  On the other hand, leaving them
out of the tree search process allows you to use these as potential methods
of preferring (at least provisionally) one tree or set of trees from the
most parsimonious total as a kind of independant check.

Oh, and John Jackson: can you demonstrate from a published analysis on
either modern insects or Cenozoic birds (both of which have produced many
flightless groups) that cladistics biases towards grouping flightless forms
together?  Otherwise your claim is extraordinarily weak, and it just sounds
like your upset because no current analysis of theropods favors your
particular phylogeny.  If on the other hand you could find such a bias
outside of the theropod taxa in question (or in the case birds, so far deep
inside that they aren't part of the Mesozoic analysis...) you might have a case.

Sorry to have to ask that of you, but you know what they say about burden of
proof and all that.

Later,

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661