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Re: when is a Lazarus Taxon not a Lazarus Taxon?

Tom Holtz wrote:

That thing that makes a critter a Lazarus taxon is our limited knowledge of its stratigraphic and distributional record, and not properties of the organism per se.

Just to emphasize this, Lazarus taxa occur when ghost lineages are extended forward in the fossil record (or even to the Present). Ghost lineages are all too common; but it's only when a stratigraphically older taxon (hitherto thought to be the youngest member of a group) is linked phylogenetically to a significantly younger taxon that the latter is termed a "Lazarus taxon". The choristodere _Lazarussuchus_ (Oligocene) was named with this in mind. Further, if _Lazarussuchus_ is a basal choristodere (as some phylogenetic analyses have found), it implies a very long ghost lineage indeed.

I can't think of any obvious Lazarus taxa among the non-avian dinosaurs. Most of the newer discoveries have been very helpful at filling in ghost lineages, not creating new ones. Then again, maybe I'm not looking hard enough.

Some "Lazarus taxa" prove not to be Lazarus taxa after all, but are instead cases of mistaken identity. In other words, the allegedly 'resurrected' clade is indeed extinct and the new taxon belongs to another group altogether; the new taxon just happens to have evolved a similar morphology independently (i.e., through convergence) to an extinct clade. There is a separate term for this kind of taxon: "Elvis taxon". Coined by Erwin and Droser (1993) "in recognition of the many Elvis impersonators who have appeared since the death of the King.

The therizinosaurs (segnosaurs) might constitute "Elvis taxa", given that they were once linked to prosauropods. But as maniraptoran theropods, the derived, heavy-bodied and presumably herbivorous therizinosaurs are "prosauropod impersonators". (Of course, therizinosaurs are a lot more than this; and in terms of the number of individual characters they have in common, the two groups are not really all that similar.)

Thank you very much.

Tim



If we
were to discover a _Dilophosaurus_ from the end of the Middle Jurassic
that would constitute a Lazarus taxon, as it seemingly went extinct and
then reappears in the fossil record stages later.

Hope this helps,

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796


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