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Re: definition of a species

>What is the definition of a species? Is it an organism that cannot produce
>fertile offspring with another species? How can you tell if that is the case by
>looking at fossils? What about eg. lion/tiger crosses (ligers and tigons)? Are
>they sterile and therefore not new species? How do we know that fossil animals
>also were not sterile cross-breeds? And what about dogs (Canis domesticus?).
>Aren't they one species? Yet if we found the fossil remains of a Great Dane and
>a Chihuahua, would we have classified them as one species? Would this much
>variation be encountered naturally (esp. in dinosaurs), or is this strictly the
>result of mankind manipulating a species' breeding. I'm confused (obviously)!

This is not as easy to answer a question as "Where dinosaurs warm-blooded?"
or "What killed off the dinosaurs?"  ;-)  The question "What is a species?"
is the center of a REALLY contentious debate in all aspects of evolutionary
biology.  There are several different approaches to the "species
definition" in use today.

The definition you allude to is known as the "biological species
definition" - species are naturally-occuring, naturally-reproducing
populations of organisms.  A critter is a member of that species if, mated
to another member of the species, they produce offpring which are in turn
capable of producing offspring of that species.  Thus, since the offspring
of a horse and donkey, or a tiger and lion, are sterile (almost all the
time), horses and donkeys, and tigers and lions, are members of different

Some other species definitions are the Specific Mate Recognition System
(SMRS) and the Phylogenetic Species definition.  Under SMRS, species are
defined by their particular (dare I say, specific) set of mating cues and
anatomy - e.g., different horns in different bovid species, different
phermones in different insects, different genital anatomy in the various
beetle species, etc.  Under the Phylogenetic Species definition, species
are defined by the branching off of populations - the ability to breed with
your sister-group is a primitive feature, but the inability to do so is a
derived feature acquired during post-split evolution.

Of course, all of these are hard enough to judge with modern critters, and
more so with fossil species.  So, to get back to the original emphasis of
the question - fossil species are defined primarily on the degree of
morphological similarity.  With lamboesaurs or ceratopsians or
pachycephalosaurs, cranial anatomy may give some indication of an SMRS-type
species definition.  Otherwise, it is generally the fine details of the
various bones of the skull and body which indicate closer relationships.

Since there is a degree of subjectivity to this, workers may disagree as to
what specimens belong in which species.  Bob Bakker, for example, is a
notorious "splitter", and believes that even slight variations in anatomy
indicate different species.  Other paleontologists accept a greater degree
of natural, intraspecific variability.

And, no, for most known species, there is no natural "Chihuahua-to-Great
Dane" degree of variation.  But, yes, fossils of the two dog breeds would
almost certainly be classified as different species if found by
paleontologists unfamiliar with dogs.

Hope this answers some of your questions,

Thomas R. Holtz, Jr.                                   
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092