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Re: dinosaur->bird when?

>This is certain to be an extremely stupid question so
>please pardon my ignorance.

        As I'm so fond of pointing out:  the only stupid questions are the
ones you don't ask.  8-)

>I have seen several dinosaur documentaries on the Discovery
>and TLC channels that discuss dinosaur extinction theories.
>There seems to be no agreement on what caused it (meteoroite
>impact, diseases, climate changes caused by volcanic eruption...)
>but they all seem to agree that something happened 65 million
>years ago that killed all dinosaurs.
>My question is, how does that fit with the dinosaur->bird
>evolution idea?  Are birds already in existence then? if not,
>then some dinosaurs must have survived so that they can
>eventually evolve into birds.  Has there been any fossils
>of half-dino-half-birds found?

        Birds were most certainly around before the dinosaurs went extinct.
Depending on who you talk to, you'll hear that either birds have been
around since the Late Jurassic or since the Late Triassic (I favor the
Jurassic date).   The earliest definite bird, _Archaeopteryx_, has been
found in sediments of Late Jurassic age (roughly 140 million years ago) in
Germany; recently, contemporaneous birds have been found in North Korea.
These early birds display a great number of dinosaur-like characeristics,
betraying their ancestry.  A great diversity of birds appears just 10-15
million years later, with fabulous fossils from China, Spain, and some
other locations.  To make a long story short, some birds went extinct along
with the dinosaurs, but obviously, others made it through the extinction
event to proliferate into the modern forms.

>Another question: Is dinosaur a species? or are there different
>species of dinosaurs?  It is pretty amazing that there are
>dinosaurs living on land, in sea and in the sky.  I do not know
>of another animal species that does that.

        "Dinosaur" is not a species.  I think the term you're looking for
is one of a higher taxonomic level, but under the Linnean system of
classification, the exact position of the Dinosauria is uncertain:  some
like an ordinal level (Order Dinosauria); others see it as a class.  Under
cladistic classification, no such terms are given:  Dinosauria is just a
group of animals that share some traits.

        The concept of a "species" is, however, valid under both
classification systems.  A species is the most...er, specific level of
classifying an animal.  As an example, the scientific name of a human,
_Homo sapiens_, contains the animal's genus (_Homo_) and the species
(_sapiens_).  There are other members of the genus _Homo_, such as _Homo
habilis_ and _Homo erectus_ (both extinct).  They're similar to _Homo
sapiens_, but different enough that they're not identical, and so warrant
their own species.  (Traditionally, a species is defined by the ability to
breed with others of it's own species, but we can't know that with fossil
forms, so the species are instead defined by skeletal characteristics.)

        To answer your question, though, yes, there are hundreds of species
of dinosaurs!  Some genera (plural of genus) of dinosaur only have one
species:  to take a common example, _Deinonychus antirrhopus_ is the only
species of _Deinonychus_.  Other genera have many species:  _Tyrannosaurus_
has _Tyrannosaurus rex_ and _Tyrannosaurus bataar_ (that last one is
contestible); _Apatosaurus_ has _Apatosaurus louisae_, _Apatosaurus ajax_,
and _Apatosaurus excelsus_.  When you're asking about different kinds of
dinosaurs, you're asking about genera, and there are hundreds!

        But to correct something else you said:  there are no flying
dinosaurs, and no purely aquatic dinosaurs!  The flying animals to which
you're referring are the pterosaurs (the Pterosauria), which are a related
group of animals to dinosaurs, but do not possess the traits that define
the Dinosauria, and so are not dinosaurs.  The swimming forms to which you
refer are the ichthyosaurs, plesiosaurs, and mosasaurs.  Ichthyosaurs are
reptiles, more closely related to modern lizards than to dinosaurs,
although their origins are unclear (as the first ones in the fossil record
are already so derived!).  They first appear in the Lower Triassic before
the dinosaurs appear, but go extinct prior to the extinction of the
dinosaurs (sometime around the beginning of the Late Cretaceous).
Plesiosaurs are likewise reptiles more closely related to lizards than to
dinosaurs, and first appear in the Late Triassic as well, and persist
throughout the Mesozoic.  Mosasaurs descended from varanid lizards in the
Early Cretaceous, and also last until the end of the Cretaceous.  None of
these are dinosaurs, despite this popular misconception perpetrated by an
endless slew of bad kid's dinosaur books!  8-)

        Hope this helps!

Jerry D. Harris
Denver Museum of Natural History
2001 Colorado Blvd.
Denver, CO  80205
(303) 370-6403
Internet:  jdharris@teal.csn.net
CompuServe:  73132,3372

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