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Riddle of the "Smallest": Quantifying "Small"



Chandler wrote:

<Just wondering...what is currently thought of as the
smallest (non-avian) dinosaur? I know it used to be
Compsognathus...but I know there are dinosaurs smaller
than that. Also, does anybody know what the smallest 
ornithopod, stegosaur, and sauropodomorph is?>

  First, I would urge you to check out T. Mike
Keesey's site at:

  http://dinosaur.umbc.edu

  Where he lists _Record-Breakers_ (i.e., biggest,
smallest, etc.).

  Second, while I know what you are essentially
asking, both Mike Keesey and Tom Holtz have urged on
this list before about quantifying smallness. Mass and
length are two different things. If what you mean by
small is overall dimensions, and how light it is,
etc., then which dinosaur out of each group has the
least mass? is the more appropriate question. There
are big sauropods that are both greater in mass yet
shorter in length than others (comp. *Brachiosaurus*
to *Diplodocus*).

  To answer this final question, least mass and all
that, in theropods excluding Aves, one first must
consider whether alvarezsaurs are within Aves or not;
assume they are not, then *Parvicursor*, at an
estimated 1.7-2ft, with an estimated total hindlimb
length of about 7.5in., may be the smallest non-avian
dinosaur _period_. *Compsognathus* is a bit longer
and, compared to *Parvicursor*, a bit bulkier, and may
have been 3+ft. long with a longer hindlimb length
(~12in.); *Eoraptor*, a suggested most-basal dinosaur
or theropod, has a relative hindlimb length of ~14in.,
and was still around 3ft. or so (incomplete). Basal
dinosaurs are the smallest among their kindred besides
the avians, which attain minimum lengths among the
trochilids (hummingbirds) and some passeriforms (not
as close to hummingbirds (as well as the fossil bird
*Lianingornis*, only a few inches long _in toto_), but
around there, as I'm sure you can guess. I will also
assume that you don't want sizes of juveniles, so I
can leave *Mussaurus* out of the discussion.

  The smallest ankylosaur: complete, or relatively so,
is *Minmi sp.*, around 12ft. or something, my own
estimation from the refered specimen. *Struthiosaurus*
may have been smaller, but specimens are hardly close
to estimating length in animals that can have quite
long tails compared to quite short ones.
*Tianchisaurus*, the animal almost named after
"Jurassic Park" and whose species name is virtually
un-pronouncable, may be smaller as well -- I have yet
to read the paper or see any of the material. [see
below for a possibly smaller animal]

  The smallest stegosaur: *Dacentrurus* and a few
other specimens, such as those refered to the few
Chinese taxa you can barely pronounce the names of
(*Chialingosaurus*, *Chungkingosaurus*, etc.), are
around 12ft. long. [see below for a possibly smaller
animal]

  The smallest theropod: discussed above.

  The smallest "prosauropod": probably
*Thecodontosaurus*, the most complete of the basal
forms, and probably a basal sauropodomorph, outside of
the *Plateosaurus* + *Saltasaurus* node. Around 4ft?.
The basal "prosauropods" were actually quite small,
and fit the model Sereno suggested that the basal
dinosaurian radiation were very small, cursorial,
omnivorous or carnivorous animals that developed
herbivory in several lineages (it is possible
sauropods alone of the sauropodomorphs were strict
herbivores, as advanced sauropods from about
Macronaria up had opisthopubic pelves, and all had a
wide thoracic region for the expanssive gut and deep
pelvic canal).

  The smallest sauropod: There seems to be a
variability in nature that allows the very big to
develop very small forms in isolation. There are
several dwarf forms of sauropods, and unlike the other
radiations of dinosaurs, the most basal forms are
still 30ft or larger. *Vulcanodon* and *Barapasaurus*
were somewhat large, the former possibly more gracile
than the latter; *Vulcanodon* was possibly the
smallest sauropod _period_. Dwarf forms of
diplodocimorphs (*Amargasaurus*, ~30ft.) and
titanosaurs (*Magyarosaurus*, *Malawisaurus*, both
~30ft.) are congruent with general interpretations of
giants becoming dwarfs in isolation, though
*Malawisaurus* and *Amargasaurus* were apparently
free-roaming, open territory animals, on large
landmasses, rather than island-bound, so the
hypothesis does not fit for those taxa.

  The smallest ornithopods and allies: hypsilophodonts
(paraphyletic?) and heterodontosaurs, along with
"fabrosaurs" produce several light, gracile to robust
forms barely clearing 3ft. in length. The list is
rather extensive, so you can just take my word for it
:) . They include *Heterodontosaurus*, *Lesothosaurus*
[= *Fabrosaurus*?], *Hypsilophodon*, *Yandusaurus*,
etc. *Leaellynasaura* and *Drinker* are also
purportedly ~3ft. long only.

  The smallest marginocephalians: psittacosaurids and
basal neoceratopians are generally quite small. they
don't exceed 6ft, for example, and at least some
species of *Psittacosaurus* were around 3ft.,
including *P. sinensis* and *P. ordosensis.*
*Archaeoceratops,* *Chaoyangsaurus,* and
*Microceratops* may have been only between 3-4ft.,
whereas an undescribed pachycephalosaur [=
"Microcephale"] is purportedly quite small. Other
pachycephalosaurs vary between 3-5ft in smallest size,
including *Goyocephale* (~5ft.) and *Tylocephale* and
*Prenocephale* (~5.6-6ft.); smaller forms are less
complete or less determinable as adults.

  A variety of taxa are known from teeth that indicate
very small forms. *Azhendohsaurus* is a sauropodomorph
similar to *Thecodontosaurus,* and may have been
smaller. Ornithischian remains include a variety named
by Lucas and Hunt over the course of the
late80's-90's, with some quite small forms similar to
teeth of other groups: *Pekinosaurus* could be an
ankylosaur, with a mesiodistal length exceeding crown
height and quite a distinctive denticulate pattern
resembling that of ankylosaurs. *Galtonia*, on the
other hand, resembles the teeth of basal stegosaurs,
along with *Gongbusaurus*, a suggestively larger form,
and these have teeth that vary distinctively from
other "hypsilophodonts." I would be tempted to say
more, but I have to look through the lit.

  So the smallest dinosaurs appear to exceed 2ft. at
least, and this is congruent with the size of proposed
dinosaur outgroups, including *Marasuchus* and
*Lagerpeton* (both around 2.5-3ft.), and basal
pterosaurs. *Scleromochlus* was also around 2ft. Mass
of these critters again, depends on measuring and
calculating, and Greg Paul (1988) has listed several
of his estimates that are viewed quite accurately by
the multitude of paleoecologists [that I have read,
including Farlow; correct me if I'm wrong, Jim]. I
couldn't site these for a good deal of animals listed
(lacking the estimates first-off) but *Parvicursor* at
least may have weighed as much as a magpie or
starling., less than a lb. Non-theropod dinosaurs
appear to have been decidedly more bulky, in the crow
or even cat range, and could generally be considered
to have exceeded a lb in weight.

  Also bear in mind this is all speculation and from
notes I haven't cross-checked with specimen comparison
and caliber measurments and details, so take it with a
grain of salt,

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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