Re: Achillobator - how big is it??

["Jaime A. Headden" <qilongia@yahoo.com>]

T. Mike Keesey wrote:

<<So 5/3 as long and three times as massive -- that
doesn't sound too weird, does it? I'm
probably/roughly/nearly three times as massive as
someone 3/5 my height.>>

Tim Williams wrote:

<Point taken. Which gives _Achillobator_ a body LENGTH
of 5m at best.>

  [snip]
 
<Actually the point of my post was not to quibble
about which dromaeosaurid is
bigger/longer/heavier/sexier, but how one would
estimate the size (length in this case) of the total
animal. Is there a tendency among maniraptorans to
have shorter hindlimbs with increasing body size?>

  There is a tendency among most animals to have
shorter, more robust limbs (especially among bipeds)
the larger they get, so that bending, torsional, and
torque stress in the bones themselves are reduced.
Further modifications include enlargement of the
epiphyses and joints of the bones to retard torsion
and torque at the joints, or locked, fused parts. This
allometric increase is apparent in ratites,
phorosrhacoid [= phorusrhacoid], mammals (see
modifications in dogs, bears, horses, elephants, from
small species to larger, and even among single taxa in
an ontogenetic sequence, such as most living
terrestrial, limbed tetrapods. Such are observations
made from a variety of publications, especially the
general texts like MacDonald (1985), Janis et al.
(1994), Weishampel et al. (1990), Romer (1966), and so
forth.

<Like the growth series of a tyrannosaurid, for
example.>

  Not completely known, and approximated only in
*Gorgosaurus libratus.* (e.g., Paul, 1988) Fairly
young juveniles are not known (validly).

-----

  For comparisons, though, figures differ. Hindlimb to
total length ratios for several taxa are as follows,
with sources:

  *Deinonychus* (Ostrom, 1990) YPM 4371
  .282

  *Ingenia* (Barsbold et al., 1990)
  ~.443-.354

  *Dromeciomimus* (Russell, 1972) type
  .406

  *Tyrannosaurus* (Paul, 1988)
  .35

  *Archaeopteryx* (Paul, 1988) JM SoS 2257 (Solnhofen)
  .495

  *Troodon* (Russell, 1969)
  .331

  The highest is Archie's, Dromec's and Ing's
(estimated), due in part to the short vertebral
columns. On the other hand, Dromec's, Troo's, Tyrant's
and Deino's are lower, with longer columns. There is a
proportionate increase in length in the femur, tibia
and tarsus/pes, in succession, in Deino, Dromec,
Archae and Troo; Tyrant has a longer femur than tibia,
and Ing has a shorter tarsus/pes than tibia. Wherever
*Achillobator* fits, let him such be compared. Making
a comparison of other limb and vertebral elements to
other dromies and maniraptoriforms should be more
conducive to estimating Achilles' full measurements.


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