Horner and Goodwin on Triceratops

[Mike Taylor <mike@indexdata.com>]

Jaime,

Just wanted to say that this is fascinating, and you should wrap it up
as a short paper and send it out pronto!  It's too good for the DML.


Jaime A. Headden writes:
 >   In Horner and Goodwin's latest paper*, they offer a table of ten osteologic
 > features they suggest may lead to ontogenetic identity in *Triceratops* 
 > skulls.
 > They listed 31 specimens, of which ten could not be given a positive on one 
 > of
 > those ten features. These specimens range from "baby" to "subadult," but do 
 > not
 > include "adult."
 > 
 >   In the paper, Horner and Goodwin note that "Forster [_JVP_ 16:259-270]
 > re-evaluated *Triceratops* and recognized two species: *Triceratops horridus*
 > and *Triceratops prorsus*" and further mention that they "would expect the
 > cranial ontogenetic characters identified in our study for *Triceratops* to 
 > be
 > consistent, regardless of the number of species or genera accepted, because
 > cranial ontogeny is conserved in closely related taxa." This may have some
 > substantial bearing on the nature of the identifications, since the 
 > assumption
 > of similar ontogeny is presumed _a priori_ and was not tested specifically.
 > Indeed, since its first mention, the names of species do not come up again 
 > for
 > *Triceratops.* This is worth the testing, to some degree, to see if, even in
 > closely related taxa, suture closure, fusion of extraneous elements, and the
 > appearance and shape of sexually dimorphic or visual display structures would
 > differ from species to species, based on either 1) age of the species 
 > relative
 > to the other, 2) environment of the species relative to the other, or 3)
 > different maturation periods and phase intervals. These, as in (say) African
 > antelopes of even the same "genus," can vary to a diagnostic degree. I began 
 > by
 > evaluating the data as I would a data matrix, so this review is very limited 
 > in
 > scope and certainly not a comprehensive review in the least.
 > 
 >   First, the ten "characters" noted by Horner and Goodwin are the following:
 > 
 > Scalloped margin of the parietal and squamosal
 > Frill margin scalloped to wavy
 > Epoccipitals merged to frill
 > fan-shaped frill
 > Horns curved only posteriorly
 > Base of horns excavated internally
 > Inter-nasal fusion
 > Fusion of the nasal horn onto the nasals
 > Supraoccipital separated by the exoccipitals
 > Horns curved forward at the base
 > 
 >   If this were ran as a cladistic series using the skulls as operating
 > taxonomic units, these characters might thusly be coded:
 > 
 > 1. Parietal/Squamosal, margin, shape: scalloping pronounced (0), slight
 > scalloping to wavy margins (1).
 > 2. Epoccipitals, relative to the frill margin: separate (0), fused (1).
 > 3. Frill, shape: low and semicircular in profile (0), tall and "fan-shaped",
 > more than 1/3 circular in profile (1).
 > 4. Postorbital, horn, curvature: curved only caudally (0), curved anteriorly 
 > at
 > the base, then dorsally distally (1).
 > 5. Postorbital, horn, internal cone-shaped basal excavation: absent (0),
 > present (1).
 > 6. Nasals, fused: absent (0), present (1).
 > 7. Nasal, horn: separate from nasals (0), fused onto nasals (1).
 > 8. Exoccipitals, position: meeting at the midline above the foramen magnum 
 > (0);
 > separated by the supraoccipital (1).
 > 
 >   And if these characters were made into a matrix, the 31 specimens thus 
 > coded
 > would be rendered thus:
 > 
 > ucmp_154452  00000001
 > mor_652      00000000
 > mor_1199     10000000
 > ucmp_136306  10000000
 > mor_1110     10001000
 > ucmp_150234  10000000
 > mor_539              00000000
 > ucmp_137263  10010000
 > mor_1120     11111111
 > mor_699              10110001
 > ucmp_136092  101?0001
 > ucmp_137266  00110000
 > ucmp_173739  00010000
 > mor_1604     01010110
 > mor_004              01110110
 > mor_1625     01010110
 > ucmp_113697  01011100
 > ucmp_174838  01010110
 > ucmp_136589  01010000
 > ucmp_140416  01010000
 > ucmp_129205  01010000
 > 
 >   Included were only the specimens for which a coding was given in Horner and
 > Goodwin's table, but this still left MOR 652, considered a "baby" by the
 > authors, without any positive coding. As such, it would be the "outgroup" by
 > default, though the resultant trees all treat the two included "babies" in 
 > the
 > table as closest to the root anyway, along with a variety of other specimens,
 > due to the singularity of their given codings.
 > 
 >   This matrix, thus produced, resulted in 25 different trees, a testament to
 > this matrix's completeness and thorough handling of the characters. The
 > consistency index (CI) comes out at a flat 0.5, as does the homoplasy index.
 > These are not encouraging numbers, but the matrix is so much smaller than the
 > sampled taxa! I ran the tree under DELTRAN optimization and with all 
 > characters
 > unordered, and recieved a strict consensus tree where only two real patterns
 > emerged: MOR 699 and UCMP 136092 always grouped together (both of these 
 > listed
 > as "subadult"), and all specimens listed by Horner and Goodwin as "adult"
 > grouped together as a unit with MOR 1120 (the winner of the matrix as the 
 > only
 > taxon to score on all characters with a "1", listed as a "subadult") paired
 > consistently with MOR 004. The latter was the "quintessential" poster child 
 > for
 > the adult phase in Horner and Goodwin's view, and this might be saying
 > something.
 > 
 >   As I look at the resultant trees, I find them unsatisfying for the purpose,
 > but note that the ontogeny is not measured against skull length, provenance, 
 > or
 > age of the strata in which they were recovered. This information, when 
 > checked,
 > might produce a new set of constraints that may actually allow 
 > differentiation
 > of species, or destroy the species limits, or find that as species, these 
 > taxa
 > do not age any differently. That the "iconic" adult, MOR 004 has very short,
 > nearly unrecurved horns and a prominent nasal horn, suggests it might be a 
 > VERY
 > old adult, which has had some time to grow its schnoz, and the postorbital 
 > horn
 > cores began to dissolve with onset of osteoporosis (I'm throwing this out 
 > from
 > my butt, so beware as this is based only very partly on Horner and Goodwin's
 > comments).
 > 
 > * Horner, J. R. & M. B. Goodwin. 2006. Major cranial changes during
 > *Triceratops* ontogeny. _Proceedings of the Royal Society, B_ 273:2757-2761.
 > 
 >   Abstract:
 >   "This is the first cranial ontogenetic assessment of *Triceratops*, the
 >    well-known Late Cretaceous dinosaur distinguished by three horns and a
 >    massive parietal–squamosal frill. Our analysis is based on a growth series
 > of
 >    10 skulls, ranging from a 38 cm long baby skull to about 2 m long adult
 >    skulls. Four growth stages correspond to a suite of ontogenetic characters
 >    expressed in the postorbital horns, frill, nasal, epinasal horn and
 >    epoccipitals. Postorbital horns are straight stubs in early ontogeny, 
 > curve
 >    posteriorly in juveniles, straighten in subadults and recurve anteriorly 
 > in
 >    adults. The posterior margin of the baby frill is deeply scalloped. In 
 > early
 >    juveniles, the frill margin becomes ornamented by 17–19 delta-shaped
 >    epoccipitals. Epoccipitals are dorsoventrally compressed in subadults,
 >    strongly compressed and elongated in adults and ultimately merge onto the
 >    posterior frill margin in older adults. Ontogenetic trends within and
 > between
 >    growth stages include: posterior frill margin transitions from scalloped 
 > to
 >    wavy and smooth; progressive exclusion of the supraoccipital from the
 > foramen
 >    magnum; internal hollowing at the base of the postorbital horns; closure 
 > of
 >    the midline nasal suture; fusion of the epinasal onto the nasals; and
 >    epinasal expansion into a morphologically variable nasal horn. We
 > hypothesize
 >    that the changes in horn orientation and epoccipital shape function to 
 > allow
 >    visual identity of juveniles, and signal their attainment of sexual
 >    maturity."
 > 
 >   This paper is a follow-up to Goodwin et al. 2006, _JVP_ 26(1):103-112,
 > published earlier this year, describing the skull of UCMP 154452.
 > 
 > Jaime A. Headden
 > http://bitestuff.blogspot.com/
 > 
 > "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
 > 
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