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Horner and Goodwin on Triceratops
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*
> They listed 31 specimens, of which ten could not be given a positive on one
> those ten features. These specimens range from "baby" to "subadult," but do
> 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
> 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
> 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
> *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
> 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
> evaluating the data as I would a data matrix, so this review is very limited
> 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
> 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
> separated by the supraoccipital (1).
> And if these characters were made into a matrix, the 31 specimens thus
> 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
> 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
> unordered, and recieved a strict consensus tree where only two real patterns
> emerged: MOR 699 and UCMP 136092 always grouped together (both of these
> 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
> 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
> the adult phase in Horner and Goodwin's view, and this might be saying
> 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,
> age of the strata in which they were recovered. This information, when
> might produce a new set of constraints that may actually allow
> of species, or destroy the species limits, or find that as species, these
> 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
> old adult, which has had some time to grow its schnoz, and the postorbital
> cores began to dissolve with onset of osteoporosis (I'm throwing this out
> my butt, so beware as this is based only very partly on Horner and Goodwin's
> * Horner, J. R. & M. B. Goodwin. 2006. Major cranial changes during
> *Triceratops* ontogeny. _Proceedings of the Royal Society, B_ 273:2757-2761.
> "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
> 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,
> posteriorly in juveniles, straighten in subadults and recurve anteriorly
> adults. The posterior margin of the baby frill is deeply scalloped. In
> 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
> growth stages include: posterior frill margin transitions from scalloped
> wavy and smooth; progressive exclusion of the supraoccipital from the
> magnum; internal hollowing at the base of the postorbital horns; closure
> the midline nasal suture; fusion of the epinasal onto the nasals; and
> epinasal expansion into a morphologically variable nasal horn. We
> that the changes in horn orientation and epoccipital shape function to
> visual identity of juveniles, and signal their attainment of sexual
> 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
> "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
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