O rodu Gobiceratops znovu a podrobněji

12. listopadu jsem na blog napsal o novém rodu a druhu bagaceratopida, Gobiceratops minutus (zde). Původní abstrakt obnášel jen velmi malé množství informací, naštěstí se více podrobností vyskytlo na DML (Dinosaur Mailing List), konkrétně v příspěvku Jaimeho A. Headdena. V něm je citován Vladimir Alifanov, ruský paleontolog, který je autorem původní studie. Citace Headdenova příspěvku:

Alifanov, V. R. 2008. [The tiny horned dinosaur *Gobiceratops minutus*
gen. et sp. nov. (Bagaceratopidae, Neoceratopsia) from the Upper
Cretaceous of Mongolia.] _Paleontologicheskii Zhurnal_ 2008(6):49-60.
(Reprinted in _Paleontological Journal_ 42(6):621-633.)

One look at the skull and your first impression is "That's a juvenile!".
Alifanov addresses this topic in his discussion of the taxon, which ordinarily
you'd think would be referred to *Bagaceratops* as a juvenile of the type
species (as it is even from the same formation):

"The mature age stage of the holotype of *Gobiceratops minutus* gen. et
sp. nov. is supported by the solid connection of bones of the braincase
and skull roof; incidentally, this provided insignificant deformation of
the specimen in the burial. In addition, the premaxillae are fused
ventrally (this is not observed in some larger specimens of
bagaceratopids), the nasal and prefrontal are fused on the right side,
the prefrontal and lachrymal are fused on the left side, and fusion is
also observed in the vomers, pterygoids, palatines, and ventral bones
of the braincase. Partial fusion is also observed between the
pterygoids and between the quadrates and quadratojugals.

"The young age of the holotype of *Gobiceratops minutus* gen. et sp.
nov. is supported by the predentary bone, which is broken in the lower
part at the suture because of incomplete fusion with the dentaries.

"The data considered suggest that the holotype is a young individual."

I can only say that one cannot be "young" and osteologically "mature" in this
sense, or if they can, it's gotta be weird. Skeletal maturity usually follows
sexual maturity in mammals, reptiles, but not birds, such that one should
expect suturing and fusion of elements in an adult, but its clear that the
differential fusion is odd for this animal, especially with loose regions of
the mandible skull roof, etc., that simultaneously concluding the animal to be
mature and young (a subadult?) is inconsistent. The skull is clearly juvenile
in its aspect, but does the bone support this conclusion? Does it preserve
finished bone or is the surface still highly textured?

Most interesting is that Alifanov has not looked at the possibility that
ceratopsians fuse many cranial bones relatively early in their ontogeny, a
respect that occurs to the frontals and parietals in virtually all
ceratopsians, and the nasals in the basal ones. This should make it certainly
harder to compare *Gobiceratops* positively against taxa and make a
differential diagnosis that does not conclude closely that the specimen belongs
potentially to another taxon.

Indeed, the diagnosis reads like a description fo a juvenile skull:

"Tiny horned dinosaur, with large orbits and large lower temporal fenestrae.
Preorbital region of skull short and low, and occipital region high and wide.
Rostral bone absent. Antorbital fossae located under orbits; subnarial
fenestrae located in line with their anterior margin, wider and slightly higher
than nares. Supraoccipital large and wide. Dorsally, prefrontals lacking
contact with frontals because of long posterior processes of nasals, which
reaching orbital border. Nasal horn in shape of slightly projecting tubercle
with wide base. Frontals with medially concave orbital borders and massive
posterolateral processes. Nasofrontal suture V- or U-shaped. Postorbitals and
parietal adjoining each other, excluding frontals from formation of border of
upper temporal fenestrae. Parietal wide, with short medial crest in occipital
part of bone. Fenestrae in parietal absent. Occipital process of maxillae long,
reaching line of posterior orbital border.
Quadratojugal process of jugals having well-pronounced crest with wide base.
Quadratojugals large, open laterally. Dentaries lacking angular process.
Surangulars and angulars approximately equal in area of lateral surface.
Splenials reaching occipitally line of middle of adductor fossae. Each jaw with
six functioning teeth."

Lack of fusion of some elements, including the rostral (leading to its
absence), size of the orbits, orbits overslinging the antorbital
fossa/fenestra, low tooth count, relatively triangualr skull with broad
parietal region and extremely short antorbital region, extent and size of the
maxilla, are all juvenile aspects. Some of these (no parietal fenestra, nasals
fussed) are plesiomorphies.

Alifanov continues to justify the taxonomy by supporting the variability and
uniqueness of other named taxa, such as *Lamaceratops* and *Platyceratops*,
*Breviceratops*, by arguing that the specimens are different, and thus unique,
arguing that dissenters such as Makovicky are underassessing taxonomic
variability. I can only say that this specimen is a juvenile, is quite young,
and does not look as though supportive information on its taxonomic identity
can substantively separate it from *Bagaceratops*. The occipital region, which
Alifanov uses to primarily distinguish it from *B. rozhdestvenksii*, is
disarticulated (unfused and juvenile), and clearly not useful in distinguishing
the taxa.

Cheers,

Jaime A. Headden

Podle Alifanova holotyp gobiceratopa náleží mladému jedinci, a to i přes pevné spojení kostí mozkovny a lebeční klenby. Oproti tomu čelistní kosti nejsou navzájem zcela spojené. Alifanov také argumentoval ve prospěch zavedení nového rodu a druhu, a to i přes názory, že se jedná pouze o juvenilního jedince rodu Bagaceratops. Tento skeptický postoj zaujímá i Headden, který odmítá Alifanovovo "ospravedlňování taxonomie" výčtem jedinečných znaků a odlišností od jiných primitivních ceratopů z rodů Platyceratops, Lamaceratops a Breviceratops. Podle Headdena je nalezený jedinec příliš mladý na to, aby přinesl nějaké jednoznačné důkazy pro nutnost separace od rodu Bagaceratops. Týlní oblast nalezené lebky, kterou Alifanov hlavně využil k odlišení od druhu B. rozhdestvenksii, je podle Headdena k tomuto účelu nepoužitelná. Přestože jde o subjektivní názor, který nemusí paleontologové s J. Headdenem sdílet, zůstává zde možnost situace, kdy by se Gobiceratops stal mladším synonymem rodu Bagaceratops, nebo dokonce vleklého a nedořešeného problému, na jehož řešení se paleontologové neshodnou ani po desítkách let (Nanotyrannus kontra Tyrannosaurus).

Zdroj:

Jaime A. Headden; http://dml.cmnh.org/2008Nov/msg00089.html