A new name in the New Mexico Museum of Natural History and Science Bulletin #14 Kranz, P. M., 1998. "Mostly Dinosaurs: A Review of the Vertebrates of the Potomac Group (Aptian Arundel Formation), USA," in Lucas, Kirkland & Estep,eds., 1998: 235-238.

COMMENTS BY MICKEY MORTIMER

"Capitalsaurus" Kranz, 1998 "C." potens (Lull,1911) = Creosaurus potens (Lull,1911) = Dryptosaurus potens (Gilmore 1921)

Holotype

(USNM 3049) (7-10 m) proximal caudal centrum (140 mm)

Diagnosis

Proximal caudal centra slightly opisthocoelous and show the combination of a single ventral keel and almost straight ventral edge.

Description

This species is only known from a proximal caudal centrum, but comparisons to other large theropods suggest it was seven to ten meters long.

The centrum is slightly opisthocoelous (perhaps platycoelous would be a better description), lacks pleurocoels and is much taller than wide (1.3 times taller than wide).  The ventral edge is almost straight and has a single ventral keel.

Relationships

This species has been referred to Allosaurus and Dryptosaurus in the past, but is stratigraphically closest to Acrocanthosaurus.  It will be compared to these three genera first, then to other genera that may be similar.  Comparisons to most genera are difficult due to poor descriptions.

The proximal caudals of Allosaurus are amphiplatyan to slightly procoelous, the opposite of "Capitalsaurus".  Also, they are about as wide as tall, sometimes wider, and the ventral edge is much more concave.  The ventral surface has a slight groove instead of a keel.

Those of Dryptosaurus share the straighter ventral edge and are slightly taller than wide (~1.05 times), but no further details can be discerned.

Acrocanthosaurus has caudal pleurocoels (like Carcharodontosaurus, but not Giganotosaurus), a concave ventral margin and amphiplatyan or amphicoelous centra.  The ventral surface is grooved and the centra are 1-1.2 times taller than wide.

Ceratosaurus has procoelous centra with deeply concave ventral margins and a deep ventral groove.

The centra of Carnotaurus are slightly wider than tall, amphicoelous and have deeply concave ventral margins.

Torvosaurus also has amphicoelous centra with deeply concave ventral margins.  In addition, its ventral surface is grooved and the centra are subcircular.

A few proximal caudal centra are known from Baryonyx.  These are amphicoelous, taller than wide (1.1 times) and deeply concave ventrally.

Monolophosaurus has centra taller than wide with deeply concave ventral margins.  The ventral surface has a shallow groove.

Sinraptor has a centra taller than wide (1.3 times) and a single ventral keel like "Capitalsaurus", but its centra have deeply concave ventral margins.

Nedcolbertia has amphicoelous centra with deeply concave ventral margins.  They are slightly taller than wide and have ventral grooves.

Gorgosaurus has amphicoelous or amphiplatyan centra with slightly concave ventral margins.

Ornithomimids have centra with deeply concave ventral margins and shallow ventral grooves that are slightly wider than tall.  Those of Gallimimus are slightly procoelous while Archaeornithomimus has amphicoelous centra.

Microvenator and Chirostenotes have centra that are much wider than tall with ventral sulci.  Those of Chirostenotes are amphicoelous and those of Microvenator are platycoelous.  Both Chirostenotes and oviraptorids have pleurocoels.

Unfortunately, segnosaurs have poorly described caudal vertebrae, although those of Segnosaurus are said to be platycoelous.

Bagaraatan has slightly procoelous centra much taller than wide (1.3 times), some of which have almost straight ventral margins.

Eumaniraptorans have proximal caudal centra distinctively rectangular in section, except alvarezsaurids which have strongly procoelous centra.

Thus, no theropods have a strong resemblance to "Capitalsaurus" and it is probably not a carcharodontosaurine, caenagnathoid or eumaniraptoran.  The single ventral keel is only known in Sinraptor.  It is unknown whether other sinraptorids exhibit this condition.  Dryptosaurus, tyrannosaurids and Bagaraatan have nearly straight ventral margins.  Several taxa (Monolophosaurus, sinraptorids, carcharodontosaurids, Bagaraatan) have centra about 1.3 times taller than wide, like "Capitalsaurus". 

I know of no theropods with opisthocoelous caudal centra, but Segnosaurus and Microvenator have platycoelous centra.  "Capitalsaurus" seems to be unique, and thus a valid genus, but it's relationships remain obscure.  I recommend it remain Neotheropoda incertae sedis.

This example shows that even an isolated bone can be differentiated from other taxa, although future comparison is needed to securely validate this genus.  Hopefully Kranz will properly describe it soon.