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Smitanosaurus agilis

Reptilia

Taxonomy
Morosaurus agilis was named by Marsh (1889). Its type specimen is USNM 5384, a partial skeleton (Braincase and partial skull roof, proatlases, and first three cervical vertebrae), and it is a 3D body fossil. Its type locality is Felch Quarry 1, Garden Park (YPM), which is in a Kimmeridgian/Tithonian channel sandstone/claystone in the Morrison Formation of Colorado.

It was recombined as Camarasaurus agilis by Hay (1930) and Steel (1970); it was recombined as Camarosaurus agilis by Johnson (1931); it was recombined as Smitanosaurus agilis by Whitlock and Wilson Mantilla (2020).

Synonymy list
YearName and author
1889Morosaurus agilis Marsh p. 334 fig. 3
1890Morosaurus agilis Zittel p. 711
1897Morosaurus agilis Marsh p. 498
1898Morosaurus agilis Williston p. 173
1901Morosaurus agilis Nopcsa p. 206
1902Morosaurus agilis Hay p. 483
1910Morosaurus agilis Versluys p. 214
1916Morosaurus agilis Mook p. 139
1920Morosaurus agilis Pompeckj p. 113
1930Camarasaurus agilis Hay p. 191
1931Camarosaurus agilis Johnson p. 358
1970Camarasaurus agilis Steel p. 72
1999Morosaurus agilis Carpenter p. 9
2020Smitanosaurus agilis Whitlock and Wilson Mantilla p. 3–5

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RankNameAuthor
kingdomAnimalia()
Bilateria
EubilateriaAx 1987
Deuterostomia
phylumChordataHaeckel 1874
subphylumVertebrata
superclassGnathostomata
Osteichthyes()
subclassSarcopterygii()
subclassDipnotetrapodomorpha(Nelson 2006)
subclassTetrapodomorpha()
Tetrapoda
Reptiliomorpha
Anthracosauria
subclassAmphibiosauriaKuhn 1967
Cotylosauria()
Amniota
Sauropsida
classReptilia
subclassEureptilia()
Romeriida
Diapsida()
RankNameAuthor
Archosauromorpha(Huene 1946)
Crocopoda
ArchosauriformesGauthier 1986
Eucrocopoda
Archosauria()
informalAvemetatarsalia
Ornithodira
Dinosauromorpha
Dinosauriformes
Dinosauria()
Saurischia()
Sauropoda()
Gravisauria
Eusauropoda
Neosauropoda
Diplodocoidea()
Diplodocimorpha
Flagellicaudata
Dicraeosauridae()
genusSmitanosaurus
speciesagilis()

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

Smitanosaurus agilis Marsh 1889
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Diagnosis
ReferenceDiagnosis
J. A. Whitlock and J. A. Wilson Mantilla 2020Smitanosaurus agilis is diagnosed by an atlas lacking the ‘lip’ on the anteroventral margin (interpreted as a reversal), an axial neural spine that is both tall (height exceeds that of pedicle; convergently acquired by Amargasaurus and Dicraeosaurus) and subvertically oriented, and an axis with postzygapophyseal articular surfaces that face ventrally rather than posteroventrally. Presence of a small but conspicuous posteriorly projecting spur on the squamosal does not appear to be present in other dicraeosaurid taxa, but the feature is subtle and may not be immediately recognizable depending on preservation. Smitanosaurus agilis is distinguished from Suuwassea by supratemporal fenestrae that are narrower than the maximum height of foramen magnum, the presence of a sharp-lipped pneumatic fossa on the lateral surface of the axial intercentrum, and pleurocoels in anterior cervical vertebrae that extend onto the parapophyses. Smitanosaurus agilis is distinguished from Kaatedocus by the presence of a pineal foramen, a foramen magnum without contribution from the basioccipital, a relatively short, stubby posterior process of the proatlas, the presence of a ‘neck’ on the atlantal neural arch between the pedicle and the postzygapophyses, the presence of diapophyses on the atlantal neural arch, the presence of a strongly arcuate PODL on the axis, the subvertical neural spine of the axis (n.b., the axis associated with the holotype of Kaatedocus is not from the same individual and may not pertain to the same species: E. Tschopp, pers. comm.), the presence of an EPRL in post-axial cervical vertebrae, the absence of the ‘lateral spine cavity’ and an accessory lamina on the neural spine of cervical vertebra 3, the relatively more prominent, subvertical neural spine in cervical vertebra 3, and the absence of a concavity on the posterior ventral surface of the centrum in cervical vertebra 3. Smitanosaurus agilis is distinguished from diplodocids by the presence of a ventrally directed prong on the squamosal.
Upchurch et al. (2004:299) noted that Smitanosaurus agilis has “a craniodorsally directed neural spine of cervical 3.” The preserved anterior margin of the neural spine is indeed inclined anterodorsally, as in dicraeosaurids (Fig. 13), but the damaged posterior and dorsal margins make it difficult to interpret the orientation of the spine as a whole.