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Perhaps this sauropod will prove less controversial
than "Nanshiungosaurus" bohlini :-)
Isanosaurus Buffetaut, Suteethorn, Cuny, Tong, Le
Loeuff, Khansubha and Jongautcharlyakui 2000
I. attavipachi Buffetaut, Suteethorn, Cuny, Tong,
Le Loeuff, Khansubha and Jongautcharlyakui 2000
Etymology- "Attavipach's notheastern Thailand
lizard", P. Attavipach being the former Director General of the Thai Department
of Mineral Resources, who supports paleontological research, and Isan being the
local name for northeastern Thailand.
Late Norian-Rhaetian, Late Triassic
Nam Phong Formation, Thailand
Holotype- (Department of Mineral Resources,
Thailand CH4) posterior cervical centrum (154 mm), posterior dorsal centrum (85
mm), posterior dorsal neural arch, fragmentary ribs, six caudal centra, two
chevrons, scapula (573 mm), sternal plate (197 mm), femur (760 mm)
Diagnosis- very prominent acuminate S-shaped fourth
trochantor
Description-
The posterior cervical centrum is opisthocoelous,
with a strong ventral ridge and deeply concave sides. The posterior dorsal
vertebra also lacks pleurocoels, but has a tall neural arch. The neural
spine is transversely compressed with incipient posterolateral laminae
(suprapostzygopophyseal lamina?) and a lamina extending from the transverse
process to the base of the spine (spinodiapophyseal lamina?). The caudal
centra are amphicoelous.
The scapula has a small rounded proximodorsal
expansion (50% of scapular length, ). The sternal plate is subtriangular,
with a low ridge on the outer surface. Unfortunately, it's orientation in
the figure is unknown.
The femur is straight and the shaft is compressed
anteroposteriorly. The femoral head is prominent and dorsomedially
inclined. The greater trochantor is massive and the lesser trochantor is
lacking. Far from being deflected medially, as in titanosaurs, the
proximal femur is deflected laterally. The fourth trochantor is placed
proximally on the posteromedial surface of the shaft and has a distinct
S-shaped morphology. It is intermediate between the hook-like shape of
prosauropods and the low rounded form of other sauropods. The distal femur
has a well-developed ectipicondyle and lacks a longitudinal crest proximal to
the lateral condyle.
Relationships-
Buffetaut et al. referred Isanosaurus to the
Sauropoda, but simply stated it was a basal member probably more derived than
Gongxianosaurus, as basal sauropod relationships are poorly known.
Isanosaurus is obviously sauropod, based on the
presence of- tall dorsal neural arches; suprapostzygopophyseal laminae; straight
femur; dorsomedially projected femoral head; femoral shaft compressed
anteroposteriorly; reduced fourth trochantor.
The exact relationship among basal sauropods
(Barapasaurus, Gongxianosaurus, Kotasaurus, Kunmingosaurus, Ohmdenosaurus,
Shunosaurus, Vulcanodon, Zizhongosaurus) is currently controversial as several
genera have yet to be included in phylogenetic analyses, many are poorly known
and Shunosaurus might belong to the Euhelopodidae. Isanosaurus can be
compared to most of these to help determine where it might be placed, although a
study of basal sauropod relationships is not within the scope of this
post.
Gongxianosaurus lacks opisthocoelous cervical
centra, which characterize Kotasaurus, Shunosaurus, Isanosaurus and
Barapasaurus. Both Barapasaurus and Zizhongosaurus have transversely
expanded dorsal neural spines, unlilke Shunosaurus and Isanosaurus.
Isanosaurus and Barapasaurus have spinodiapophyseal laminae on the posterior
dorsal neural arches, unlike Shunosaurus and Zizhongosaurus. Vulcanodon
has a lesser trochantor, unlike Gongxianosaurus, Shunosaurus, Isanosaurus and
Barapasaurus. Isanosaurus cannot be compared with Kunmingosaurus or
Ohmdenosaurus. Based on the characters listed above, Isanosaurus would
seem to be more derived than Vulcanodon, Gongxianosaurus and Shunosaurus, but
less derived than Barapasaurus. It's position relative to other species is
equivocal. Of course, this is only
involving the four characters which vary in basal sauropods and which are
known in Isanosaurus. I'm sure an analysis including characters that
are currently unknown in Isanosaurus would improve our understanding
immensly. For now, I recommend classifying Isanosaurus as
Sauropoda incertae sedis until a more detailed study is carried
out.
There are also several aspects of Isanosaurus's
morphology that deserve comment. The posteromedially placed fourth
trochantor was thought by Upchurch (1998) to be a synapmorphy of a
Cetiosaurus-Neosauropoda clade, convergently developed in Vulcanodon. The
presence of this character in Isanosaurus and Kotasaurus shows it needs more
study. Perhaps the three form a clade based on it, although Vulcanodon
does seem more basal than Isanosaurus. The fourth trochantor is more
prominent than Vulcanodon, Barapasaurus or Shunosaurus, which may support a more
basal position for Isanosaurus. Finally, the ridged sternal plate is
reminiscent of euhelopodids, although it is on the ventral surface as opposed to
the dorsal surface. The condition is unknown in other basal sauropods
however, which may have something to say regarding the monophyly of the
Euhelopodidae.
Those who want figures of the material (half a
page) can contact me offlist. Dan Bensen has suggested I write about
Achillobator next. The description is sixty pages long, with good figures
so this will take a while.....
Mickey Mortimer
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