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Details on Mamenchisaurus youngi



This time we look at Mamenchisaurus youngi, asked for by Timothy Williams.

Mamenchisaurus Young 1954
M? youngi Pi, Ouyang and Ye 1996
= Mamenchisaurus "yangi" anonymous 1993
= Mamenchisaurus "youngsi" Ouyang 1996
etymology- "Young's Mamenchi lizard", Young being the original describer of
the genus and Mamenchi being a ferry that was close by on the Yangtze River.
Late Jurassic?
Sichuan?, China
holotype- (ZDM 0083) (19-20 m) skull (510 mm), lower jaw (470 mm), eighteen
cervical vertebrae (seventh 411 mm), cervical ribs, twelve dorsal vertebrae
(twelfth 200 mm), seven dorsal ribs, sacrum, eight proximal caudal vertebrae
(fourth 224 mm), three proximal chevrons, six mid caudal vertebrae,
scapulocoracoids, humeri (~770 mm), radii (~500 mm), ulnae (~540 mm), mani,
ilium, pubis, ischia, femur, tibiae (one proximal) (~590 mm), fibula (~590
mm), pes
Diagnosis- pronounced anterior process on maxilla; squamosal and
quadratojugal contact (also in Camarasaurus).  Also see below for
differences from other Mamenchisaurus species and what the implications are.
Description-
This sauropod is described only in Chinese, but has a few figures
representing the skull, skeleton, cervical vertebra, dorsal vertebra and
caudal vertebra.  Comparing the presacral length to M. hochuanensis gives a
total length of 19-20 meters.
The skull is very similar to Euhelopus.  Like this genus and M.
jingyanensis, the premaxilla has a laterally facing subnarial foramen and
unreduced subnarial process, the last two characters excluding them from the
Neosauropoda.  The naris is 108% of orbital length (or 37% of skull length,
going by Upchurch's characters), a supposed macronarian character (which M.
jingyanensis lacks).  The squamosal and quadratojugal clearly contact,
otherwise only known in Camarasaurus (Wilson and Sereno, 1998).  There are
22-24 dentary teeth, compared to 19 in M. sinocanadorum and 20 in M.
jingyanensis.  The external mandibular fenestra is plesiomorphically
present, as in other Mamenchisaurus species and unlike neosauropods.  The
surangular is more than twice as deep as the angular, another supposed
macronarian character absent in the other two species, but probably present
in Euhelopus.  Besides the above mentioned characters, M. youngi differs
from Euhelopus in the longer snout, maxilla with pronounced anterior
process, twice as many maxillary teeth (18 vs. 9), more elongate antorbital
fenestra, pointed posterior postorbital process, anteriorly expanded dentary
and low coronoid process of the lower jaw.  M. jingyanensis is a much more
plesiomorphic species than M. youngi, with a naris not extending above the
orbit, robust postorbital, elongate main body of jugal, infratemporal
fenestra that only extends anteriorly to the middle of the orbit and
external mandibular fenestra twice as long, in addition to characters noted
above.  The jaw of M. youngi is very similar to M. sinocanadorum, besides
the surangular-angular proportions noted above and the reduced external
mandibular fenestra.  Comparison with M. fuxiensis is limited, but M. youngi
would appear to have more maxillary and dentary teeth (12-14 and 15-17
respectively for M. fuxiensis).
There are eighteen cervical vertebrae with opisthocoelous centra and divided
pleurocoels.  The neural arches are deeper than the centrum, unlike
Omeisaurus, and the cervical ribs are at least three centra long.  There are
twelve dorsal vertebrae, the last contains a pleurocoel and is strongly
opisthocoelous (which is a macronarian character).  There are five sacral
vertebrae, which is more than M. hochuanensis and M. fuxiensis.  The
anterior caudal centra are procoelous, like other Mamenchisaurus species and
the anterior chevrons have bridged articulations, unlike macronarians (and
Shunosaurus and Haplocanthosaurus).
The metacarpus is about 38% of radial length, so less than macronarians.
The metacarpals appear to be bound, which Sereno and Wilson used as a
neosauropod character.  The femur is partially covered by the ilium, so
humerofemoral values are not available.
Relationships-
This is complicated by the fact that Sereno and Wilson (1998) and Upchurch
(1998) have different opinions as to the relationship of Chinese sauropods.
Sereno and Wilson have Shunosaurus very basal, Omeisaurus (and
Mamenchisaurus) right below the Neosauropoda and Euhelopus as a titanosaur.
Upchurch has these genera in a single basal family, the Euhelopodidae.  I'm
currently combining aspects of the two analyses to see which view is
supported by more evidence, but several interesting observations can be made
when comparing M. youngi with M. jingyanensis and Euhelopus.  M.
jingyanensis is the most plesiomorphic species, based on a small external
naris anterior to the orbit, infratemporal fenestra whose anterior margin
lies only below the midpoint of the orbit, surangular and angular subqual in
height and large external mandibular fenestra.  M. sinocanadorum still has
the last two characters.  M. youngi shares with Euhelopus a naris extending
above the orbit, an infratemporal fenestra extending anteriorly almost to
the rostral border of the orbit, a reduced external mandibular fenestra and
surangular much deeper than angular.  Does this mean Mamenchisaurus is
paraphyletic?  To complicate matters further, Datousaurus lacks a mandibular
fenestra, but is plesiomorphic otherwise while Abrosaurus exhibits all but
the first derived character.  Shunosaurus and Omeisaurus are plesiomorphic
in all listed characters.  Thus, I think that based on skull characters,
Shunosaurus, Omeisaurus, Datousaurus, M. jingyanensis and M. sinocanadorum
are more plesiomorphic than a group containing M. youngi, Euhelopus and
Abrosaurus.  M. youngi and Euhelopus share nares that reach over the orbit,
while Euhelopus and Abrosaurus share a high coronoid process.  Then again,
Abrosaurus has only thirteen cervical vertebrae, which would place it
outside of the Omeisaurus, Mamenchisaurus, Euhelopus group.  What
complicates this even further is the fact that M. constructus and M.
hochuanensis lack skull material, while I lack a figure of M. fuxiensis's
skull.  M. hochuanensis and M. fuxiensis have only four sacral vertebrae,
however, unlike M. youngi and Euhelopus.  Also, regarding Euhelopus's
placement as a macronarian by Sereno and Wilson, M. youngi shows several
macronarian characters (large naris, deep surangular, strongly
opisthocoelous posterior dorsals) and lacks others (bridged anterior
chevrons, short metacarpus), while lacking several neosauropod characters
(large subnarial premaxillary process, laterally facing subnarial foramen,
open mandibular fenestra) and exhibiting others (infratemporal fenestra
extending far anteriorly, five sacral vertebrae, bound metacarpus?).  I
think that until a major study is done utilizing both Sereno and Wilson's
and Upchurch's analysis, M. youngi should be only tentatively assigned to
Mamenchisaurus, and the case for Euhelopus convergently acquiring
macronarian-like states is strengthened.
Reference- Pi, Ouyang and Ye, 1996. >unknown title<. in Papers on
Geosciences
Contributed to the 30th International Geological Congress, pg. 87-91.

The skull of M. youngi is still available, as is the skeleton and figures of
the seventh cervical, twelfth dorsal and fourth caudal.
That was more work than I was expecting!  Thanks to those of you who have
been requesting "Details on" posts.  Next, I'll cover some dromaeosaurs.

Mickey Mortimer