[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Euhelopodidae



Nick Gardner wrote-

> Dino_Rampage wrote-
> >1) What is the status of the Euhelopodidae? Is it considered basal to the
> >Neosauropoda? And are Euhelopus, Omeisaurus & Mamenchisaurus still
> >considered a monophyletic clade? Would Shunosaurus be considered closely
> >related to, or part of Euhelopodidae?
>
> There seems to varying ideas as to its monophyly.

Well, not really.  Though Upchurch placed Shunosaurus and omeisaurids in the
Euhelopodidae in the past (1995, 1998), more recently he agrees with Wilson
(Upchurch and Barrett, 2001) in the main areas of their polyphyly.
Shunosaurus basal to cetiosaurids, and Euhelopus more derived than
omeisaurids.  However, they have Mamenchisaurus slightly more derived than
Omeisaurus, and Euhalopus sister to Lourinhasaurus + Neosauropoda (not as a
titanosaur).
Wilson (2002) did a great job arguing against the results of earlier
analyses (Upchurch 1995, 1998; Wilson and Sereno, 1998), so the latter
really need not be mentioned as viable alternatives.  Sereno's 1999 topology
was based on the Wilson and Sereno paper mentioned above and Pisani's
supertree was, well... a supertree.

> Upchurch (1995, 1998, 1999) supported a monophyletic Euhelopodidae (with
> Datousaurus, Euhelopus, Mamenchisaurus, Omeisaurus, & Shunosaurus)  does
> not.
>
> Refer to http://www.cmnh.org/dinoarch/1999Oct/msg00060.html for a listing
of
> the characters.  They appear to be often acquired in other clades.  Maybe
> someone might comment on that post.

No one better than Wilson himself, so here are his thoughts on these
characters from his 2002 paper-

""Caudal end of prefrontal in dorsal view . . . is acute, subtriangular, and
inset into the rostrolateral corner of the frontal" (character 28).
Upchurch scored 'euhelopodids', Rebbachisaurus (based on Rayososaurus), and
diplodocids with the derived state for this feature (character CI =0.33).
The prefrontal of Diplodocus and Apatosaurus does have an unusual
posteromedially oriented hook at its posterior extreme (Berman & McIntosh,
1978: fig. 3a, d), but this feature characterizes neither 'euhelopodids' nor
Rayososaurus. Published dorsal views of the skull of Omeisaurus indicate
that the prefrontal is rounded posteriorly, without any trace of the hook
that characterizes diplodocids (He et al., 1988: figs 8, 9). Similarly,
based on published illustrations and personal observations, the condition in
Shunosaurus appears primitive (Zhang, 1988: fig. 8). Euhelopus was scored on
the basis of an element that Mateer & McIntosh (1985: fig. 1C, D) identified
as a conjoined frontal and prefrontal. This element, however, is probably
incorrectly identified, as the frontal portion of the element has neither
the roughened orbital margin nor the anterolaterally orientated ventral
ridge that forms the inner margin of the orbit (pers. observ.). The presence
of a hooked prefrontal is restricted to the diplodocids Apatosaurus and
Diplodocus."

"Thirteen or more cervical vertebrae (character 76).
Upchurch (1998) assumed ordered changes for all vertebral count characters
and resolved the presence of 13 cervical vertebrae as diagnostic for
'Euhelopodidae'.  This is despite the fact that only Shunosaurus was scored
as possessing 13; Omeisaurus, Mamenchisaurus, and Euhelopus were scored as
having 17.  Ordering of this and other vertebral count features predispose
them to act as synapomorphies for taxa that do not share the same character
state. In addition to dependence on a particular coding assumption, two
other considerations weaken support for increased cervical count being
unique to 'euhelopodids'. First, presence of 13 cervical vertebrae is an
ambiguous synapomorphy
of 'euhelopodids' because vertebral counts are not known for the basal
sauropods Vulcanodon,
Barapasaurus, Patagosaurus, and Cetiosaurus. Therefore, an increase to 13
cervical vertebrae could have arisen in more basal nodes on the cladogram.
Second, the presence of 13 or more cervical vertebrae characterizes nearly
all known sauropods and has been considered to be the primitive condition
for Eusauropoda (Wilson & Sereno, 1998: fig. 47). Of the sauropods scored by
Upchurch, only Camarasaurus was scored as primitively lacking 13;
'euhelopodids', Brachiosaurus and diplodocids were scored as derived, and
all other sauropods were scored as unknown. Had other derived sauropods been
scored appropriately (e.g. Haplocanthosaurus, Hatcher, 1903), the presence
of 13 cervical vertebrae would be resolved as primitive for Eusauropoda.
Based on its ambiguous character distribution among basal sauropods,
generality within more derived sauropods, and dependence on an ordered
coding strategy, presence of 13 or more cervical vertebrae cannot be held as
a 'euhelopodid' synapomorphy.
Presence of 17 cervical vertebrae (characters 77-79), however, is unique to
Omeisaurus, Euhelopus,
and Mamenchisaurus, regardless of coding strategy.  Although some argue that
Omeisaurus has only 16
(e.g. McIntosh, 1990), a large increase in the number of cervical vertebrae
can be regarded as a potential
synapomorphy of this 'euhelopodid' subgroup."

""Height : width ratio of cranial cervical centra . . . is approximately
1.25" (character 85).
This is the only feature clearly shared by Shunosaurus, Omeisaurus,
Euhelopus and Mamenchisaurus to the exclusion of other known genera. The
distribution of this character amongst other basal sauropods (e.g.
Barapasaurus, Vulcanodon, Patagosaurus), however, remains unknown."

"'Centroparapophyseal lamina' present on middle and posterior dorsal
vertebrae (character 105).
There are two laminae that may connect the centrum and parapophysis: one
projects forward from the parapophysis to the anterior portion of the
centrum (anterior centroparapophyseal lamina, acpl), and the other projects
backward to the posterior portion of the centrum (posterior
centroparapophyseal lamina, pcpl).  Upchurch (1998: 60) states that this
lamina "supports the parapophysis from below and behind", identifying it as
the pcpl. This feature was scored as derived for 'euhelopodids' and all
neosauropods except Camarasaurus (character CI =0.33). Salgado et al. (1997:
19) list the presence of a pcpl as a synapomorphy of Titanosauria,
contending that it is absent in all other sauropods. Wilson (1999a)
reevaluated the distribution of vertebral laminae in sauropods, and found
that the pcpl characterized all titanosaurs (as stated by Salgado et al.,
1997), as well as Brachiosaurus (Janensch, 1950: fig. 53), Euhelopus (Wiman,
1929: pl. 3, fig. 4.; pl. 4, fig. 2), Apatosaurus (Gilmore, 1936: pls. 25,
33), and Diplodocus (Osborn, 1899: fig. 7). No pcpl was identified in
Shunosaurus, Dicraeosaurus, or
Omeisaurus from the figures in Zhang (1988), Janensch (1929b), and He et al.
(1988), respectively.
Wilson (1999a) considered presence of a pcpl an unambiguous synapomorphy of
Titanosauriformes, independently acquired in diplodocids."

"Size of the 'cranial process' (characters 147-8) and presence of a 'ventral
slit' (character 149) in middle and distal chevrons.
All are homoplastic but unambiguous synapomorphies of 'Euhelopodidae'.
Coding for the characters 148 and 149 is identical, and character 147
differs from these two only in coding Camarasaurus derived. Character 147
codes the presence of the 'cranial process', whereas the second (character
148) codes a "prominent cranial process resulting in craniocaudal length of
the chevron greatly exceeding its height."
Together, these two binary characters act as an ordered three-state
character (Table 6). Presence of an
enlarged cranial process and a ventral slit are surely independent, despite
their identical codings. Other
dinosaurs have chevrons that have anteriorly and posteriorly elongate blades
but lack a ventral 'slit' (e.g. Deinonychus; Ostrom, 1969: fig. 41).
Upchurch's treatment of negative evidence for all three characters is
problematic. For example, Patagosaurus, Cetiosaurus, Brachiosaurus, and
Haplocanthosaurus were scored as primitive for all three characters, but
distal tails are not known for any of these taxa. In some taxa, only the
distal tail bears chevrons with cranially directed processes
(e.g. Camarasaurus, Gilmore, 1925: pl. 14). Isolated chevrons of
Barapasaurus are forked and have a
ventral slit (pers. observ.), but the distribution of caudals that have this
type chevron is unknown. Scoring Barapasaurus with the derived condition and
sauropods lacking distal tails as unknown resolves this feature as a basal
sauropod synapomorphy that was reversed in Titanosauriformes. Upchurch
(1998: 87) mentions this possibility."

"'Parasagittally elongate ridge on dorsal surface of the cranial end of the
sternal plate' (character 157). This is the second of two features that
Upchurch (1998) resolved as unambiguously unique to 'euhelopodids'.  Unlike
the other unambiguous 'euhelopodid' synapomorphy (character 85: height/width
ratio of cranial cervical centra), however, this feature cannot be scored in
Euhelopus, Mamenchisaurus, or in the basal sauropods Vulcanodon and
Barapasaurus. Moreover, no 'longitudinal ridge' could be identified from
figures of Omeisaurus (He et al., 1988: fig. 42) or Shunosaurus (Zhang,
1988: fig. 44). However, both have a small prominence at the anterior
extreme of the sternal plate. This prominence is present in most sauropods
(e.g. Apatosaurus [Marsh, 1880]: fig. 2B and Alamosaurus
[Gilmore, 1946: pl. 9]) and may represent a syna-pomorphy of Eusauropoda."

Mickey Mortimer