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

Re: Age of Gobi formations

Tim Williams (sirius531@yahoo.com) wrote:

<Basalt. Btw, Kurzanov mentioned eggshells identical to those from Bayn Dzak;
the Djadokhtan age isn't based solely on Pinacosaurus.>

  I'm sorry, basalt? A deep sea product of volcanism? In an apparent
tributary-fed system? On shore? I don't think so. What is the reference? As for
eggshells, well then, great evidence of oviraptorosaurs having the same egg
type, considering eggshells in the *Macroelongatoolithus* oogenus should be
virtually identical in prism structure and surface ornament with pore
diverticulation, as befits ootaxonomy practices (see Carpenter's _Dinosaur Eggs
and Babies_, a wonderful resource and textbook on the subject of ootaxonomy and

<No evidence that it did. Only the larger, more escalated ankylosaurs e.g.
Saichania, persisted into the upper Barungoyotian strata. This is true not only
at Khulsan and the red beds of Khermeen Tsav but also Toosgot, whereas
Pinacosaurus is known only from the very bottom of the local strata at Ereelhz.>

  Local restriction to identify broad speciation distribution? We need a better
corollary than this. Ukhaa Tolgod has been identified as a largely "Djadokhtan"
locality, but also includes a great majority of fossils in common with the Barun
Goyot, and may be correlated to both of them; suggestions that the Djadokhta and
Barun Goyot may have overlapped have not been responded to, as well, and the
presence of a distinct fauna shared (the so-called "Barungoyotian") versus the
Nemegt fauna belies the absence of a conformity between any level of the
Djadokhta that is securely correlated to Bayn Dzak and Khulsan (Barun Goyot type
section). Khulsan and Khermeen Tsav are both correlated vertically to underlie
the Nemegt, but neither preserve a contact for the Djadokhta, and the faunal
similarities suggest horizontal overlap within the "Barungoyotian," which is one
of the reasons they have been relegated to a faunal age. See Gao and Norell
(2001) for commentary. *Pinacosaurus* may easily occur in Barun Goyot based on
this apparent lack of unconformity, but the absence of the identification of the
older boundary for the Barun Goyot leaves the "clear" separation of taxonomic
progression a very tricky, and very speculative, issue. Similarly, and again, if
*Velociraptor* did it, why cannot any other taxon?

<The margin of error for the IGEM date was considerable but this is not true of
more recent research, which clearly establishes a late Campanian age for a
Djadokhtan locality.>

  But does not prevent younger beds from being Campanian. Even with an admission
for margins of error.

<You mean RED beds.>

  I had thought I meant white beds; my mistake. Problems with the presence of
both at Khermeen Tsav, the original identification of the Barun Goyot/Nemegt

<You mean middle Maastrichtian. The Nemegt spans an entire normal subchron and
at least parts of two reverse subchrons IIRC. The amount of time it encompasses,
together with the Barun Goyot, must be fairly substantial.>

  So if we know the temporal length by magnetic polarity in subchrons, why
cannot be relate this age more drastically? And the length of a magnetochron, as
a subchron, can be of any length, as the magnetic scales in most stratigraphic
maps will illustrate.

<Bordzongyin Gobi. Shuvalov's information is in THE AGE OF DINOSAURS IN RUSSIA

  Thanks; a book I need to buy already.

<Yes. Carr wrote: "Both species (i.e. T. bataar and T. rex) are united by
absence of a lacrimal horn, inflated descending paroccipital process,
transversely oriented occipital plate and rostrocaudally restricted basisphenoid
recess among other features." They may not be congeneric but these similarities,
together with the obvious Asia-America connections at the time e.g. Saurolophus,
certainly suggest Tarbosaurus was closest to the ancestry of Tyrannosaurus.>

  As has been noted before, however, these features also imply part of a series,
and *Daspletosaurus* has previously been united with the taxa (Paul, 1988; Carr,
1999; Currie, Hurum & Sabath, 2003) on the basis of many of these, in that the
relative reduction of the facial ornamentation and expansion of the adductor box
with more robust teeth either indicate a parallel, or a member of a series;
Hurum and Sabath (2003) similarly restored and described *T. bataar* as being
distinct from previous descriptions, as it appears to be more gracile than
*Tyrannosaurus rex,* rather than the bulky form described by Paul (1988);
several details of the skull have been redescribed (e.g., they describe the
paroccipital process as non-inflated; similarly, Currie (2003) notes that in a
skull of *Daspletosaurus* bears an internally inflated paroccipital process.
Descriptions of the basisphenoid recess are difficult due to compression, but
the basisphenoid in *D. torosus* has been illustrated by some (Bakker, Williams
& Currie, 1988) as being transversely as wide as it is rostrocaudally long, a
condition that is along the same trend. A similar basisphenoid is known in the
ambiguous theropod *Itemirus,* linked by some (Kurzanov, 1976; Molnar, Osmólska
& Kurzanov, 1990) to a possible tyrannosaur or maniraptoran affinity. This
reduces the relative use of these features as exclusive conditions, especially
given recent work on the convergence of similar head-use and biting strategy
forming similar cranial morphologies.

<Probably got them. Consider the primitive Tsagayan types e.g. Amurosaurus,
despite the putative Maastrichtian age.>

  A "primitive" species can succeed another in series that of an "advanced"
species. Note the presence of Lancian *Leptoceratops;* though recently
recognized as the outgroup to ceratopsids and putative Ceratopsomorpha (which
may as well be stem ceratopsids to some, or basal ceratopsids to others), these
forms occur in younger strata than do true ceratopsids, such as the lower
Judithian *Styracosaurus,* or the coeval *Monoclonius.* *Amurosaurus* can be a
primitive form in a young formation, its not hard. Godefroit et al. are working
on the levels as are other Russian-led people, so we will see when the dating of
these formations are resolved, probably based on foram/pollen and invert

<There were plenty of lambeosaurs in the Tsagayan and Wangshi etc so I suspect
the Nemegt documents their virtual demise. There were plenty of hypacrosaurs
alongside Saurolophus in America-B. sicinskii shows some similarity to
Hypacrosaurus btw-so the Nemegt may well represent the slightly later period in
which they waned drastically.>

   *Barsboldia* also shows similarities to *Lambeosaurus* proper, but is unique
and apparently more basal on the basis of the rather non-lambeosaur-like
distally expanded neural spines. It doesn't help that the material is not very
diagnostic when it comes to differentiating various hadrosaur groups and its
relation to them.

<But Saurolophus is known only from the Maastrichtian, and Norman wrote that the
American and Nemegtian species are "virtually identical.">

  This statement leads to circular reasoning: *Saurolophus* is only
Maastrichtian, so a *Saurolophus* species must be Maastrichtian. I am afraid
this is just not enough to say that the occurrence of *Saurolophus* means it is
a Maastrichtian bed. Eggshell aside and the presence of basalt aside,
questionable usage of which, as above, leads to suspect dating. Some note that,
apart from Norman (I assume this is in 1990? you did not provide any citations
or references), the two species are distinct enough to warrant isolated use of
the species, rather than a "lumped" genus for reference; at least George
Olshevsky would have it that these are distinct genera, whereas I think some
would feel more species should be lumped into *Saurolophus.* One man's
polyspecific genus is another man's series of monospecific genera.

<Who says Altan Ula IV is Djadokhtan?? The locality is undoubtedly Nemegtian.>

  Osmólska (1997, in Currie and Padian's Encyclopedia of Dinosaurs) has listed
the Altan Ula IV fauna as Barun Goyot, whereas the fauna was listed by Barsbold
(1997) as Djadokhta, including the "primitive" "oviraptorosaur" *Avimimus* and
the leptoceratopsid *Udanoceratops,* the former from Altan Ula and Udan Sayr and
the latter from only Udan Sayr. I personally feel the assignment is peculiar and
questionable, but I do not know the data this is based on.


  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps in
the face of adversity, that a simple skip is so hard to do.  We should all learn
to walk soft, walk small, see the world around us rather than zoom by it.

  "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)