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Astrodon johnsoni, was Sereno's (2005) definitions



  In the recent omnibus, _Thunder Lizards_ (edited by Tidwell and Carpenter),
is a paper on the tooth-based taxon *Astrodon johnsoni*.

  Carpenter, K. & V. Tidwell. 2004. Reassessment of the Early
    Cretaceous sauropod *Astrodon johnsoni* Leidy 1865
    (Titanosauriformes). pg. 78-114 in Tidwell & Carpenter (eds.)
    _Thunder Lizards: the Saurorpodomorph Dinosaurs_ [Life of the
    Past series] (Indiana University Press, Bloomington).

Abstract:
  "Sauropod material from the Arundel Formation (Aptian-Albian
   boundary) of Maryland has been variously referred to *Astrodon
   johnsoni* Leidy 1865 or to *Pleurocoelus nanus* Marsh 1888.
   Most of the specimens are juvenile as demonstrated by the small
   size of the bones, the lack of neurocentral fusion, absence of
   an olecranon, and underdevelopment of muscle scars. Contrary to
   some recent statements, the Arundel sauropod is diagnostic.
   Only a single sauropod taxon is present in the Arundel
   Formation, to which the name *Astrodon johnsoni* must be used
   under the Principle of the First Revisor of the International
   Code of Zoological Nomenclature."

  This paper asks the question, is *Astrodon johnsoni* Leidy 1865 valid?
Johnson himself (Johnson, 1859) proposed the name, but did not provide a
discussion of the specimens in any detail, and the name is essentially a nomen
nudum. Leidy took Johnson's name and converted it into a binomen. Carpenter and
Tidwell consider that Hatcher's original thesis, as the first revisor of the
taxon *Astrodon* is strong enough to consider his hypothesis that
*Pleurocoelus* was a synonym to be strong enough to argue for synonymy here.
The argument essentially regards the likelihood of dental similarity and the
presence of more than one sauropod in a single formation to preclude the
possibility of more than one sauropod in the Arundel. 

  This is, of course, some surprise to the sauropods from, say, Tendaguru or
the Morrison Formation, but the sauropod material similarly also nearly all
derives from the Muirkirk quarry, near Muirkirk, Maryland, while the types
apparently derive from Bladensberg, Maryland. The types are very large, and
show a distinct marginal wear facet. Referred teeth include many smaller,
possibly juvenile (see abstract) crowns, and with varying conditions of wear.
In fact, the type crowns have wear facets indicating imbrication (teeth occlude
between one another), while other crowns have either no wear, marginal wear, or
distal, apical wear, including producing a star-shaped wear facet. 

  Furthermore, the authors state "Isolated teeth from Utah and Texas have been
referred to *Astrodon* (Cifelli et al. 1997), although this is probably
incorrect because similar teeth occur in other taxa." (pg. 78) If this is true,
why should we be confident all the teeth from the Arundel are even consistent
with a single taxon? Of particular curiousity is that the type provenance of
the Bladensberg teeth, the type, are in fact unknown, and are simply reported
as deriving from Arundel deposits in Bladensberg. Is this true? How can we
know?

  Much work is done on the first revisor principle of the ICZN, begining with
the statement (pg. 79) that "Hatcher also notes that there was no evidence to
indicate the presence of more than one species of sauropod, a point on which we
concur." However, while I would agree we would need discrete skeletal
repitition to argue for more than one taxon (i.e., two humeri with distinct
morphologies), we cannot use a group of sauropod bones to infer a single taxon,
either, though we can propose the inference that the various skeletal remains
may belong to the same taxon, to prove it you should ideally have associated or
articulated remains, while much of the material from Muirkirk has been found
isolated and it's use in a single skeleton is extremely tricky, since much of
it also appears to be immature, juvenile, or subadult, and thus in various
states of growth that question its reliability to cohesively refer unassociated
remains to one another. 

  At the Morrison quarries and the Tendaguru quarries, we were fortunate to
find many associated skeletons, but this is simply not the case here, and the
issue must weigh on taxonomy, regardless of what Hatcher believed. Gilmore
followed Hatcher largely in recognizing all the species, *nanus*, *altus* and
*johnsoni* to belong equally to *Astrodon*, which may have more support in
using the genus name to contain the theory of skeletal inference. The lack of a
uniform generic diagnosis on this basis is especially difficult on this matter.


  Carpenter and Tidwell use the first revisor principle, which argues that
precedence or priority is fixed by the first person to revise the use or
diagnosis of the taxon, to argue that *Astrodon* is a senior synonym of
*Pleurocoelus* since the two were considered by others to be synonymous.
However, this principle does not argue that apomorphies of the type specimens
of each synonymized taxa are existant among all taxa, as in fact they are not.
The lack of teeth in the two syntype series Marsh used for his species of
*Pleurocoelus* render the ability of inference of synonymy questionable. I
would thus propose that *Astrodon* is a nomen dubium and should not be used as
a senior synonym of any taxon, even if they bore teeth in their type specimen,
which is particularly important as there are additional taxa known which bear
the same form of dentition. Additionally, *Pleurocoelus* should be treated as
separate and likely not a nomen dubium if it is possible, as Carpenter and
Tidwell argue, that the material from Muirkirk are diagnostic of a taxon
(partial crania, vertebrae, limbs and girdles).

  The authors further argue (pg. 82) that they "are cognisant that this
descision will not be unversally accepted, but not to accept *Astrodon
johnsoni* as the valid name will upset nomenclature stability because of the
implication iut would have that more than one species of sauropod is present in
the Arundel Formation." Though the authors did not consider more than one
possible sauropod available, this does not mean there were not more than one,
and the taxonomic instability that may result from associating synonyms of
non-dental material to tooth-based types is highly questionable and hardly
worth a compromise of assuming that there is only one Arundel sauropod. We find
already that the Arundel likely appeared to be a Cloverly-like formation which
has already produced more than one type of sauropod, several large theropods,
small theropods, ornithischians both large and small, etc., which argues for a
potential similar arrangement of taxa could be present in the Arundel (Ostrom,
1970). 

  Thus the burden of proof for synonymy is very high, and assuming simplicity
in favor of many taxa does not seem to work for the authors' purposes, save
possibly to reduce the taxonomic lists, or rescue *A. johnsoni* from the
dustbins. On the contrary, I argue here that this is precisely where *A.
johnsoni* belongs:

  1. The provenance of the teeth is not known, though assumed to
     derive from the Arundel and recovery from Bladensberg,
     Maryland.

  2. The syntype series of both *Pleurocoelus altus* and *P.
     nanus* lack dental material, thus making them impossible to
     compare directly to *A. johnsoni*.

  3. The argument that only one sauropod is likely from the
     Arundel is considered problematic given the Morrison,
     Tendaguru, and Cloverly sauropod faunae.

  Cheers,

Jaime A. Headden

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


                
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