Re: Journal of Vertebrate Paleontology 24(2)

["Jaime A. Headden" <qilongia@yahoo.com>]

Chris Taylor (ck.taylor@auckland.ac.nz) wrote:

<A detailed comparison of the cranial and mandibular anatomy of these
species indicates different feeding adaptations.

[...]

The second paper mainly deals with the skull (_Thalassocnus_ gives us a
nice succession of taxa from about the same place, separated in time, each
showing more adaptations for marine life than its predecessors, perhaps
one of the best and straightforward evolutionary transitions series to be
found in vertebrates).>

  And they say evolution can't be observed. Well, along with some
subjective input, it can in this case. I love these papers because they
describe the series of transformations, primarily in the skull, of a group
of sloths. De Muizon et al. describe the third, and youngest (and largest)
species to date, *T. yaucensis,* which shows the broadests snout and most
"spout-like" mandibular symphysis of *Thalassocnus* to date. Seems the
more aquatic the species, the larger and more broader the snout: perhaps
better for grazing than otherwise. The snout lacks the incisive foramen
between premaxillae in larger species, and the mandibular symphysis is
larger, indicating possible greater stresses to the jaw. This appears
similar and convergent in sirenians, and may indicate a broad fleshy
snout, rather than the narrow "muzzle" so far envisioned for sloths.

  *Thalassocnus* is refered to a new "subfamily" Thalassocninae, under
Nothrotheriidae, whereas other nothrotheres are considered
Nothrotheriinae. I am waiting to see if the cranial variation will be
described as potentially variable among species or "genera," as more
splitting has occured for less variation in the ankle and manus of sloths
than just five species of a single "genus." As de Muizon et al. move
higher in the stratigraphic column, it is more or less likely that
ancestral "thalassocnines" will be found, showing more regular
nothrotheriine proportions and cranial features. Why these are not
considered derived from other nothrotheriines, I do not know, but
apparently they split off, as in the split of the Nothrotheriidae into
sister subtaxa at the same point. The most basal nothrotheriid,
*Pronothrotherium,* is considered only a basal nothrotheriine due to this
paraphyletic schizm. Instead of applying Gaudin's older use of
Nothrotheriidae, de Muizon et al. apply the "elevation" of Ameghino's
Nothrotheriinae to -idae as "new." This was done despite a) low diversity
of the Megatheriidae or b) monotypy of the new 
"subfamily" Thalassocninae, reducing the effective use of the term when
*Thalassocnus* supports the same information. Applying this new specific
information with Gaudin's recent (2004) phylogeny based on the
craniodental information:

--Megatherioidea (sensu Gaudin)
   |--Megalonychidae
   `--Megatheria
       |--Megatheriidae (sensu Gaudin)
       `--Nothrotheriidae (sensu Gaudin)
           |--Nothrotheriinae
           |   `--+--Pronothrotherium
           |      `--+--Nothropus
           |         `--+--Nothrotheriops
           |            `--Nothrotherium
           `--Thalassocninae
               `--Thalassocnus

  As one may note, use of the term "Megatheria" _within_ Megatherioidea
and inclusive of two "families" in place of what traditional systems would
call a "superfamily" position is at odds with de Muizon et al. using such
labels for their taxa; de Muizon et al. (2002) only differ in placing
*Nothropus* and *Pronothrotherium* into a monophyletic group apart from
the "nothrotherins." One could also include under Nothrotheriidae the
"Schismotheriinae," including *Schismotherium* and *Analcimorphus* (de
Muizon et al., 2004), as well as *Hapalops,* *Synhapalops* and
*Diheterocnus* (which Gaudin, 2004, found close in relationship but
outside of the *Megalonyx* + *Megatherium* + *Nothrotherium* clade,
between them (Megalotherioidea) and Mylodontidae/Mylodontoidea. However,
de Muizon et al. refer to de Muizon and McDonald (1995) in supporting a
closer relationship between Nothrotheriidae and Megalonychidae, and
explain this reasoning thus:

  "We also propose that they are more closely related to the megalonychids
than to the megatheres, because they do not share any of the unique
derived characters present in the Megatheriidae (McDonald and Muizon, 1995
and in prep.)."

  So we must wait for the answer to "why?" apart from the paper first
describing *Thalassocnus.* And why am I spending so much time spending
this post on mammals? Well, for one thing, I think as do other
sloth-fanatics, that sloths just rock. Second, this has phenetic
connotations on arbitrary elevation of nomenclature for the sake of
preserving rank-based ideas of differentiation. As described before,
Linnaean taxonomy separates, rather than conjoins. De Muizon et al. regard
this grouping similar to Gaudin (2004), but there are some substantial
differences in application of nomenclature:

SUPERFAMILY Mylodontoidea
SUPERFAMILY  Megatherioidea (sensu de Muizon and McDonald, 1995)
  FAMILY  Megalonychidae
  FAMILY  Nothrotheriidae 
     SUBFAMILY  Nothrotheriinae
       Pronothrotherium
       Nothropus
       Nothrotheriops
       Nothrotherium
     SUBFAMILY  Thalassocninae
       Thalassocnus
  FAMILY  Megatheriidae

  So why can't *Thalassocnus* be the basal member of Nothrotheriidae? Why
does it need it's own subfamily? Phyletic reasoning says that species that
fall outside an implied ingroup should be equal in "rank" so that they can
be accessed easily by intuitive means. It's different, so it's got it's
own subfamily. Note that while most recent phylogenies place extant
*Bradypus* outside of all other sloths, as the most basal member, such
"traditional" treatment requires there to be both a FAMILY Bradypodidae
(there are no other bradypodids but *Bradypus*) and a SUPERFAMILY
Bradypodoidea, despite there being only one family. This is redundant, but
"required," nomenclature, based on ideas of phenetic distance. The same is
true for *Thalassocnus,* apparently. This is supported by some on recent
bird phylogenies (Gerald Mayr is a practitioner of "traditional" taxonomy
as the naming of families and orders for single "genera").

  de Muizon, C. & McDonald, H.G. 1995. An aquatic sloth from the Pliocene
  of Peru. _Nature_ 375: 224?227.

  de Muizon, C.; McDonald, H.G.; Salas, R.; & Urbina, M. 2002. A new early
  species of the aquatic sloth *Thalassocnus* (Mammalia: Xenarthra) from
  the Late Miocene of Peru. _Journal of Vertebrate Paleontology_ 23 (4):
  886-894.

  de Muizon, C.; McDonald, H.G.; Salas, R.; & Urbina, M. 2004. The
youngest
  species of the aquatic sloth *Thalassocnus* and a reassessment of the
  relationships of the nothrothere sloths (Mammalia: Xenarthra). _Journal
  of Vertebrate Paleontology_ 24 (2): 387-397.

  Gaudin, T.J. 2004. Phylogenetic relationships among sloths (Mammalia,
  Xenarthra, Tardigrada): The craniodental evidence. _Zoological Journal
of
  the Linnaean Society_ 140: 255-305.

  Cheers,


=====
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)


                
__________________________________
Do you Yahoo!?
Take Yahoo! Mail with you! Get it on your mobile phone.
http://mobile.yahoo.com/maildemo