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In PALAIOS ... Taphonomy of Two Western Hemisphere Assemblages



  Haven't seen these mentioned on the list yet (Palaios is a little out of
the way in access, I guess), so here's the papers:

Rogers, R.R.; Arcucci, A.B.; Abdala, F.; Sereno, P.C.; Forster, C.A.; and
May, C.L. 200 . Paleoenvironment and Taphonomy of the Chañares Formation
Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular
Preservation in Volcanogenic Concretions. _Palaios_ 16(5): 461?481.

Abstract:

"The enigmatic concretionary exposures that typify the Chañares Formation
(Ladinian, northwestern Argentina) long have defied precise
paleoenvironmental characterization. Recent work indicates that the
formation accumulated in an alluvial-to-lacustrine setting within an
active rift basin that received sedimentary detritus from surrounding
highlands, as well as copious amounts of volcanic ash. Ash- flow sheets
were emplaced presumably as secondary mass flows on alluvial surfaces
characterized by small fluvial channels and shallow lakes. Thin bentonite
beds intercalated in the Chañares Formation indicate that ash also
accumulated via direct airfall, although this mode of emplacement accounts
for a very small fraction of the overall section. A shift to widespread
lacustrine deposition is recorded by the superjacent Los Rastros
Formation, which preserves at least six shallowing-upward hemicycles, five
of which commenced amidst explosive volcanic activity as evidenced by
intercalated bentonite beds.

Volcanism played an important role in the generation and preservation of
the Chañares Formation's exceptional tetrapod fossil record. This is
especially true of the classic Los Chañares locality, where more than 100
individuals representing a diverse array of taxa (archosaurs, cynodonts,
dicynodonts) are entombed in volcanogenic concretions with matrices of
relic glass shards diagenetically replaced by calcite. Taphonomic
attributes of the Los Chañares locality are consistent with the scenario
of mass mortality, and several clues hint at the nature of the event. The
killing agent was lethal to a variety of taxa, killed both adults and
juveniles, and led to the concentration of taxa that under normal
circumstances would tend to dissociate, such as carnivores and their
potential prey. It also produced a counterintuitive bias against the
preservation of large-bodied taxa, which may have been largely
unsusceptible to the death event, or perhaps were excluded from the Los
Chañares death assemblage via post-mortem sorting. The spatial arrangement
of skeletal material in a small sample of concretions is consistent with
the stranding of tetrapod carcasses along a strandline, and it is feasible
that volcanism led to catastrophic flooding of the landscape via damming
and/or diversion of local drainages. Uncompacted skeletal elements and
relic outlines of glass shards indicate that carbonate concretions formed
shortly after skeletal material was buried in reworked volcanic ash. The
microbial decay of organic matter presumably catalyzed concretion
diagenesis. There is no indication that bone hydroxyapatite diffused into
the entombing glassy matrix and contributed to concretion formation. Bones
entombed within early diagenetic concretions were safeguarded from
subsequent destructive pedogenic and/or diagenetic processes, and were
incorporated in exquisite quality into the fossil record."

----

Ryan, M.J.; Russell, A.P.; Eberth, D.A.; and Currie, P.J. 200 . The
Taphonomy of a Centrosaurus (Ornithischia: Certopsidae) Bone Bed from the
Dinosaur Park Formation (Upper Campanian), Alberta, Canada, with Comments
on Cranial Ontogeny. _Palaios_ 16(5): 482?506.
 
Abstract:

"Bone bed 43 is one of at least eight paucispecific Centrosaurus bone beds
located in the Dinosaur Park Formation (Upper Campanian) in Dinosaur
Provincial Park, Alberta, Canada. It long has been used as a case example
for evidence of herding and social behavior in dinosaurs, but a detailed
analysis of the site has not been presented until this study. The bone bed
is dominated by the disarticulated, mostly fragmentary and slightly
abraded remains of Centrosaurus apertus, with minor occurrences of other
taxa, notably teeth from the large tyrannosaurid Albertosaurus libratus.
Fossils occur in a stacked to amalgamated succession of lag deposits,
deposited and reworked at the erosional base of a paleochannel. The most
parsomonious scenerio suggests that Centrosaurus material represents part
of a large aggregation of animals (possibly numbering in the thousands)
that died by drowning on the alluvial plain. Disarticulation occurred at a
point upriver from the bone-bed site. Scavenging by theropods, primarily
Albertosaurus, at or near the original site of death is suggested by the
high number of shed theropod teeth. A subsequent event prior to
fossilisation moved the material to its present location removing many
juvenile-sized and hydrodynamically light elements from the original death
assemblage. Evidence for distinct size classes amongst the preserved
elements is not supported by the data, but the size range of elements
preserved are representative of living individuals that would have ranged
from small juveniles to mature adults. The large data base of specimens
from bone bed 43 allows for the illustration of the ontogenetic changes
that occurred in the diagnostic cranial elements of Centrosaurus."

=====
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.

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