Vertebrate taphonomy in loess-palaeosol deposits: A case study from the late Miocene of Central Argentina.

CLAUDIA MONTALVO,; RICARDO NESTOR MELCHOR; GRACIELA VISCONTI,; ESPERANZA CERDEÑO

Barcelona, España

Congreso; 2nd International Meeting TAPHOS'05 / 4ª Reunión de Tafonomía y Fosilización; 2005

Universitat de Barcelona

Taphonomic studies of Tertiary continental vertebrate assemblages in Argentina are scarce. This contribution presents the results of a taphonomic study on a section of the Cerro Azul Formation exposed at the town of Telén (36º15’13’’ S, 65º30’41’’ W), in the north of La Pampa Province, central Argentina. The outcrops of the Cerro Azul Formation are scattered over the central and eastern areas of La Pampa. This is a continental unit mainly composed of loess and paleosols, yielding a very rich continental vertebrate fauna consisting mainly of mammals of late Miocene age (Goin et al. 2000). The sedimentological section studied is 6 m thick and shows an homogeneous lithology composed of massive siltstones and fine-grained sandstones with pervasive micritic cementation, pedogenic slickensides and clastic dikes. Two stacked paleosols (about 1.7-1.9 m thick) interpreted as calcic vertisols are separated by 2 -7 cm thick, discontinuous beds of laminated, mudcracked and grayish pink marl. Paleosol profiles show poor horizonation, although the upper preserved horizon of each paleosol shows decreasing amounts of carbonate cementation, a sharp drop in the intensity of the magnetic susceptibility profiles, fine prismatic peds, more clayey composition, and contains fossil bee cells (Celliforma isp.). Associated micromorphological features are root traces, pendant hypocoatings, calcite nodules, pellets and septaria. The lower horizon displays irregular, dominantly vertical carbonate concretions (possibly rhizoconcretions) and rare iron glaebules. It is interpreted that sedimentation was dominantly eolian in an open low-relief setting under a semi-arid and seasonal climate. However, it is envisaged that secondary contribution by sheetflooding is possible. A seasonal climate is required for the pervasive development of the pedogenic slickensides common in vertisols. In addition, the interval of mudcracked and laminated marl and the iron glaebules suggest periods of positive hydrologic balance. In contrast, the presence of carbonate in the section indicates limited rainfall because soil carbonate generally does not accumulate in areas where rainfall excedes 1 m/yr. The faunal assemblage includes reptiles (turtles and lizards), unidentified birds, and 44 mammal taxa (marsupials, rodents, xenarthrans, litopterns, notoungulates, and a carnivore). The studied sample, recovered from an area of 48,000 m2, comprises 11,647 remains, of which 5,598 were anatomically and taxonomically identified. The collected fossils were randomly scattered over the outcrops with a low density of remains and lacking preferential orientation. There are 1,300 disarticulated postcranial elements, 2,894 xenarthran carapace scutes, five turtle scutes, and 1,399 cranial elements (including isolated teeth) of different mammals and Teiidae reptiles. Most specimens correspond to small individuals, less than 5 kg of estimated body mass (29 taxa), with a MNI of 246; in contrast, macromammals (18 taxa) have a MNI of 27. Among microvertebrates, 81.3% of individuals are grouped in six taxa of rodents and notoungulates. Except for the abundance of lower jaws, the relative abundance of other skeletal remains is very low, appendicular skeletal bones are very scarce, and mainly correspond to those of microvertebrates. The articulation degree between skeletal elements is very low (0.25%) and it is limited to some carapace scutes and autopodial bones. Most remains are broken, only some more resistant bones, such as metapods and phalanxes, are complete. There is a correlation between the weathering degree and the specimen size. Among the whole macromammal sample, only a few specimens (1.40%) show weathered bone areas. Bones are fully weathered as they exhibit loss of compact bone and are intensively cracked and flaked. Cranial and postcranial micromammal remains are better preserved than those of macromammals, and evidence for modifications on bone surface is generally related to other processes, such post-burial corrosions. Tooth marks are observed on 0.85% of microvertebrate remains. On the skulls, the loss of the occipital region, the dorsal plate and the zygomatic arches are frequent, the half maxilla being an abundant element. Lower jaws do not preserve the articular area or the ascendant ramus. Among appendicular skeletal bones epiphyses are frequent. The abundance of specimens with transverse smooth fractures on mineralized bones (considered post-burial fractures) makes it difficult to evaluate the breakage degree, but a pattern does exist. Corrosive activity of pedogenic origin is very strong; in 27.06% of the cases it is attributed to chemical action of plant roots. Surface marks attributed to lichens were also observed on 2.69% of remains, which imply that they were reworked. Calcareous rhizoliths are present on 0.45% of elements; 17.51% have adherent calcareous concretions or are included within them, and 1.32% of the remains have a fine micrite coating. The most frequent modification that affects fossils is black stains of manganese oxide mineralization, which are slight in 16.20% (most macromammal elements), moderate in 32.74%, and very strong in 51.06% of the remains, the latter being completely blackened by manganese. Among microvertebrates, selection of prey, breakage pattern of different skeletal elements, and tooth marks suggests that the accumulation was produced by predator activities. Even though there are no modifications assigned to digestive activity, these could have been masked by diagenetic processes. After a brief period of exposure previous to burial, evidenced by the scarce weathered remains, these were redeposited and dispersed from the original accumulation area. Evidence of abrasion during transport is lacking, thus probably precluding a significant aqueous transport. For this reason, and considering the small size of these remains, their high areal dispersion, and the lack of preferential orientation, it is proposed that they were mostly wind-transported from the original nearby area of accumulation. A different history affected the macromammal remains. In this case, an important loss of skeletal remains was produced, according to the ample taxonomic representation. Given the presence of very fragmented, highly weathered elements, indicating a very long exposure to weathering agents, it is probable that this loss took place prior to burial. Therefore, a natural and gradual death in an open environment is proposed for this group of vertebrates, whose body remains would have been exposed to weathering and dispersing agents. In this context, just a few elements could have become buried, thus increasing their preservation potential. Remains of both groups, once buried, suffered further pedogenic and diagenetic processes, including corrosion, manganese oxide mineralization, development of micrite coatings and rhizoliths, inclusion in carbonate concretions, and transverse smooth fractures in mineralized bones. The presence of two groups of remains with different taphonomic features suggests contrasting taphonomic histories. In consequence, the fossil assemblage from Telén would represent a condensed assemblage corresponding to two distinct time spans, i.e., the accumulation of microvertebrates took place in a short time interval whereas that of macromammals occurred over a longer span, coincident with the development of both soils.