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Chubutese dinosaurs



I have read the paper of Rauhut et. al. in press about dinosaur from the Lower 
Cretaceous of Chubut province (Argentina) mentioned yesterday by Thomas R. 
Holtz, Jr.

Perhaps better than the studied remains (partial tibia of Titanosauria indet. 
and two caudal vertebrae and a tooth fragment of a ?probable abelisaurid? from 
uppermost Hauterivian-Barremian of La Paloma Member), the most interesting part 
of this paper is the new datings of several Chubutese dinosaur: eg. 
Chubutisaurus is Late Cretaceous (Senonian) and Carnotaurus is also from Late 
Cretaceous (Maastrichtian). 

J. I. Ruiz-Omeñaca

The following is taken directly from the sciencedirect full-text version:
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2. Geological and palaeontological setting
      The Chubut Group comprises the lower Los Adobes and the upper Cerro 
Barcino formations (Codignotto et al., 1978; Figari & Courtade, 1993; Page et 
al., 1999). It unconformably overlies Middle?Upper Jurassic units usually 
grouped in the Cañadón Asfalto Formation, and older Jurassic units, and is 
overlain unconformably by uppermost Cretaceous units, the 
Campanian?Maastrichtian La Colonia, Paso de Sapo, and Lefipan formations and 
their lateral equivalents. Both the Los Adobes and Cerro Barcino formations are 
composed mainly of fluvial and volcaniclastic sediments (Figari & Courtade, 
1993; Page et al., 1999; Manassero et al., 2000), although considerable 
differences in detailed sedimentology exists between the distinct members of 
the two formations. The Los Adobes Formation is considered to be Late 
Valanginian?Hauterivian in age( Cortiñas, 1996; Page et al., 1999), whereas the 
Cerro Barcino Formation might represent the latest Hauterivian?Senonian 
(?Campanian), although the youngest part may be Cenomanian in most areas ( Page 
et al., 1999).
      Both formations can be subdivided into several members. The Los Adobes 
Formation begins with the Arroyo del Pajarito Member, which is mainly composed 
of coarse fluvial sediments and pyroclastics. This member is overlain by the 
Bardas Coloradas Member, a unit consisting mainly of reddish fluvial and 
fine-grained overbank deposits. The base of the Cerro Barcino Formation is 
marked by a notable change in sediment composition, indicating a climatic 
change from the wet conditions of the Bardas Coloradas Member to the more arid 
environments of the La Paloma Member. The latter is dominated by pyroclastic 
and fluvial deposits with some intercalated dune sediments. It is overlain by 
the Cerro Castaño Member, which indicates a return to more humid conditions, 
with mainly fluvial and overbank deposits. Several higher members, the Las 
Plumas, Bayo Overo and Puesto Manuel Arce members, have been distinguished to 
the east and north-east of the area studied. These are probably latest 
Early?Late Cretaceous in age (?Albian??Campanian), and might be separated by 
small unconformities (Cortiñas, 1996).
      Although the sediments of the Chubut Group cover vast areas of Chubut 
Province, the vertebrate fauna from this unit is still poorly known. This might 
be owing to the comparative rarity of vertebrate fossils in most members of the 
two formations, but the relatively small amount of palaeontological fieldwork 
carried out in these areas, as compared to the Cretaceous of Neuquén, for 
example, is certainly another factor.
      No vertebrates have so far been recorded from the Los Adobes Formation. 
The La Paloma Member has yielded some undescribed turtle remains. The only 
dinosaur fossil previously reported from this unit is a single theropod tooth 
fragment from the locality `Turtle Town', which was originally thought to be 
within the Cerro Castaño Member (Rich et al., 2000). Several dinosaur remains 
have been reported from the Cerro Castaño Member, including two partial 
skeletons of a large, undescribed carcharodontosaurid and several theropod 
teeth (Vickers-Rich et al., 1999; Rich et al., 2000). Recently discovered 
vertebrate material in this member also includes remains of a titanosauriform 
sauropod, a probable coelurosaurian theropod, crocodile remains, and a partial 
turtle carapace (Rauhut, unpublished data). The Bayo Overo Member has yielded 
the sauropod Chubutisaurus (Del Corro, 1975), an undescribed sauropod, and 
several theropod teeth ( Rich et al., 2000). Though frequently cited as being 
derived from the Aptian?Albian (e.g. Weishampel, 1990; Bonaparte, 1996; Novas, 
1997) Chubutisaurus is thus most probably of Late Cretaceous age (Page et al., 
1999; Rich et al., 2000). Likewise, the abelisaurid Carnotaurus, originally 
thought to be from the Albian of the Cerro Barcino (=Gorro Frigio) Formation 
(Bonaparte, 1985; Bonaparte et al., 1990), is now known to have come from the 
La Colonia Formation, which, according to new palynological evidence, is 
Maastrichtian in age (A. Archangelsky, pers. comm. 2001).

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