PDF (Lo-Res) - Smithsonian Institution Libraries

A Fossil Screamer (Anseriformes: Anhimidae) from
the Middle Tertiary of Southeastern Brazil
ABSTRACT
A new genus and species of anhimid is described based on 14
fragmentary, partially associated bones collected from the shales
of the Tremembe Formation of the Taubate basin in southeastern
Brazil. The age of this formation is either upper Oligocene or
lower Miocene. An isolated, almost complete left coracoid was
chosen as the holotype. This bird, the first paleospecies described
for the family Anhimidae, was smaller and more gracile than
Chauna chavaria (Linnaeus), the smallest species of living
screamers.
Introduction
The screamers (Anseriformes: Anhimidae) are distinguished
from the more widespread family Anatidae by the narrow,
downwardly hooked bill lacking filtering fringes, long legs,
and large, unwebbed feet with strong hind toes. The skeleton of
anhimids is very noticeably pneumatized. There are three modem
species of screamers in two genera, Anhima cornuta (Linnaeus),
Chauna chavaria (Linnaeus), and C. torquata (Oken),
all of which are endemic to South America. There are no fossil
species described for this family (Olson, 1985), but undescribed
fossils of possible screamers are known from the early
Eocene of Wyoming and England (S.L. Olson, pers. comm.,
1995). In the present study, a new genus and species of anhimid
is described, the first record for this family in the Tertiary of
South America.
The Tremembe Formation, in the town of the same name in
the Taubate basin of southeastern Brazil (Figure 1), is a lacustrine
deposit of small extent. The sediments are composed of
alternate layers of thinly foliated bituminous shales, 6 to 10
meters thick, and an almost homogeneous montmorillonitic
Herculano M.F. Alvarenga
Herculano M.F. Alvarenga, Departamento de Zoologia, Instituto de
Biociencias, Universidade de Sao Paulo, Caixa Postal 11294, Sao
Paulo-SP, 05422-970 Brazil.
223
clay of about the same thickness (Figure 2). Both of these layers
have produced a large diversity of fossil vertebrates, including
representatives of six families of birds (Alvarenga, 1982,
1985, 1988, 1990, 1995). The age of the Tremembe Formation
is either upper Oligocene or lower Miocene, as discussed by
Soria and Alvarenga (1989), Alvarenga (1990), and Vucetich
etal. (1993).
The presence of flamingos, such as Palaelodus and Agnopterus
(Alvarenga, 1990), a cathartid vulture (Alvarenga,
1985), and a large number of small fossil fishes and Crustacea
suggest the interpretation of the site as an old lake of shallow,
alkaline water. Some large and small mammals (Soria and Alvarenga,
1989; Vucetich et al., 1993) and a large phorusrhacid
(Alvarenga, 1982) also are known from these sediments.
MATERIALS AND METHODS.—Among the birds described
from the Tremembe Formation, some specimens were found
articulated in the shales, whereas others, in the layer of montmorillonitic
clay, often occurred as fragmented and dissociated
bones, which caused some difficulties in relegating them to a
particular taxon. The fossils described herein were collected by
the author from the montmorillonitic clay on different occasions
from 1978 to 1993. They belong to at least three individuals,
all identified as anhimids of a size compatible with that of
a single species.
The bones were compared with those of skeletons of almost
all families of birds, especially Anhimidae {Chauna chavaria,
C. torquata, Anhima cornuta), Anatidae {Anseranas semipalmata
(Latham)), and some Gruidae, including Grus, Balearica,
and Anthropoides. The single skeleton of Chauna chavaria
used in this study was obtained on loan from the National Museum
of Natural History (USNM; collections of the former
United States National Museum), Smithsonian Institution, and
all others are from the author's collection (HA).
Anatomical terminology follows that of Howard (1929) and
Baumel et al. (1979), although I also follow Olson (1987) in
using the term "procoracoid foramen" in the description of the
coracoid. The fossil specimens herein described are housed in
the Vertebrate Paleontology collection of Museu Nacional da
Universidade Federal do Rio de Janeiro (MNRJ), Brazil.