2010 Overboard in the Mojave - Biological Science - California State ...

fragment recovered from within 0.5 m above the base of
Member D (Fig. 6). Anodonta shells from 0.7 m above
the base of Member D have produced infinite 14 C ages of
47+ ka (Y-1993) (Stuiver, 1969; Bassett and Jefferson,
1971) and 35+ ka (R. Berger, 1982, personal commun.).
A U/Th age of 74.0 ka on mammalian bone and a U/Th
age of 60.3 ka on Anodonta shell are reported by Budinger
(1992) 1.5 and 2.0 m above the base of Member D
respectively (Table 1) (Fig. 6). Given its stratigraphic
position, the former age appears to be too old. An Anodonta
horizon, in the uppermost exposures of Member
D south of Manix, has yielded a 14 C age of 19.1 ± 0.25 ka
(QC-1467) (R. Pardi, 1983, personal commun.). Bivalve
remains from stratigraphically correlative strata in eastern
Afton basin have produced 14 C dates that range in age
from ca. 31 ka to 28 ka and from ca. 21 ka to 18 ka (Table
1) (Bassett and Jefferson, 1971; Hastorf and Tinsley,
1981; Meek, 1990, 1999).
Paleontology of the Manix Formation
The aquatic invertebrate, and aquatic and terrestrial
vertebrate taxa that compose the fossil assemblage from
Lake Manix (Table 2) have been recovered from the
fluvial sediments of both the lower and upper wedges of
Member B, the littoral deposits at the base and middle
of Member C, and the fluvial sediments of Member D of
the Manix Formation. Invertebrate fossils are restricted to
Members C and D, and not unexpectedly, the remains of
fish and aquatic birds are restricted to Member C.
The fossiliferous part of the section ranges in age from
>350 ka to ca. 20 ka, and spans late Irvingtonian through
late Rancholabrean North American Land Mammal Ages
(LMA). Most of the mammalian genera represented in
the assemblage range through this entire period, and are
found in Members B through D. However, the scimitartooth
cat Homotherium and Antilocapridae occur only
at the base of Member C, and Bison is restricted to the
youngest deposits, Member D. Most vertebrate remains
have been recovered from the base of Member C.
Jefferson (1968) named the mammalian fossil assemblage
from the Manix Formation the “Camp Cady local
fauna” after the historic Union Army post that was located
on the Mojave River 3.7 km southwest of Manix siding
(Fig. 2). Camp Cady was occupied during and shortly after
the United States civil war (Chidester, 1965). In 1968,
Lake Manix deposits were though to encompass only the
Wisconsin, and the entire assemblage was assigned to
the Rancolabrean LMA (Winters, 1954; Howard, 1955;
Jefferson, 1968). If retained, this name (see Walsh, 2000,
p. 268–270) should apply to only those fossils recovered
from the upper wedge of Member C and Member D.
george t. jefferson
Figure 10. Partial skeleton of Aechmophorus occidentalis,
western grebe (Natural History Museum of Los Angeles
County specimen LACM 123458). The synsacrum and
articulated left and right femora (46.1 mm in length) are to
the lower left. Body and neck vertebrae occur to the right.
A nearly complete skeleton of this animal was recovered
from the base of Member C, Manix Formation (Jefferson,
1985b). Note the excellent preservation of delicate
osteological structures that is typical of the avian remains
from Member C.
The Lake Manix assemblage is significantly biased
taphonomically toward large-sized mammals, although,
excluding lower vertebrates, water fowl compose ~20% of
the assemblage. Amphibians, lizards, snakes, insectivores,
bats, small carnivores, and small ungulates are absent.
However, these animals occur locally in other late Pleistocene
assemblages like that from Schuiling Cave (Downs
et al., 1959; Jefferson, 1983) ~10 km west of the southern
edge of Troy Lake playa basin, or from the ancestral Mojave
River deposits at Daggett and Yermo (Reynolds and
Reynolds, 1985) ~20 km southwest of Camp Cady.
Avian remains from the lacustrine sedments are often
partially articulated (Howard, 1955; Jefferson, 1985b)
(Fig. 10). Those from the littoral deposits are not. Most
mammalian fossils occur as single, isolated skeletal elements
that often exhibit some abrasion due to fluvial
transport and/or wave action. Few mammalian osteologic
elements are articulated. Under fluvial or littoral
high energy depositional conditions, estimates of relative
taxonomic abundance within the assemblage (Table 3
and 4) are better represented as a percentage of the total
number of identified specimens (NISP) for each taxon,
rather than on a calculated minimum number of individuals
(Horton, 1984; Badgley, 1986).
Large mammalian herbivores far outnumber carnivores
in modern faunas (23:1, Mech, 1966; >250:1,
Schaller, 1972). The herbivore/carnivore ratio for the
larger mammals (larger than Canis latrans) in the Lake
Manix assemblage is relatively well balanced with 672
46 2010 Desert Symposium