DePaul Discoveries
Volume 2 | Issue 1
Article 6
2013
Fossil Marine Vertebrates from the Middle Part of
the Upper Cretaceous Graneros Shale in
Southeastern Nebraska
Alexander D. Meglei
[email protected]
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Acknowledgements
Faculty Advisor: Dr. Kenshu Shimada; Department of Environmental Science and Studies and
Department of Biological Sciences
Recommended Citation
Meglei, Alexander D. (2013) "Fossil Marine Vertebrates from the Middle Part of the Upper Cretaceous Graneros Shale in
Southeastern Nebraska," DePaul Discoveries: Vol. 2: Iss. 1, Article 6.
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Meglei: Fossil Marine Vertebrates from the Middle Part of the Upper Cretaceous Graneros Shale in Southeastern Nebraska
D E PA U L D I S C O V E R I E S ( 2 O 1 3 )
Fossil Marine Vertebrates from the Middle Part of the Upper
Cretaceous Graneros Shale in Southeastern Nebraska
Alexander D. Meglei*
Department of Biological Sciences
ABSTRACT
The Graneros Shale is a rock unit formed in the Late Cretaceous Western Interior Seaway of North
America. In this study, I report an assemblage of fossil vertebrates from the middle portion of the Graneros Shale in
southeastern Nebraska. The estimated age of this fauna is about 97 million years old, and it is dominated by carnivores
that consist of at least 11 taxa, comprising of eight sharks, one bony fish, and two reptilians (turtle and plesiosaur).
The fauna includes taxa indicative of a nearshore, but fully marine, environment that do not contradict with previous
paleoenvironmental inferences made for the Graneros Shale. This study is important because it represents the first
collective faunal study of fossil vertebrates from the mid-Graneros Shale at a single locality.
and rays) and 10 osteichthyan (bony fish) taxa (Jansen et
INTRODUCTION
Fossils of extinct organisms provide insight into the
al., in press).
environmental conditions of the past. The Graneros Shale
(Fig. 1A) is a Late Cretaceous rock formation deposited
Kirkland’s (1989) brief report also included one other
in the Western Interior Seaway (Hattin, 1965; for specific
fauna from the Graneros Shale at the same locality
age estimation, see below), in which vertebrate fossils,
in southeastern Nebraska focusing only on its
such as those of fish and plesiosaurs, are known to
chondrichthyan components. The fauna is from the
occur (e.g., Welles, 1970; Witzke, 1981; Russell, 1988;
middle part of the formation below the X-bentonite,
Schumacher and Everhart, 2005). However, while
but the entire fauna has remained unstudied. In this
previous reports on fossil vertebrates from the Graneros
paper, I formally describe the vertebrate assemblage. My
Shale are scattered in literature, the only collective study
goal is two-fold: 1) to formally describe the taxonomic
on the Graneros vertebrate fauna is that of the fish fauna
components of the fauna, and 2) to discuss the
from the uppermost part of the formation in southeastern
paleoenvironment of the area during its deposition.
Nebraska reported by Jansen et al. (in press) who built
upon a brief account reported by Kirkland (1989). The
LO C A L I T Y A N D ST R AT I G R A P H Y
fauna came from the uppermost horizon within the
The fossils described in this paper were collected from
formation, approximately 50 cm above a 30–45 cm thick
a roadcut along the east side of State Highway 15,
altered volcanic ash layer known as the X-bentonite, and
approximately 1.6 km north of the Kansas-Nebraska
was found to contain at least 14 chondrichthyan (sharks
boarder. As in Kansas (Hattin, 1965), the Graneros
Shale at the locality is represented largely by dark gray,
* Faculty Advisor: Dr. Kenshu Shimada; Department of Environmental
non-calcareous, silty-clayey shale and contains the
Science and Studies and Department of Biological Sciences
X-bentonite approximately 50 cm below the contact
Project completion: Winter Quarter, 2013
Author contact: [email protected]
between the Graneros Shale and the conformably
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DePaul Discoveries, Vol. 2 [2013], Iss. 1, Art. 6
FO S S I L M A R I N E V E R T E B R AT E S
overlying Greenhorn Limestone (see also Hattin, 1975).
DISCUSSION
Although the exact stratigraphic horizon or range where
Russell (1988) provided a summary of Cretaceous marine
the fossils described in this present study occurred
faunas of North America and listed several vertebrate
is uncertain, they all came from the typical dark-gray
taxa from unknown stratigraphic horizons within the
silty-clayey portion of the Graneros Shale below the
Graneros Shale in multiple areas (Iowa, Nebraska, Kansas,
X-bentonite. Whereas the Graneros Shale is about 10 m
Colorado, and Wyoming) known at that time. They
thick in the adjacent area (e.g., Kansas), several meters of
included Ptychodus occidentalis (shark), Xiphactinus
the Graneros Shale is present at the locality. Because the
sp. (bony fish), Polycotylidae indet. (plesiosaur),
fossils described here come from the middle portion of
Thalassomedon haningtoni (plesiosaur), Coelosuchus
the formation, I refer to the vertebrate assemblage as the
reedi (crocodilian), and Platypterygius americanis
‘mid-Graneros fauna’ for the purpose of this study.
(ichthyosaur) (Russell, 1988; see Schumacher and
Everhart, 2005, for additional information on plesiosaurs
M AT E R I A LS A N D M E T H O D S
from the Graneros Shale) Subsequently, Kirkland (1989)
Fossil specimens were collected by James I. Kirkland in
reported the occurrence of the following four shark taxa
the late 1980s soon after the rocks at the locality became
from the formation: Cretodus semiplcatus, Cretoxyrhina
freshly exposed due to the construction of Highway
mantelli, Squalicorax falcatus, and Odontaspis sp. My
15 (Fig. 1B). All fossils are poorly preserved and were
present study that identifies 11 taxa (Table 1) represents
collected via surface picking, and thus, it is likely that
the re-examination of Kirkland’s (1989) material based
the fossil assemblage involves collecting bias towards
on an updated taxonomic scheme. It constitutes the
larger fossils. Prior to my study, the specimens remained
first collective work of the vertebrate fauna from the
uncatalogued in the collection of the University of
mid-Graneros Shale with more constrained geographic
Nebraska State Museum (UNSM), Lincoln, Nebraska.
and stratigraphic ranges than Russell’s (1988) faunal
The specimens were subsequently loaned to DePaul
summary.
University (to K. Shimada) for the purpose of my study.
Taxonomic identifications were conducted based on
Although the exact stratigraphic horizon within the
published literature (e.g., Shimada et al., 2006; Cumbaa
middle section of the Graneros Shale is uncertain, the
et al., 2010).
mid-Graneros fauna comes from below the X-bentonite
and not immediately above the underlying Dakota
R E S U LTS
Sandstone. 40Ar/39Ar age dating analysis on the
Table 1 lists all the vertebrate taxa and specimens
X-bentonite in an adjacent area (west-central Kansas)
represented in the mid-Graneros fauna at the locality.
yielded a reliable plateau age of 95.53 ± 0.15 million years
The fauna consists of a total of 11 taxa, comprising
ago (Ma) (Shimada et al., 2006). Based on invertebrate
eight chondrichthyan and one osteichthyan fishes as
evidence, the upper part of the Dakota Sandstone is
well as two reptilian taxa. The most common taxon
inferred to be approximately 99.6Ma (Cobban et al., 2006,
is Archaeolamna sp. Although the exact taxonomic
fig. 1). Therefore, my mid-Graneros fauna is interpreted
identification of some of the taxa remains uncertain, all
to be about 97 ± 1 Ma (i.e., middle Middle Cenomanian).
other decisively identified vertebrate taxa reported here
have been documented from other similar aged (mid-
The underlying Dakota Sandstone is inferred to have
Cenomanian; see below) localities in North America (e.g.,
been deposited in a marginal-marine setting in which
Welton and Farish, 1993; Cumbaa et al., 2006; Shimada
the Graneros Shale represents the onset of a major
et al., 2006; Shimada and Martin, 2008; Underwood and
transgressive phase of the Western Interior Seaway
Cumbaa, 2010; Cumbaa et al., 2010; Everhart, 2011).
(Hattin et al., 1987). Although the exact taxonomic
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Meglei: Fossil Marine Vertebrates from the Middle Part of the Upper Cretaceous Graneros Shale in Southeastern Nebraska
D E PA U L D I S C O V E R I E S ( 2 O 1 3 )
identity of the chelonian remains (Fig. 2J) is uncertain,
also served as food for probable small to medium-sized
other vertebrates in my fauna are fully marine taxa,
carnivores such as Caracharias and Archaeolamna (e.g.,
indicating that the locality was in a true marine condition
see Shimada et al., 2006). However, the fact that the fauna
without any apparent terrestrial influence during the
is dominated by carnivorous taxa suggests that there
deposition of the mid-Graneros Shale. However, the
must have been many more small prey species present
fact that the mid-Graneros Shale consists of terrigenous
in abundance in the Western Interior Seaway during the
sediments (land derived particles) (Hattin, 1965) and
deposition of the mid-Graneros Shale.
that my fauna includes a hybodont shark characteristic
of a near-shore environment (see Welton and Farish,
C O N C LU S I O N
1993) indicates that the eastern shoreline of the Western
My study represents the first collective description of
Interior Seaway was still not so far away from the locality
vertebrate fauna during the deposition of mid-Graneros
and that the locality at that time represented a shallow-
Shale in the Late Cretaceous Western Interior Seaway
water environment relative to the ocean during the
of North America. A total of 11 taxa are identified,
deposition of the overlying Greenhorn Limestone in the
comprising
area (see also Hattin, 1975; Hattin et al., 1987).
Squalicorax, Caracharias, Archaeolamna, Cretodus,
eight
chondrichthyan
(Hybodontidae,
Cretalamna, Cardabiodon, and Cretoxyrhina), one
One notable characteristic of the fauna is the scarcity
osteichthyan (Enchodus), and two reptilian (Plesiosauria
of Squalicorax and Enchodus that are quite common in
and Chelonia) taxa. Whereas the estimated age of this
other mid-Cenomanian localities (e.g., Shimada et al.,
fauna is about 97 Ma, the fauna includes taxa indicative
2006; Shimada and Martin, 2008; Gallardo et al., in press).
of a nearshore, but fully marine, environment that is
Their underrepresentation in my sample may be due
consistent with previous paleoenvironmental inferences
to a collecting bias towards larger specimens because
for the Graneros Shale based on lithological and
remains of these taxa are generally small. Nevertheless,
invertebrate evidence (Hattin, 1965, 1975). Although
the scarcity of these two taxa also in a fauna with well-
the taxonomic representation of the fauna most likely
sampled small taxa from the uppermost part of the
suffers from collecting bias towards larger taxa, the
Graneros Shale (Jansen et al., in press) is intriguing. This
fauna is dominated by carnivorous taxa, suggesting the
observation could mean that Squalicorax and Enchodus
abundant existence of small prey taxa in the sea during
preferred offshore environments during the deposition
the deposition of the mid-Graneros Shale.
of at least middle-upper portion of the Graneros Shale,
but more thorough sampling of vertebrate remains from
ACKNOWLEDGEMENTS
the entire formation is necessary to substantiate this
I thank G. R. Corner (UNSM) for the loan of the specimens
proposition.
for the purpose of this study,
J. I. Kirkland (Utah
Geological Survey) for providing useful information,
With a probable exception of Chelonia indet., all
and K. Shimada (DePaul University) for support and
vertebrates from the mid-Graneros Shale reported here
guidance.
are considered carnivores or opportunistic feeders,
.
including scavengers (e.g., see Schwimmer et al., 1997;
Cicimurri and Everhart, 2001; Shimada, 2008; Shimada
et al., 2010). Enchodus, a probable small predator that
possibly fed on soft-bodied invertebrates such as squid
(Grandstaff and Parris, 1990), is the smallest taxon
represented in the mid-Graneros fauna and could have
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DePaul Discoveries, Vol. 2 [2013], Iss. 1, Art. 6
FO S S I L M A R I N E V E R T E B R AT E S
FIGURE 1
Simplified Upper Cretaceous stratigraphy in
southeastern Nebraska (A; gray line indicates middle
portion of Graneros Shale focused on in this paper)
and photograph of southeastern Nebraska locality
described here (B; photograph taken in April 2011).
Meglei (Page 11)
FIGURE 2
Table 1. List of vertebrate taxa from mid-Graneros Shale in southeastern Nebraska along with
Representative remains from mid-Graneros Shale in southeastern
their sample
sizes
and catalogue
List of
vertebrate
taxa numbers.
from mid-Graneros Shale in southeastern Nebraska
Nebraska. A, Hybodontidae indet. (dorsal fin spine; UNSM
TA B L E 1
along with their sample sizes and catalogue numbers.
127900); B, Squalicorax curvatus (one of five teeth in UNSM
127901); C, Carcharias amonensis (one of four teeth in UNSM
127902); D, Archaeolamna cf. A. kopingensis (one of nine teeth
in UNSM 127903); E, Cretodus semiplicatus (one of four teeth in
UNSM 127904); F, Cretalamna appendiculta (one of four teeth
in UNSM 127905); G, Cardabiodon sp. (tooth; UNSM 127906);
H, Cretoxyrhina mantelli (one of two teeth in UNSM 127907);
I, Enchodus gladiolus (right palatine bone; UNSM 127908); J,
Chelonia indet. (one of shell fragments in UNSM 127910); K,
Plesiosauria indet. (tooth; UNSM 127909). Orientations: A, left =
anterior view, middle = posterior view, right = left lateral view;
B-H, left = lingual view, right = labial view; I, lingual view (anterior
to right); J, exterior view; K, from left to right = labial, anterior,
posterior, and lingual views. Bar scale = 10 mm.
.
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Meglei: Fossil Marine Vertebrates from the Middle Part of the Upper Cretaceous Graneros Shale in Southeastern Nebraska
D E PA U L D I S C O V E R I E S ( 2 O 1 3 )
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