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Interesting excerpts

May 19, 2013

PTEROSAUR DIVERSITY THROUGH THE CAMPANIAN AND MAASTRICHTIAN

Nathan Carroll
Department of Earth Sciences, Montana State University, Bozeman, Montana, USA

Pterosaur diversity in the latest Cretaceous is traditionally depicted as
extremely low, represented only by the Pteranodontidae, Nyctosauridae, and
Azhdarchidae in the Campanian. Aside from a single late occurrence of the
Nyctosauridae, the Maastrichtian record appears to be dominated by the
Azhdarchidae (Prentice et al., 2011; Butler et al.,2012). Azhdarchids are highly
disparate in size (ranging from 2.5m to 10 m wingspans) and ecologically
distinct from other pterosaur groups as stork-like generalists, but ecologically
conservative within their group (Witton and Naish, 2008). However, a new
azhdarchid from the “Jack’s Birthday Site” suggests that there were at least
three morphologically distinct azhdarchids in the Campanian Two Medicine
Formation of Montana. This new azhdarchid possesses relatively mature bone
histology despite numerous unfused elements, suggesting that azhdarchid growth
may differ from the better known ontogenetic sequences of Pteranodon (Bennett,
1993). A controversial rostrum from the Maastrichtian Javelina Formation of
Texas may also challenge the hypothesis of reduced Latest Cretaceous pterosaur
diversity and disparity. The robust nature of the specimen has led some authors
(Kellner 2004) to suggest that it is a late surviving member of the Tapejaridae
and others (Lü et al. 2006) to suggest that it is a new robust member of the
Azhdarchidae. If the former is true, then diversity is indeed higher than
previously recognized in the Campanian and Maastrichtian. If the latter is true
the ecologic disparity of azhdarchids is greater than previously known.
Furthermore, the recent reassignment of the small (~1 m wingspan) Piksi
barburulna, also of the Two Medicine Formation of Montana, from Aves to the
Ornithocheiroidea (Agnolin and Varricchio, 2012) suggests a greater diversity
and size disparity in pterosaurs. The fragmentary nature of the specimen
prevents speculations about its ecology, but its size suggests that it at least
occupies the same morphospace as birds of this time. These recent reports
highlight the taphonomic problems associated with assessing pterosaur diversity.
Studies of pterosaur biology, diversity and disparity have long suffered from
the incomplete and fragmentary preservation of most specimens. Lagerstätten
provide views into growth, ecology, biology and diversity with reasonable
clarity, but such localities are missing from the Latest Cretaceous. Recent
arguments for the decline in pterosaur diversity, especially between the
Campanian and Maastrichtian, point to the relatively high number of
pterosaur-bearing formations in the Latest Cretaceous yet very low diversity of
pterosaur remains (Butler et al., 2012). Pterosaur specimens from Maastrichtian
formations are scarce and typically assigned to the terrestrially adapted
Azhdarchidae. However, of the 21 pterosaur-bearing formations recognized in the
Maastrichtian, only 4 represent marine environments, one of which produced the
latest occurrence of a nyctosaur. The terrestrial preservation of pterosaurs is
inherently challenging and has produced a much more fragmentary record. Thus the
apparent drops in diversity and disparity may reflect either a greater abundance
of terrestrial versus marine units or the overall poorer quality of pterosaur
remains within terrestrial versus marine units.

THE HIDDEN DIVERSITY OF SMALL-BODIED ORNITHISCHIANS

Caleb M. Brown1 and Clint A. Boyd2
1Department of Ecology and Evolutionary Biology, University of Toronto, Toronto,
Ontario, Canada
2South Dakota School of Mines and Technology, Rapid City, South Dakota, USA

Recent work quantifying taphonomy (processes of decay and preservation) in
well-sampled Late Cretaceous North American dinosaur assemblages has illustrated
strong preservational biases against the completeness, articulation, discovery,
and familial diversity of small-bodied (less than approximately 60 kg) dinosaur
taxa (Figure 1A). The discovery rates of large taxa are best described by
logarithmic curves nearing, or at a plateau, while small taxa show either a
steady or increasing rate of discovery (Figure 1B). This suggests that, in
well-sampled formations, our current knowledge of the large bodied dinosaur
assemblage is reasonably representative of the true biological fauna; however,
small taxa are greatly underestimated in both their diversity and abundance,
with many more discoveries expected to be made with greater sampling. This is
particularly true for small bodied ornithischians, as they lack the diagnostic,
and abundant, teeth possessed by small-bodied theropods, making them much more
problematic to identify to species level. Full understanding of dinosaurian
faunal assemblages requires work concentrating on small species, which tend to
be both rare and fragmentary.
Here we present on research specifically focused on the remains of small bodied
ornithischians, based mainly on isolated elements or partial skeletons, which
has revealed a higher diversity of these small animals throughout the Late
Cretaceous of North America. New small `hypsilophodontid’ taxa have been found
in the Campanian Kaiparowits, Oldman, and Dinosaur Park Formations, as well as
the Maastrichtian Prince Creek and Frenchman formations. The morphological data
provided by these recent discoveries facilitates the resolution of many of the
basal relationships within Ornithischia, which have long remained problematic.
This work has resulted in the division of North American `hypsilophodontids’
into two clades: one consisting of small bodied taxa typified by the Campanian
taxon Orodromeus, and a second clade of medium to large sized taxa typified by
the Maastrichtian taxa Parksosaurus and Thescelosaurus. Similar recent discovery
or description of other small-bodied ornithischians has included
leptoceratopsids and pachycephalosaurs from multiple Late Cretaceous formations,
also based on isolated elements.
The work described here has resulted in moving a step closer in our
understanding of the diversity and relationships of these small taxa and their
potential ecological significance in the diverse dinosaur dominated faunas.

DROMAEOSAURS AT THE END OF THE CRETACEOUS: THEIR EVOLUTION AND EXTINCTION

Alan H. Turner
Department of Anatomical Sciences, Stony Brook University, Health Science
Center, Stony Brook, New York, USA

Dromaeosaurid theropods, such as Velociraptor and Deinonychus, are some of the
most iconic of dinosaurs. Yet dromaeosaurid fossils are rare. In the past
decade, however, we have seen a remarkable increase in the group’s diversity,
morphological disparity, and geographic extent. Numerous new species have been
described, including several fragmentary forms that are ambiguously
dromaeosaurids. Until recently the group was best known from the Upper
Cretaceous of Asia and North America, but these new discoveries expanded their
geographic and temporal range into the Lower and Upper Cretaceous of Asia,
Europe, North America, and South America.
Dromaeosaurids are very closely related to troodontids and avialans
(collectively known as paravians), and although their interrelationships have
remained remarkably stable despite the rapid pace of discoveries, our estimate
of what the ancestral paravian looked like has radically changed. Instead of a
Velociraptor-like animal, it was much more bird-like than previously thought.
Indeed, the morphological gap between the paravian clades has blurred to the
point that basal dromaeosaurids, troodontids, and avialans are nearly
indistinguishable from one another, and in life these animals would appear
extremely similar. However, important morphological divisions exist that allow
us to understand the evolutionary history of these three clades. This provides
the evolutionary context to understand the origins of the dromaeosaurid fauna
that existed up to the K/Pg extinction.
Heavily regionalized sampling and Lagerstätten effects bias attempts to quantify
diversity and patterns of regional extinction. Nearly half of all named
dromaeosaurids are from Campanian or Maastrichtian deposits. Yet this belies the
fact that dromaeosaurid diversity likely peaked at the end of the Early
Cretaceous and dromaeosaurids were nearing extinction by the beginning of the
Maastrichtian, prior to the K/Pg event. What remained of dromaeosaurid diversity
by the close of the Late Cretaceous looked quite different from that which
preceded it in the Early Cretaceous, with the differences driven by multiple
instances of body size increase, differential extinction, and/or long period of
geographic isolation.

THE DINOSAURS OF ISLAND AFRICA: A NEW LATEST CRETACEOUS FAUNA FROM KENYA AND THE
EVOLUTION OF GIGANTISM IN ABELISAURID THEROPODS

Joseph J. W. Sertich
Department of Earth Sciences, Denver Museum of Nature & Science, Denver,
Colorado USA

The African fossil record of dinosaurs and their Cretaceous ecosystems has
expanded significantly over the past two decades. Unfortunately, these
discoveries have been limited to the Early and “middle” Cretaceous, with a
conspicuous absence of fossils from the latest Cretaceous. In fact, much of the
record of terrestrial evolution on the African continent following its isolation
from the other Gondwanan landmasses approximately 100 million years ago remains
a mystery. Dinosaur fossils, long known from the Lapurr Mountains of
northwestern Turkana, Kenya, promise to provide the first glimpse at life during
this prolonged period of African isolation during the end Cretaceous. A
relatively diverse non-marine fauna from the Lapurr sandstone (“Turkana Grits”)
includes rocodyliforms, pterosaurs, and dinosaurs. Crocodyliform fossils are
referable to at least two distinct taxa, an unusual, massively-built form likely
mistaken for a spinosaurid theropod dinosaur in early reports, and a
long-snouted dyrosaurid similar to taxa found in Maastrichtian nearshore
deposits elsewhere in Africa. Though mostly fragmentary, the dinosaur record
includes at least two taxa of iguanodontian ornithopods. Distinct vertebral
morphologies indicate up to three sauropod taxa including two lithostrotian
titanosaurians. Among the most common and complete dinosaur fossils are those of
theropod dinosaurs, with both cranial and postcranial remains hinting at the
presence of at least two distinct abelisaurid taxa. One taxon is known from
multiple unassociated cranial and postcranial specimens that significantly
expand the upper limits of body size in ceratosaurian theropods. A comprehensive
phylogenetic analysis places this giant among derived abelisaurids, while
comparisons with material from Madagascar and South America indicate that it
likely exceeded 11-12m in length. Though efforts to establish solid
chronostratigraphic dates for the deposits have long been met with little
success, recently acquired data are suggestive of a latest Cretaceous age. If
this age hypothesis is confirmed, these fossils will represent the first
significant terrestrial vertebrate remains from Africa during this critical
interval and provide important evidence for understanding the ecology and
evolution of dinosaurs following African isolation.

2 Comments leave one →
  1. June 1, 2013 8:33 pm

    Seems I was right about the ornithopods

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