Vega Island Stratigraphy, Biostratigraphy, and Chronostratigraphy

All of the latest Cretaceous dinosaurian and avian specimens have been
recovered from the shallow marine deposits within the James Ross Basin,
northeastern Antarctic Peninsula. The dinosaur specimens have been recovered
from Santonian to Late Maastrichtian-aged sediments, while, all of the avian
specimens have occurred in the Maastrichtian-aged rock sequence primarily from
Vega Island.

The stratigraphic sequence for Cape Lamb and Sandwich Bluff areas on Vega
Island was described by Pirrie et al. (1991) and then by Marenssi et al. (1998)
with biostratigraphic data detailed by Pirrie et al. (1991) and
chronostratigraphic data by Crame et al. (2004). To this framework, our
projects on Vega and James Ross Islands have added vertebrate data including
several dinosaur taxa as well as bird taxa from the region.

A partial skeleton of a hysilophodontid dinosaur was recovered from the Early
Maastrichtian deposits of the Cape Lamb Member of the Snow Hill Island
Formation on Vega Island (Hooker et al. 1992). Within the Gunnarites zone, the
hypsilophodontid was recovered in the middle of the Diplomoceras lambi (a
heteromorph ammonite) horizon. Additionally, the Naze dromaeosaur weathered
from the lower portion of the Cape Lamb Member again within the Diplomoceras
subzone on the Naze of James Ross Island. This places these two dinosaur
localities, near the middle of the Gunnarites assemblage zone, just above the
level where Crame et al. (2004) recorded a 87Sr/86Sr datum of 71.0 Ma.

The charadriiform (shorebirds) bird localities where two different bird
specimens were recovered (Case & Tambussi, 1999; Cordes, 2001; Cordes, 2002),
are placed into the upper half of the Gunnarites assemblage zone, at level
between 37-40 meters above the level where Crame et al. (2004) recorded a
87Sr/86Sr datum of 70.3 Ma. Based on the rate of sedimentation (Pirrie et al.,
1991) it would place the two Cape Lamb localities of the supposed charadriiform
specimens, into a very Early Maastrichtian time frame of approximately 70
million years ago.

The two specimens of presbyornithids (Noriega and Tambussi, 1995) are
now known to be one, an anseriform (ducks, geese and relatives) Vegavis (Clarke
et al., 2005) and the second is a proposed gaviiform (loon) assigned to the
genus Polarornis (Chatterjee, 2006; Fig. 1). Both specimens were recovered
from concretions coming from the lower part of the Sandwich Bluff Member of
Pirrie et al. (1991).

In the amphitheater where the fossils were collected, the horizon that produces
the fossil is a concretionary layer, some 20 meters above the unconformity,
which represents the base of the Sandwich Bluff Member of the Snow Hill Island
Formation. This concretionary horizon is associated with the Maorites
Assemblage Zone (Pirrie et al., 1991) and the Manumiella (dinoflagellate) Zone
of Riding et al. (1992) which are both indicators of a Late Maastrichtian age.
Higher up in the Sandwich Bluff section, dental and pedal material of a
hadrosaurine hadrosaur, was recovered from the Late Maastrichtian Sandwich
Bluff Member on Vega Island (Case et al., 2000). Additional and abundant avian
material has been collected from the same horizon as the hadrosaur. More
recently, between the hadrosaur horizon and the lower bird concretionary
horizon has come material of a cursorial bird that adds another Antarctic bird
to the growing list of avian taxa from Vega Island and the James Ross Basin in
general.