An 110 Ma Crocodilian-Bearing Vertebrate Assemblage Preserved Between Basalt Flows on a mid-Pacific Seamount, ODP Site 865, Allison Guyot
Abstract
Crocodilian teeth, fish teeth, scales, and bones, and teeth and bones of uncertain affinities occur within a mudstone deposit sandwiched between basalt flows at the base of ODP Hole 865A on Allison Guyot in the middle of the Pacific Ocean. Age control from large benthic foraminifers, Sr-isotope stratigraphy, and radiometric ages of the basalt place the age of this assemblage at 110 \mp 1.2 Ma, or late Aptian/early Albian. This deposit contains the oldest known crocodilian remains in the Pacific Realm. The vertebrate-bearing interval lies within a 30 cm mudstone unit overlying lithified fossiliferous limestone and underlying fossiliferous clayey limestone. The surface of the lithified limestone contains fish scales, vertebrate teeth and bones, and a carapace of the ostracod Cytherella. The lower 17 cm of mudstone is the prime vertebrate-bearing layer. It has a fining-upwards grain size trend, and an upward increase in dark coloration. The sand sized fraction is primarily pelletal glauconite, pyrite and fish debris. Limestone clasts up to several mm in diameter occur through the lower portion, whereas the higher portion contains sand laminae. A pristine 1 cm long crocodilian tooth, along with several smaller crocodilian teeth from this interval, attest to the presence of a saltwater crocodilian within the lagoon of this drowned mid-Pacific atoll. Common and varied fish teeth include both slender forms for piercing and grasping mobile prey and blunt flat-topped forms for crushing and grinding hard-shelled bottom living organisms such as oysters. The crushing teeth resemble those of the modern Black Drum fish, a bottom feeder common in shallow coastal environments. Abundant fish scales include both heavy phosphatic ganoid scales, a type present on many Mesozoic fish, and thin flexible scales of a type present on most modern fish. Some teeth and bone fragments are currently unidentified, and may represent vertebrates other than fish or crocodilians. No calcareous fossils are found within this unit. The lower unit grades upwards into a 5 cm thick black organic mudstone consisting of mostly amorphous organic debris, colonial algae, and smaller amounts of terrestrial spores, carbonized plant debris, and amber. No vertebrate remains are observed within the black mudstone. This unit in turn grades upwards into an 8 cm unit of mudstone with a coarsening-upwards grain size trend. This upper 8 cm unit contains only rare vertebrate remains, but has numerous oyster fragments and abundant, well preserved, articulated ostracod shells. The ostracod assemblage consists entirely of Cytherella (Suborder Platycopina). The entire 30 cm unit contains common pyrite, glauconite, and plant debris, and lacks calcareous fossils except in the uppermost part. This, as well as the occurrence of abundant Cytherella indicate sediment deposition in a restricted marine environment with low oxygen conditions. This discovery represents possibly the only known record of mid-Cretaceous reptilian remains from the mid- Pacific region, and the entire fossil assemblage provides valuable data for understanding the history of biotic migrations across the Cretaceous Pacific oceanic island realm. The intercalations of basalt flows with restricted marine and terrestrial sediments can be found not only in this ODP drill core but in others around the Pacific and shows the possibility of future discoveries of this type by scientific drilling of old seamounts.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V13A0651F
- Keywords:
-
- 0459 Macro- and micropaleontology (3030;
- 4944);
- 3036 Ocean drilling;
- 4950 Paleoecology;
- 9355 Pacific Ocean;
- 9610 Cretaceous