BIOSTRATIGRAPHIC AND MORPHOMETRIC ANALYSIS OF THE CALCAREOUS NANNOFOSSIL TRIBRACHIATUS ORTHOSTYLUS AND TWO NEW SPECIES/MORPHOTYPES OF TRIBRACHIATUS IN LOWER EOCENE SEDIMENTS

A preliminary biostratigraphic and morphometric analysis of lower Eocene sediments from the South Dover Bridge (SDB) core, Maryland, sediments from the Bass River core, New Jersey, and outcropping sediments from the mid-Waipara River in New Zealand, reveals the presence of two new species/morphotypes of the genus Tribrachiatus. These two new morphotypes have perceptibly different morphologies and biostratigraphic ranges than Tribrachiatus orthostylus. Tribrachiatus orthostylus has three arms of approximately equal length and an average circumference that ranges from 50–250 µm with rare gigantic specimens. Specimens that display bifurcation of ray tips are dominant in Zone NP11, while those from the basal Zone NP12 more commonly have curved tips. Tribrachiatus orthostylus exhibits low birefringence under polarized light. Tribrachiatus morphotype A has two arms of the same length that curve into a crescent shape and a third arm that is shorter and extends perpendicular to the other two arms. The width between arm tips has an average range between 17–30 µm and the minor arc angle is highly variable with a range of 80–165°. Tribrachiatus morphotype A exhibits high birefringence under polarized light. Tribrachiatus morphotype B is characterized by two arms of equal length that are flattened on the upper and lower sides and has a vaulted inner angle that averages between 120–140. The tips of the two arms in well preserved specimens have a claw-like structure; the third arm is short and stubby. The width between arm tips is <15 µm. Tribrachiatus orthostylus has its first occurrence (FO) near the top of Zone NP10 and disappears at the top of Zone NP12. Morphotypes A and B have near simultaneous FOs just below the FO of marker species Discoaster lodoensis at the base of Zone NP12. Comparison of the relative abundances of T. orthostylus and Tribrachiatus morphotypes A and B to the bulk ¹³C curve of the SDB core suggests that these species were sensitive to changing paleoclimate conditions during the Eocene hyperthermal events and could potentially be useful marker species.