Anatomy of an extinction revealed by molecular fossils spanning OAE2

The Cenomanian–Turonian mass extinction (Oceanic Anoxic Event 2-OAE2) was a period of profound ecological change that is recorded in the sedimentary record in many locations around the globe. In this study, we provide a new and detailed account of repetitive changes in water column ecology by analyzing the organic geochemical record preserved within the OAE2 section of the Greenhorn Formation, Western Interior Seaway (WIS) of North America. Results from this study provide evidence that OAE2 in the WIS was the result of the cumulative effect of reoccurring environmental stresses rather than a single massive event. During OAE2, extreme variations in biotic composition occurred erratically over periods of several thousands of years as revealed by molecular fossil (biomarker) abundances and distributions calibrated to sedimentation rates. These cycles of marine productivity decline almost certainly had follow-on effects through the ecosystem and likely contributed to the Cenomanian–Turonian mass extinction. While the causes behind organic productivity cycling are yet unproven, we postulate that they may have been linked to repeated episodes of volcanic activity. Catastrophic volcanism and related CO2 outgassing have been interpreted as main drivers for OAE2, though this study provides new evidence that repetitive, punctuated environmental stresses were also important episodes within the anatomy of OAE2. Following OAE2, these cycles of productivity decline disappeared, and the WIS returned to conditions comparable to pre-OAE2 levels.

Scanning electron microscope examination of the dental enameloid of the Cretaceous durophagous shark Ptychodus supports neoselachian classification

Although Ptychodus teeth are well known in Late Cretaceous marine deposits in North America and Europe and a few specimens with jaw elements have been discovered, the taxonomic position of the shark genus Ptychodus is enigmatic due to the lack of preservation of diagnostic material other than teeth. These sharks possessed a pavement dentition suited to a diet of hard-shelled macroinvertebrates (durophagy), leading several studies to variously describe Ptychodus as a batoid, a hybodont shark, or a selachimorph. Members of the Selachimorpha share one dental synapomorphy, a triple-layered enameloid (TLE) consisting of an outer shiny-layered enameloid (SLE) of randomly oriented hydroxyapatite crystallites, a middle layer of parallel-bundled enameloid (PBE), and an inner layer of tangled-bundled enameloid (TBE). Batoids and hybodonts both have teeth with single crystallite enameloid (SCE). We examined teeth from Ptychodus collected from the Lincoln Limestone of the Greenhorn Formation of Barton County, Kansas, and compared their enameloid ultrastructure with that of a Carboniferous hybodontiform and the Cretaceous lamniform shark Squalicorax curvatus Williston, 1900. Scanning electron microscopic examination of Ptychodus shows that crystallite bundling in the form of a TLE is evident in these teeth. The PBE is most apparent at transverse enameloid ridges of Ptychodus teeth. Columns of dentine penetrate into the tooth enameloid, and the crystallites near the dentine are randomly oriented. These observations bolster the argument that Ptychodus is a genus of highly specialized selachimorph shark, rather than a hybodont or batoid.

A note of caution concerning the application of quantitative palynological data from oxidized preparations

Kerogen extracted from rock samples for palynological studies frequently requires oxidative treatment in order to liberate palynomorphs from amorphous organic matter (AOM). Treatment with nitric acid (70% HNO3) is sometimes adequate though many Mesozoic organic-rich samples require more severe oxidation to break up clumped AOM. A widely used method involves the repeated oxidation of a preparation in Schulze’s solution (70% HNO3 supersaturated with KClO3), followed each time by rinsing with 2% potassium hydroxide solution (KOH) (Jones, 1994).The mid-Cretaceous Greenhorn Formation, exposed near Pueblo, Colorado, USA, is composed of interbedded bioturbated limestones and laminated calcareous shales (Cobban &amp; Scott, 1972). In the middle part of the Bridge Creek Limestone Member, kerogen extracted from these two lithologies is markedly different. Diverse, predominantly gonyaulacineaen dinocyst assemblages are found in kerogen from the bioturbated limestone samples. These either require five minutes of nitric acid treatment or no oxidation at all. Most of the AOM is finely disseminated and passes through a standard 10 μm sieve mesh.Dinocysts in the >10μm kerogen fraction from the laminated calcareous shale samples are outnumbered by clumped AOM at a ratio of between 10 and 1000:1. Prior to oxidation, gonyaulacineaen and peridiniineaen dinocysts are seen to be present. However, if Schulze’s solution and KOH are applied until most of the clumped AOM has disintegrated (a process which takes from 1 to 48 hours), Gonyaulacineae are rare (<5%) in resulting dinocyst assemblages (which are dominated by well-preserved Peridiniineae). . . .