|Barremian||Early Cretaceous I||Early Cretaceous II|
|Albian||Late Cretaceous I||Timescale|
Early Cretaceous I (Neocomian)
Early Cretaceous II (Aptian-Albian)
The Aptian was a long age, the second-longest of the Mesozoic. It was also an eventful time. Unlike some geological ages, it is not at all difficult to find important events in the Aptian. A spike occurred in an already rapid rate of ocean spreading in the Atlantic. This, for the first time, opened the Atlantic Ocean wide enough to allow significant mixing of waters across the equator. This is associated, in some as-yet undetermined manner, with: (a) the first in a series of Ocean Anoxic Events which extended into the Santonian, and (b) with a probable massive release of methane about 117 Mya. This last event -- or something else -- caused a dramatic increase in temperatures in the mid-Aptian (Jenkyns & Wilson, 1999), and a significant turnover of land fauna in the Late Aptian and across the Aptian-Albian boundary. The Aptian saw the first ceratopsians, almost the last stegosaurs, the enormous Sauroposeidon and the diminutive Gobiconodon, the simultaneous evolution of the first of the swift, unarmored iguanodont- hadrosaur group, and the definitive members of nature's ultimate tanks, the nodosaurs.
Image: Gobiconodon from Validosta State University's Virtual Museum of Fossils.
ATW040915. Text public domain; no rights reserved.
Aptian reefs were typically composed of rudists. In spite of their name, rudists were probably no less civil than the scleractinian corals they replaced. Indeed, it is the rudists who have suffered from a singular lack of courtesy from ill-bred geologists, who rejoice in pointing out that rudist masses are not really "reefs." Technically, this may be correct. The rudists tended to trap sediment between their long, ribbed lower valves, which stood up from the sea floor like the stalks of some horrible mutant celery. Thus, rudist reefs were not entirely composed of biogenic carbonates like a coral reef. They contained substantial amounts of sand and mud, in addition to encrusting calcareous algae and other oddments. Yet it is surely impolite to harp on these occasional indiscretions of the distant past.
Rudists were, in fact, aberrant bivalve mollusks. Consider an ammonite, or nautiloid. Now, since this is a bivalve, imagine a creature with two such shells curling in opposite directions, like the horns on a ram's head. Finally take one (or sometimes both) of the coiled horns and straighten it out. The result should look something like Caprinula, shown in the image. The left valve became an elongate stem anchored in the sea floor. The right valve could take on any number of morphologies -- from a simple, flat, operculum (like the "lid" on the exhaust pipe of a diesel truck) to any one of the peculiar morphologies adopted by single-shelled mollusks. "Teeth" mounted on the valves formed the articulation between them. The internal structure, so far as it has been worked out, was more or less like that of conventional bivalves. See the image of Requienia. The animal itself occupied only the upper part of the lower valve, the remainder of the lower section being sealed off with horizontal partitions.
Rudists diverged from other bivalves some time in the Jurassic. The earliest were Requinidae which were still usually somewhat coiled. The formation of Aptian rudist reefs may have been driven by -- or may have driven -- the evolution of the Capridae, like Caprinula, and other new forms of rudist. This brief bloom of rudist diversity ended before the close of the Aptian-Albian and was not resumed until well into the High Cretaceous. Rudist diversity collapsed during the Maastrichtian, and the entire taxon became extinct at the end of the Mesozoic.
These earlier rudists are frequently associated with other faunal indicators to form a characteristic "Urgonain" facies. In Urgonian deposits, rudists are often found in association with masses of tests from large foraminifera, such as Orbitolina and Choffatella, the presence of which is an indicator of Aptian-Albian age. Lehmann et al. (1999). A well-known example is the reef enclosing the Shuaiba Formation limestone reservoirs of the United Arab Emirates, one of the richest oil-producing systems in the world. Urgonian facies are also characterized, negatively, by the absence of ammonites and belemnites. These absence indicators may simply reflect the fact that these early rudist reefs were usually products of shallow water, or even intertidal, environments.
Corals may also be associated with rudist reefs -- possibly only those reefs far from the equator. It has been argued at some length that equatorial surface waters during the Aptian heat wave approached 30° C. and were lethal to corals. Thus, this first generation of rudist reefs may have gotten its start because the seas were simply too hot for corals to thrive.
Links: Drome and Provence however, we question the statement that Hippurites was present in the Aptian), Patch reefs in the limestones of the Altamira Formation ... the formation which also created the canvas for an unknown Late Pleistocene artistic genius) , Bild constratatal great site on rudists), Hippurites taburnii more on rudists).
ATW040917. Text public domain. No rights reserved.