Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean food chains

Then as now, the phytoplankton was the base of the food chain in the ocean, being the single most important support for all ocean life. The multitude of single-celled photosynthesising organisms, that were mostly microscopic, were part of the plankton. The product of the photosynthesis was glucose, an energy source in its own right; it is also used to construct more complex carbohydrates. These organisms that produce their own food are collectively autotrophs. These primary producers are fed on by heterotrophs, and then up the food chains of the ocean. These herbivorous primary consumers are in turn fed on by secondary consumers, and so on.    

New types of primary producers in the phytoplankton arose in the ocean during the Jurassic. The first to appear in the fossil record are coccoliths, and later diatoms became very abundant in the surface waters. These organism built their skeletons from lime and silica, in a variety of intricate shapes. In warm waters dinoflagellates are the main producers of the present, and they were abundant in the Jurassic seas. As the phytoplankton proliferated explosively there was also an explosive radiation of zooplankton at this time. Among these planktonic organisms were foraminifers (forams) that built their skeleton from lime, and radiolarians that built their skeletons from silica.

The rejuvenation of the ocean that was occurring during the Jurassic was an opportunity for the marine invertebrates, and they proliferated in a rapid radiation of forms. Molluscs were among those proliferating, among which were oysters and clams, and a type of thick-shelled bivalve (rudists) that built reef-like mounds of their shells. There were also cephalopods that were very effective free-swimming shelled molluscs. Also appearing in the warm shallow seas of this time are new types of corals and bryozoans. On the seafloor there were crabs and lobsters that could crack shells to get at the molluscs inside and carnivorous snails that burrowed though the shells of other molluscs. Burrowing became a common form of avoidance at this time.

There are a large variety of ammonites with coiled shells that date from this time, some of which reached a metre in diameter, and belemnites that had straight shells that are related to squids of the present. According to the author³ ammonites were named for their resemblance to the horns of Ammon, an Egyptian god. The author³ states that the name is actually that of a Greek oracle god whose sanctuary was at Siwa oasis in the Libyan Desert. Tribes of the Libyan Desert first worshipped a god in the shape of a ram, Amun. After its adoption and passage through a number of civilisations the name and the meaning changed slightly. The Greek word ammos means sand.

The ammonites were a very successful group of cephalopods that are now extinct; the only cephalopods remaining to the present are squid, octopuses and nautiloids. The extant cephalopods of the present all use jets of water for propulsion, and their eyes, brains and nervous systems are all well developed. To protect against predators that crush shells nautiloids have strong shells and chambers that are filled with varying proportions of liquid and gas for buoyancy. It has been assumed that ammonites had attributes that were similar, such as generally being good swimmers, at least those in which the shells were streamlined, occupying the same niches as fish of the present.

The speed of their evolution in a short time period is indicated by the thousands of species that have been described, and they diverged to fill many of the niches that fish do at the present. The smaller ones were no bigger than a euro coin, a quarter in the US, and the largest grew to the size of the wheel from a 40-tonne truck. The combination of ammonites and belemnites form a cosmopolitan population of predators and scavengers throughout the ocean.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.