Fish of the Tethys Ocean

The world began the path to ice-house conditions, forcing life to adapt to the new, cooling conditions as the continents were approaching their present pattern of distribution and the oceanic circulation patterns were changing dramatically. The sharks and teleosts (modern bony fish) appear to have mostly passed the KT boundary event unharmed, and as the new era opened there were many vacant niches previously occupied by now extinct animals to be filled.

The author³ suggests Bolca, a small village in Verona, Italy, on the lower slopes of the Italian Alps, is one the best place to view a portrait of life in the Tethys Seaway in the Cainozoic.

At Monte Bolca there is a limestone succession that is 19 m thick that is packed with fossils that are preserved in intricate detail. As the site has been well known since the 16^th century there are many stories told about it. About 250 fish species from 82 families are known from the site, as well as a crocodile, a sea snake, and many marine invertebrates. There are levels in which the fossils have been exceptionally well preserved due to lime muds that were oxygen-starved at the time of deposition, in which the internal organs of the fish and some indications of skin colour have been preserved. There are a wide variety of fossils from larval fish to large battoid stingrays more than 1 m long. According to the author³ the presence of a nearby coral reef is indicated by the variety and numbers of marine animals that add to evidence from other sites indicating that following the KT extinction event the hexacorals were fully recovered.

Assessments of specific evolutionary developments towards the modern forms are made possible by these sorts of deposits, the mastery of movement in the 3D underwater world being one of the most important.

It is clear from the number of fish species, as well as the sheer number of remains in the Bolca site, that they had evolved a range of modes of swimming. Drag resistance was one of the most severe problems associated with movement through the water, particularly when higher speeds were required. Fish have evolved ways of minimising the 3 types of drag that were slowing them down when they swam, the surface friction being reduced by producing slime on the surface of their bodies as a lubricant, though this must of course be assumed for the fossil fish at Bolca. Drag was also reduced by developing a shape that was rounded at the front and tapered at the rear and the thickest part of the body being 1/3 of the distance from the front end. Fast-swimming fish of the present such as tuna demonstrate this; the tuna being the fastest swimmer among fish of the present, other species that are designed for sustained speed are swordfish and mackerel.

Some fish have evolved forms that suit them to being ambush predators, hiding where possible, then moving with high acceleration when prey comes close enough to be taken by a sudden burst of speed, a hunting method used by a number of reef fish of the present. Some fish have developed the ability to be very maneuverable. This is exemplified by butterfly fish and the blue gill that use their manoeuvrability to surprise smaller prey, as well as for escape from predators. This ability is achieved by the arrangement and nature of their fins.

The fish in the Monte Bolca deposit show that by this stage of fish evolution they were using many different types of fin, such as dorsal fins along the back, tail (caudal) fins, and anal fins on the ventral surface just to the rear of the anus. The intricate motion of the vertical fins that is coordinated with the movement of the pectoral and ventral fins, allow for stability and steering, as well as for forward and backward movement and braking. The speed of the fish is indicated by the shape of the caudal fin, being lunate for fast cruising speed, broad and flat for acceleration, and manoeuvrability is indicated by the number and delicacy of the other fins.

In the Monte Bolca deposit there are a number of sharks, some of which resemble tiger sharks that are often found among reef communities of the present. Sharks have survived all the extinction events, major and minor, that have occurred since they first evolved, in the process becoming very highly evolved predators. They have an excellent sense of smell that can detect blood or food in the water at concentrations as low as a few parts per hundred million and up to 15 m their vision is excellent. Along with the other elasmobranchs, the skates, rays and chimaeras, they have sense organs on the tops and sides of the head region, the ampullae of Lorenzini, with they can detect electricity. The ampullae consist of small sacs in which there is a jelly-like substance that coats folds of tissue that are very sensitive, connecting to the surface by tiny ducts that form tiny pores on the surface, and can detect voltages as low as low as 1 millionth of a volt that they can interpret. The high quality of the Monte Bolca fossils has allowed these ampullae to be found in the sharks, showing that they had already reached this level of sophistication.

Sources & Further reading

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