Dinosaurs of the Sea


Artist depiction of an elasmosaur from Oceans of Kansas

Our coverage of the evolution of marine organisms leaps forward here with the introduction of the most beloved fossils on Earth, the dinosaurs. Most of the dinosaurs we know lived in the Mesozoic Period, which includes the Triassic, the Jurassic, and the Cretaceous (from 225 to 65 MYA). Before we examine a few of the inhabitants, we first need to quickly review the "state" of the world at this time.

Towards the end of the Paleozoic (about 280 MYA), the continents (we can't forget the continents) assembled into a kind of giant supercontinent known as Pangaea. As we will see in another lecture, the continents literally float in the molten mantle of the Earth. Since their formation 4.5 billion years ago, the continents have performed a kind of balletic movement, coming together, disbanding, and coming together again. While the early history is not well documented, we now have pretty good evidence of the movement of the continents since the Cambrian.

At about 245 MYA, the continents had begun to split up again, forming a new ocean called Tethys. The opening of the Tethys Ocean began a process which led to the formation of the Indian Ocean and the Atlantic Ocean, which features one of the largest mid-oceanic ridges on our planet. This ocean and the Panthallassa Ocean, the precursor of the Pacific Ocean, constituted the major oceans of the time.

It was in the Tethys Ocean that one of the more interesting oceanic dinosaurs has been found. Known as Cryptocleidus, this plesiosaur ruled the seas during the late Jurassic (190 - 136 MYA). Cryptocleidus was an impressive predator, boasting outwardly angled teeth that let it deftly grab its prey. Most likely, this dinosaur was an excellent swimmer, flying through the water with the greatest of ease, much like a sea turtle or sea lion.

Another denizen of the deep at this time was a type of marine crocodile. Looking very much like their modern counterparts, these beasts had flippers instead of claws. Their strong tails and flippered appendages allowed them to swim rapidly to feed on their favorite foods.

One of the favorite foods of many oceanic dinosaurs was the ammonites, which flourished in the Tethys Ocean during the Cretaceous (136 - 65 MYA). Relatives of the chambered nautilus, squids, and octopus, these creatures carried a calcified shell on their back filled with air pockets that allowed them to move through the water using a form of jet propulsion. Fossils of ammonites are found everywhere that ancient seas once existed.

At the end of the Cretaceous, another catastrophe of epic proportions hit our planet, wiping out nearly 70% of the Earth's species at this time. The demise of the dinosaurs has been one of the great riddles of our time, and many scientific and popular theories have been proposed for their disappearance. However, there is now strong evidence that the extinction of the dinosaurs was caused by the impact of a major meteorite in the Gulf of Mexico. Known as the Chicxulub structure, named after the small Mayan village in its proximity, this crater has been dubbed the "smoking gun" of the Cretaceous-Tertiary boundary when the mass extinction occurred.

While Mexican geologists had long suspected that the circular geophysical anomalies in the area were of extraterrestrial (i.e. meteoric) origin, it wasn't until the bold proposition of Luis Alvarez and his geologist son, Walter, that the idea began to gain favor. Their proposition stemmed from their discovery in 1980 of a centimeter-thick layer of clay in limestone deposits dated at about the time of the mass extinction. By a fortuitous sequence of events, Walter learned of the Mexican scientists' samples and began a more intensive investigation of the area. By 1990, teams of scientists from the United States and Mexico had assembled an impressive body of evidence that the Chicxulub structure was indeed an impact crater.

Announced in the journal Nature in 1992, the theory has gained many converts. Subsequent measurements of the crater from the space shuttle and from marine seismic-reflection instruments have established that the crater diameter ranges from 110 to 180 miles across. This dimension makes it the largest of all impacts known to have occurred on our planet. More work remains to establish the complete dimensions of the impact, which lies beneath 1,000 to 3,000 feet of limestone. Still, this exciting "discovery" in our lifetime provides a sobering message for the power of the Universe in altering the course of life on our planet.

The Cenozoic (65 - 2 MYA)

As we move from the Paleozoic to the Cenozoic, we begin to see many of the species of organisms that are familiar to us today. By this time, the three major oceans as we know them -- the Pacific, the Atlantic, and the Indian -- were well established. In addition, the formation of the Mediterranean Sea and the separation of Australia from Antarctica was completed during this period. Of the animals that came into being during this period, I will restrict our discussion to two of our favorites -- great white sharks and whales.

The oldest known fossils from the ancient relative of the great white are teeth from a shark known as Carcharodon megalodon. An inhabitant of the Miocene (27 - 7 MYA), this monster of the sea reached lengths of 40 - 100 feet. Its teeth -- up to 7 inches in length -- were sharp and triangular with serrated edges that cut like a Ginsu knife. Without much trouble, this massive shark could stretch its jaws to accommodate a full grown man (or woman...I don't imagine it was particular). Fortunately, our present-day great whites, Carcharadon carcharias, have a maximum known length of 21 feet, as evidenced by a 7,300-pound specimen caught in Cuba in 1945. Their teeth have shrunk, too, down to a mere 2.5 inches. Still, their bite has been measured at 21 tons per inch, something I don't care to experience.

Question: Who are the modern day dinosaurs of the sea?

A Summary of Geological Time:

Please be familiar with the major divisions of Precambrian time and the eras of the Phanerozoic.

The Geologic Timeline Compressed to One Calendar Year
from Elementary Astronomy by Nick Strobel
http://www.bc.cc.ca.us/programs/sea/astronomy/book.htm

Origin of the Universe--Jan. 1 Origin of our galaxy--Jan 24
Solar system origin--Sept. 9 Earth Solidifies--Sept. 14
Life on Earth--Sept. 30 Sexual reproduction advent--Nov. 25
Oxygen atmosphere--Dec. 1 Cambrian explosion (600 mil years ago when most complex organisms appear, fish, trilobites)--Dec. 17
Land plants & insects--Dec. 19, 20 First amphibians--Dec. 22
First reptiles & trees--Dec. 23 First dinosaurs--Dec. 25
KT impact, mammal age, birds--10:00 AM Dec. 30 First primates--Dec. 30
Australopithicenes (Lucy, etc.)--10:00 PM Dec. 31 Homo habilis--11:25 PM Dec. 31
Homo erectus--11:40 PM Dec. 31 Early Homo sapiens--11:25 PM Dec. 31
Neanderthal man--11:57 PM Dec. 31 Cro-Magnon man--11:58:38 PM
Homo sapiens sapiens--11:58:57 PM Dec. 31 Human history--11:59:39 PM
Ancient Greeks to now--last five seconds Average human life span--0.15 seconds

Useful Links:

Kid's Guide to Dinosaurs
http://www.enchantedlearning.com/subjects/dinosaurs/dinos/Plesiosaur.shtml

Oceans of Kansas: Awesome resource for ocean-swelling dinosaurs
http://www.oceansofkansas.com