LongevityĪmmonoids were common in the oceans of the late Paleozoic and Mesozoic eras, from about 400 to 65 million years ago, becoming extinct at the same time as the dinosaurs. Some paleontologists believe that extensive predation so reduced the numbers of ammonoids that there were not enough to survive the mass extinction of 65 mya. Another theory is that because ammonoids grew faster and had thinner shells, the shells were not as strong as those of the nautilus. In contrast, in the ammonoids it was located next to the outer margin along the keel of the shell, where it was potentially more susceptible to injury. In the nautilus, the tube passed through the central part of each partition and was therefore positioned in the better protected central part of the whorl. Many believe the reason may be related to the difference in the anatomic placement of the siphuncle, the tube which allowed transfer of fluid and air inside the chambers of the shell. The reason why the ammonoids that had adapted for millions of years became extinct while their more primitive relative, the chambered nautilus, survived is a puzzle to scientists. The differences in the suture patterns help paleontologists to study ammonoid evolution. AdaptationĮarly in the evolution of ammonites the sutures were gently wavy lines but over time they became convoluted and more complex. It attacked the ammonite and extracted its body, leaving the shell to eventually sink to the ocean floor where it became fossilized. One reptile was the mosasaur, a giant snake-like reptile that measured up to 18 m (59 ft). It is believed that by secreting gas and fluids into the hollow shell chambers through their permeable siphuncle that they could regulate the air pressure in the chambers and thus control their balance, buoyancy, and depth in the water column.īy examining bite-marks on ammonoid shells, scientists have determined that they were preyed on by large marine reptiles and fish. Like their modern day relatives, ammonoids swam by using jet-like propulsion. Little is known about the reproduction of this species. Their radula, a file-like feeding structure, further shredded the food before it was swallowed. It is believed that they captured prey with their retractable tentacles and passed it to their mouth where a beak-like jaw tore it into pieces. They probably migrated at night from the ocean depths to shallower areas to feed, just as the chambered nautilus does today. Voracious predators, ammonoids fed on a variety of marine creatures including fish, crustaceans, and, especially during the Paleozoic, trilobites. The size of these animals ranged from less than 2.5 cm (1 in) to 3 m (9 ft) in diameter. The living animal added chambers to the open end of the coil as it grew and its body occupied only the newest chamber. Attached to the inside of the keel of the shell wall was a thin calcareous tube-like structure called a siphuncle which passed through each of the animal’s empty internal shell chambers. The junction of each septum with the outer shell wall is called a suture and it is these sutures that reveal the internal complexity of the ammonoid shell. Their shells were generally flat spirals containing a series of progressively larger chambers divided by thin walls called septa. Physical CharacteristicsĪmmonoids are descendants of the extinct, primitive coiled nautiloids and they are extinct relatives of modern squid, octopus, cuttlefish, and nautilus. These mollusks are believed to have lived in ocean depths from 50-250 m (164-820 ft). Geographic DistributionĪmmonoid fossils are found around the world in a variety of marine sedimentary rocks that range in age from the Early Devonian (415 mya) to the Late Cretaceous (65 mya). To view ammonites visit the Los Angeles County Natural History Museum. The fossil ammonites are no longer on display. CLIMATE CHANGE: Not Applicable At the Aquarium
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