Placenticeras meeki was a member of the extinct marine taxon Ammonites in the Phylum Mollusc and Class Cephalopoda.Cephalopods are the most intelligent, mobile, and the largest of the molluscs. As hinted in their name, Cephalopods have a well-developed head which is attached to a ring of arms and tentacles. Their habitat is not located only around coasts but span from the surface to the sea bed. As life continues to evolve some species become extinct while others adapt traits or interbreed with other species to create a new one. Cephalopod molluscs are exclusively marine, and as a result of 500 million years, have become incredibly diverse and abundant. With Ammonites and their closest related descendants being extinct, to understand how these beings lived we look to their distantly related extant soft-bodied descendants Coleoids as well as the hard shelled Nautiloids. Due to spiral shape of their fossilized shells, it is probable that Ammonites such as Placenticeras meeki closest living relatives are the modern Nautilus to which they closely resemble.
FUN FACT: Ammonoids have intriguingly played a role in folklore. Ammonite fossils were first discovered by early Romans and were mistaken for horns. Subsequently, they were called “ammonites” because of its similarity to the coiled horns of the Egyptian ram-god Ammon.
The photo on the left shows a Nautiliod. Notice the similarity in the overall morphology between the Nautiliod and the Ammonite fossil in the center. The Coleoid on the right lacks a hard shell but possess tentacles like the Nautiliod and the Ammonite when it lived.
MORPHOLOGY, MOTILITY, MACROCONCHS & MICROCONCHS
The shell or chambered portion of ammonites is referred to as a phragmocone. It contains a series or spiralled chambers called camerae which are divided from each other by thin walls known as septa. The chambers progressively got larger until the last chamber or the body chamber was reached. Since soft body parts do not fossilize well little beyond digestive organs and an ink sac, like their octopus and squid descendants, are known. Phragmocone is the only portion of Ammonites to biomineralize and contribute to the fossil record.
Each camerae separated by septa can be seen. The biggest and final camerae is where the body of the Ammonite once was
At first glance, Ammonites may appear identical to fossilized Nautiloids but as one looks closer, differentiating features such as the location of their siphuncle can be observed. Unlike the centered siphuncle of Nautiliods , the siphuncle of Ammonites such as Placenticeras meeki was situated away from the center on its shell’s periphery. The siphuncle, a thin living tube, controlled the volume of water, ions and gases movement within each of camerae and allowed the Ammonite to float or sink. The Ammonite move salt ions into blood of the siphuncle and water would follow via osmosis. As ions and water move into the siphuncle, gas from the blood of the siphuncle would diffuse into the chambers of the shell. With a decrease of water in the chambers and increase in gas, the overall density of the Ammonite decreased while its buoyancy increased allowing it to float. To sink, the reverse process happened: ions and water move into the chambers while gas diffuses back into the blood of the siphuncle. As a living tissue, siphuncles unfortunately do not preserve well however, the space it once occupied can be seen in many fossils and is known as siphuncle notches.
Watch as a Nautilus regulates its buoyancy. Although the location of the siphuncle was different, its purpose remains the same for both Ammonites and Nautiloids.
Another differentiating feature between Ammonites and Nautiliods are the shapes of their sutures. Sutures are the intersection between the septa and the out shell of wall of Nautiloids and Ammonites. While Nautiloids and early Ammonites have simple, slightly curving sutures, later Ammonites have complex sutures which form peaks and valleys or saddles and lobes. These differences in sutures are significant as they give indications as to where and how Nautiliods and ammonites lived. The simple sutures of Nautiliods withstand great pressure but have poor buoyancy and so most likely wear slow moving bottom dwellers. The complex saddles and lobes of Ammonites provide great buoyancy control but did not withstand pressure quite well and so this is why many ammonites are believed to have lived in the open sea of ancient world. Most Ammonite fossils are found amongst rocks where no bottom dwelling life are found are further implications that these creatures lived in the open sea. It is interesting to note evolution of Ammonoid lineages going from moving slowly and living in deep water depths to fast moving in shallower ones due to the morphology of the shell. The most prevalent shell shape was the planispiral shape however; there were other shell shapes such as wide, open-coiled, twisted, and hooked. Research has shown that planispiral Ammonites lived in an active, pelagic lifestyle and wider shelled Ammonites lived closer to the ocean bottom and were slow movers
Top: Simple sutures of the Nautiliod vs. the complex lobes and saddles of an Ammonite.
Bottom: Peripheral siphuncle of an Ammonite vs. Nautliod's central siphuncle.
Mincroconch (left) is noticeably smaller than the macroconch (right).
FUN FACT: The Ammonites almost became extinct at the end of the Perimian and Triassic period. However, it is important to note that they successfully diversified after these fragile moments in time! Sizes began to vary and shells were becoming coiled, secondarily uncoiled, or partially uncoiled.
Apart from telling us where they lived, the shells also indicate the gender of an Ammonite. Ammonite possess sexual dimorphism which refers to physiological differences between females and males of the same species due to inheritance of genes that code for either one of the sexual patterns. Macroconch , females ammonites, are found to be larger than the microconch or males. It is believed that with the increase in shell size, female had more room to accommodate her brood of eggs. Upon reaching sexual maturity the macroconch would have migrated to shallow waters to lay their eggs while microconch remained below.