![]() One pattern of change in the evolution of ammonoids as a clade is that their suture morphology became more complex with time. The most undulated, complex sutures are found in the prolific ammonoids of the Cretaceous, the ammonites (below right). In contrast, ammonoid sutures dip and fold in undulations called lobes and saddles (below left). In nautiloids these lines are straight and are called simple sutures (above center and right). Sutures are contact lines between shell chamber walls (called septa) and the inner shell wall of nautiloid and ammonoid shells. In nautiloids, it runs through the center of the shell chambers, while in almost all planispiral ammonoids, it is found along the shell's outer edge (above left). The siphuncle is an internal tube that runs through and connects the chambers of the shell. ![]() The other two are of nautiloids exhibiting the simple sutures typical of the group. The photo on the left shows the position of the siphuncle in ammonoids. Though nautiloids and ammonoids may appear the same, they are easily distinguished by the location of their siphuncle and the shapes of their sutures. This morphology is very similar to many of the ammonoids, which first appear in coiled form in the Devonian Period. All have shells with nacre and interconnected internal chambers, similar to what we see in the modern nautilus. The earliest forms were orthoconic (having straight shells), but during the Ordovician the nautiloids experienced a rapid diversification and evolved a planispiral (coiled in a single plane) shell shape. Nautiloids are the earliest cephalopods found in the fossil record, appearing by the Late Cambrian. More familiar to us in the fossil record are the nautiloids, ammonoids, and belemnites. They flourished in Paleozoic oceans between the Ordovician (488 mya) and Triassic periods (200 mya) with shells that, in some species, reached nearly 10 meters in length. All members of these clades were squid-like, but had straight external shells called orthocones. So diverse in fact, that paleontologists have identified three distinct fossil clades that are entirely extinct: Endoceratoidea, Actinoceratoidea, and Bactritoidea (cladogram A, at right). Clearly the lineages of extinct taxa were prolific and diverse. There are about 17,000 named species of fossil cephalopods, compared to the 800 identified living species of cephalopods. Today, biologists and paleontologists continue to captivate the human mind and imagination with details of these molluscs' behavior, natural history, and evolution. From myths about their enigmatic fossilized remains to fantastic accounts of tentacled sea monsters, cephalopods also figure prominently in the literature and folklore of human societies around the world. Their lengthy evolutionary history spans an impressive 500 million years and the abundant fossils they've left behind (mostly shelled nautiloids and ammonoids) record repeated speciation and extinction events. These "brainy" invertebrates have evolved suckered tentacles, camera-like eyes, color-changing skin, and complex learning behavior. ![]() Squid, octopuses, cuttlefish, the chambered nautilus, and their relatives display remarkable diversity in size and lifestyle with adaptations for predation, locomotion, disguise, and communication. FUN FACT: From the dark abyss to shallow tide pools, research has recently revealed some of the mysterious behaviors of two famed cephalopods, the Giant Squid and the deadly Blue-ringed octopus (click image below for an enlargement).Ĭephalopods are the most intelligent, most mobile, and the largest of all molluscs.
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