Cephalopods

WHAT ARE CEPHALOPODS?

Cephalopods are a group of ocean animals that include modern squid, octopuses, and chambered nautiluses, as well as extinct forms like ammonoids and fossil nautiloids. Cephalopods swim using jet propulsion—drawing water into their bodies and pushing it out through a tube called a siphon or funnel. Many have chambered shells, either inside or outside their bodies, which help them control their buoyancy. Cephalopods also include the largest invertebrate animals on Earth—giant and colossal squids that can grow over 40 feet long! There are three major groups of cephalopods: coleoids (which include living squids and octopods and the extinct belemnoids), nautiloids (including the modern nautilus and many fossil forms), and the extinct ammonoids. 

The Shell (and life without one!)

The only living cephalopod with an external shell (outside of the fleshy body) is the chambered nautilus. All other cephalopods around today have reduced internal shells, or no shell at all. The fossil record tells us that in the past many cephalopods – nautiloids and the extinct ammonoids – had external shells. Cephalopod shells are divided into chambers by walls called septa (singular septum). The chambers are connected by a tube called the siphuncle, through which gas and fluid move to regulate the animal’s buoyancy in water.  

Inside the Shell

As in all mollusks, a cephalopod’s body is covered by the mantle, and gills are located in the space under the mantle. All cephalopods also have large, well-developed eyes. Many coleoids, which lack a large external shell, avoid predators by changing color to provide camouflage. Many cephalopods have ink sacs that produce a dark cloud in the water while the animal escapes. The mouth is equipped with a beak shaped like a parrot’s.  All cephalopods have multiple tentacles or arms protruding from the head, but the number varies. Coleoids have eight or ten tentacles, which have suckers or hooks to grab prey, while nautilus may have more than 100 simpler narrow tentacles. 

Cephalopod Ecology

Almost all cephalopods are active swimming predators. Modern nautilus is the exception; it is a slow swimmer that feeds mostly by scavenging. While many species of squid live in huge schools (groups), the larger species are mostly solitary, as are octopus and cuttlefish.  

Fossil Cephalopods

The oldest known cephalopods are from the Late Cambrian Period, about 490 million years ago. Nautiloids show extraordinary diversity of species and shell form throughout most of the Paleozoic, and include the largest animals in the world at the time. Nautiloids are common Paleozoic fossils, including here in the Devonian rocks of central New York. Ammonoids evolved from nautiloids in the Early Devonian Period and were one of the most diverse and abundant groups of marine animals in the Mesozoic Era, before becoming extinct at the end of the Cretaceous (at the same time as non-avian dinosaurs). Ammonoids are very helpful for dating rocks precisely. Species without shells, like octopuses, are much rarer in the fossil record because soft tissue doesn’t preserve as well.  

Family Tree of Cephalopods

Living cephalopods (squid, octopods, cuttlefish, Spirula, nautilus) are only part of the diversity of body forms illustrated in the fossil record. Nautiloids, represented today only by 6 species of chambered nautilus, were once much more diverse in number of species and body form, especially in the Paleozoic. Ammonoids lived from the Devonian to the end of the Cretaceous, but are difficult to place in a family tree with other cephalopods because they are extinct. Most paleontologists think that they are more closely related to coleoids than to nautiloids, because of similarities in the siphuncle (tube connecting the chambers of the shell) placement. Coleoids include living squids, octopods, Spirula (ram’s horn squid), and the extinct belemnites. Coleoids have reduced or no shells, and when present the shells are internal rather than external as in nautiloids and ammonoids. 

COLEOIDS

Living coleoids include the familiar squid and octopuses, but also cuttlefish and Sepia, a small squid-like form with a coiled, chambered shell. 

Coleoids all have reduced or absent shells, and are well adapted to be highly mobile, swimming predators. They also have sophisticated behaviors and extremely well-developed senses, especially vision. Their large image-forming eyes resemble those of vertebrates, an excellent example of the independent (or convergent) evolution of similar characteristics for a similar function.  

Many coleoids have features that allow them to avoid predators, such as the ability to change color and release inky fluid to confuse pursuers. Octopuses are also famous for their intelligence, showing extraordinary abilities to solve puzzles and escape enclosures. Most coleoids are small, but others are enormous, including the Pacific giant octopus, which can reach more than 20 feet from arm tip to arm tip, and the giant and colossal squids, which can reach lengths of more than 40 feet and are the largest invertebrate animals known. 

Belemnites are an extinct group of coleoids that lived during the Jurassic and Cretaceous periods (about 200–66 million years ago). As in other coleoids with shells, belemnite shells were internal, and probably functioned as ballast for the animal as it swam. Belemnite shells consist of a backward-pointing, solid, bullet-shaped portion with an open forward-pointing cone-like center. This center has chambers separated with septa, and a thin flat rim projected from the opening. The chambers are usually broken in fossils and only the distinctive “bullet” is found.  

AMMONOIDS

Ammonoids are an extinct group of cephalopods with an external shell. They lived from the Early Devonian Period, around 400 million years ago, to the end of Cretaceous Period, around 66 million years ago. They were diverse and abundant in the Paleozoic, but became even more so in the Mesozoic, when they were likely major predators in oceans worldwide. Their fossils are very common in many Mesozoic rocks, especially the Cretaceous. The name “ammonoid” comes from the Egyptian god Ammon, who was frequently depicted with ram’s horns on his head. 

The intricate suture (septal) patterns on ammonites make them particularly fascinating, both aesthetically and scientifically. One question is what, if any, function these patterns had. Paleontologists aren’t sure, but these complex structures may have functioned to strengthen the shell and/or assist in retaining fluid in the shell which helped regulate the animal’s buoyancy. 

Ammonoids Can Tell Geological Time!

Their distinctive shells make ammonites very useful for determining geological age using biostratigraphy. Different ammonites, distinguished by the features of their shells, lived at different times. Once scientists established these patterns of occurrence in thick sequences of sedimentary rock, finding a particular ammonite in a rock of unknown age can tell you how old the surrounding rock is, often with very high precision. (For example, Cretaceous ammonites, in rocks more than 70 million years old, can be used to determine ages down to the nearest 100,000 years or even less!) 

“Ammon-oid” or “Ammon-ite”?

 Ammonoid is the name given to the larger group, within which three suture types are recognized that distinguish different subgroups from different times. Goniatites are Paleozoic (Devonian-Permian), Ceratites are Triassic, and Ammonites, which have the most complex suture types, are Jurassic and Cretaceous. 

“What did ammonoid tentacles look like?”

Ammonoids became extinct at the end of the Cretaceous Period (about 66 million years ago), at the same time as non-avian dinosaurs. So no human has ever seen a living ammonoid, and all of our knowledge about them comes from fossils. Soft body parts, like tentacles, do not usually fossilize. In fact, there are no known fossils of ammonite tentacles. This means that paleoartists (artists who make images of what extinct species looked like when they were alive) must use living cephalopods as models when reconstructing ammonoid tentacles.  

But which living cephalopods should a paleoartist use? Many reconstructions of ammonoids show narrow pointed tentacles similar to those of living nautiloids. Most paleontologists, however, think that ammonoids were more closely related to modern coleoids (octopuses and squids), but squid and octopus tentacles can look very different. 

Some paleontologists have recently suggested that ammonoids may have had tentacles very different from any living cephalopod, and worked with paleoartists to create images unlike any animal in modern seas. Until we find fossils that preserve ammonoid tentacles, we will likely never know exactly what they looked like. 

Learn more about ammonoid paleoart at this link!

Heteromorphs: Ammonites of very different forms

Most ammonoid shells are coiled in a single plane – a shape called planispiral. But in the Late Jurassic and Cretaceous periods (about 200-66 million years ago) some ammonites evolved very different shell shapes, including uncoiling and coiling in different geometries. These are called “heteromorphs”, which means “different form”. They include some of the strangest looking creatures in the fossil record. 

Like most ammonoids, heteromorphs likely floated and/or swam in the open ocean, rather than sitting on the bottom. The diversity of shapes among heteromorphs reflects their exploitation of different ecological niches in Mesozoic oceans. It is possible that differently shaped heteromorphs floated at different depths or different heights off the ocean floor, eating different prey. But paleontologists remain uncertain about exactly how deep each species lived, and what its orientation was when it was floating.

Learn more about ammonoids at: https://earthathome.org/quick-guide-common-fossils/#ammonoids 

NAUTILOIDS

Nautiloids are today represented by only a few species of chambered nautilus, which live in the Indo-Pacific ocean. But in the Paleozoic Era, between 500 and 250 million years ago, there was a huge diversity of nautiloids. This includes what were at the time the largest animals in the world—straight nautiloids that reached almost 30 feet long!