Episode 16 – Cephalopods

Find Episode 16, as always, on PodBean, iTunes, or Stitcher!

In this episode, we explore an odd group of highly intelligent, wriggly-armed marine invertebrates. We look into the modern diversity, fossil history, and unique adaptations of the Cephalopoda.

In the News:
Huge Titanosaur Receives a Name
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The Mysterious Chilesaurus
It has long been unknown where Chilesaurus fits on the dinosaur family tree. A new study attempts to sort out this small biped’s relationships to other groups. [Report]
Ancient Atmosphere In Ice
A 2.7-million-year-old ice core from Antarctica is now the oldest ever recovered, providing insights into the atmosphere and climate at the beginning of the Pleistocene Ice Age. [Report]
A Croc With a Famous Name
From a museum collection, a Jurassic marine crocodylomorph has been re-identified and re-named Lemmysuchus in honor of the Motorhead singer. [Report]

What is a Cephalopod?

Cephalopods are a very unique group of animals. Today this group includes two main branches; the coleoids (octopuses, squids, and cuttlefish) and the nuatiloids.

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Common Octopus (Top Left), Caribbean Reef Squid (Top Right), Giant Cuttlefish (Bottom Left), Palau Nautilus (Bottom Right). Image modified from Wikimedia Commons.

Unlike their fellow mollusks (including clams, snails, and many more), cephalopods have shifted to a more streamlined and mobile lifestyle. All are active predators and hunt a variety of prey, from crabs to small sharks.

Cephalopods are by far the most intelligent of invertebrates. They can even rival many vertebrate animals with their problem solving skills.

And on top of that, many of the coleoids have incredibly flexible bodies, malleable skin, color-changing chromatophores in their skin, and an ink sac for defense. These all combine to make them masters of camouflage and escape.

Now this has been heavily focusing on the octopus, which deserves all the attention it gets, but there are other cephalopods to be mentioned. Squids and cuttlefish share many features with the octopuses, but have the addition of two more arms (for a total of 10) called tentacles. These are retractable hunting appendages with suction cups only at their tips.

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The tentacles shown extended and separate from the typical arms. Image modified from Wikimedia.

Nautiluses are the only living cephalopods that still have a calcified external shell. This shell has chambers where gas can be moved in and out, allowing the nautilus to control its buoyancy, a feature that was common among ancient cephalopods.

File:NautilusCutawayLogarithmicSpiral.jpg
A cross section of a nautilus shell shows the chambers it uses to control where it floats in the water column. Image by Chris 73, Wikimedia.

Fossil History

Cephalopods first appear in the late Cambrian over 500 million years ago. Their earliest members had large cone shells and faced downward. They are thought to have evolved from a group of mollusks very similar to monoplacophorans, creatures with a foot like a snail and a single, simple shell on their back. This mollusk foot specialized into the many arms of the cephalopods.

From there, and throughout the Paleozoic, cephalopods diversified intensely. During this time we see the chambered shell take on a vast variety of shapes. These included everything from straight cones (the orthocones) to highly curved shells.

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Two heteromorph ammonites (Right and Top Left) and a pair of orthocones (Bottom Left). Image modified from Wikimedia Commons.

Many groups arrive at a spiral design that now shifts them to be facing forward, allowing for a more active lifestyle. The most famous fossil forms of this shell is the ammonites, which appear over 400 mya in the Devonian Period and last until the end of the Mesozoic.

File:Ammonites Dactylio Ceras Commune Schleifhausen Germany Jura.jpg
The classic ammonite spiral shell is one of the most common marine fossils throughout the Mesozoic, making them very helpful for biostratigraphy. Image by Richie Diesterheft Wikimedia Commons.

Another famous extinct group that found success in Mesozoic oceans are the belemnites. These were squid-like, with a chambered shell much like an orthocone, but this shell was internal. This reflects another major trend we see in cephalopods.

Losing The Shell

Many groups began to internalize their shells to streamline the body and have more control over their motion. We still see an example of this with the modern cuttlefish’s chambered cuttle bone, a calcified structure along the top of the inside of their body.

Some groups continued to reduce the shell, decalcifying it all together. Most squids now only have a flexible rod and octopuses have barely any hard structure to their body at all. This allows for unparalleled maneuverability, but makes them much less likely to fossilize. For as common as shelled cephalopods are in the fossil record, soft bodied ones are incredibly rare. But every now and then specimens are found. Here are some wonderful examples of fossil coleoids; 1, 2, 3, 4.

For more info on fossil and modern cephalopods, please feel free to check out the links below!
Science Friday: Cephalopod Week
University of California Museum of Paleontology: The Cephalopoda
The Cephalopod Page
Cephalopod Origin and Evolution

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