An ancient cephalopod fossil may be about to rewrite octopus history, but it depends on who you ask. At the very least, itâ€™s offering up a lesson in how hard it is to classify some fossils.
Because their soft bodies decay easily, itâ€™s rare to find well-preserved fossils of cephalopods, a group that includes octopus, squid and cuttlefish. The relatively slim pickings of fossils have made establishing the animalsâ€™ family tree a headache for paleontologists.
Enter Syllipsimopodi bideni, an approximately 330-million-year-old fossil with exquisitely preserved suckers and 10 arms. The specimen was donated to the Royal Ontario Museum in Toronto in 1988 after its discovery in Montanaâ€™s Bear Gulch Limestone, a treasure trove for soft-bodied fossils. A closer look suggests that the fossil is a type of cephalopod called a vampyropod, researchers from the American Museum of Natural History in New York City report March 8 in Nature Communications.
If true, that would make this newly designated species the oldest ancestor of octopuses by about 80 million years. This would suggest that some ancient octopus features evolved much more quickly than previously thought. â€œThis is overturning about 100 years of science in cephalopod evolution,â€ says invertebrate paleontologist Christopher Whalen. But not everyone is convinced.
The classification hinges on the fossil having a gladius, a hard internal body part shaped like a Roman sword of the same name. The gladius can be identified by slender growth lines along the fossilâ€™s edge, as well as a rib running down the center of the fossil.
But where Whalen and paleontologist Neil Landman see a gladius, others see something else.
â€œThatâ€™s not the gladius, Iâ€™m sorry,â€ says Christian Klug, a cephalopod paleontologist at the University of Zurich. He argues that the slender lines are actually evidence of a flattened phragmocone, the series of chambers found in the shells of early cephalopods. And if thereâ€™s no gladius, as Klug suggests, the fossil would not be a vampyropod after all.
Different interpretations of fossils are not uncommon in paleontology. A famous example is Tullimonstrum, more commonly known as the Tully monster. First discovered in 1955, paleontologists still disagree about whether itâ€™s a vertebrate (SN: 3/6/17).
â€œTheyâ€™re all looking at the same fossils and the same features,â€ says Roy Plotnick, an invertebrate paleontologist at the University of Illinois Chicago. But something as simple as orientation can affect the interpretation of a fossil. Plotnick is working on a study about a fossil that was classified as a jellyfish for almost 50 years; upon flipping it upside down, he realized itâ€™s actually a sea anemone.
Identifying fossil features is much more than eyeballing. For starters, paleontologists have a deep-seated knowledge of anatomy, biology and zoology. â€œMany of us know animal anatomy better than most biologists do,â€ Plotnick says. Paleontologists also need to understand the processes of fossilization and how animals decay. If a feature is missing, a paleontologist will consider whether it was absent in the animal when it was alive or just not preserved.
â€œYou need to come up with a frame of reference, some sort of interpretive framework, which is based on what you see,â€ Whalen says. For instance, the preserved suckers allowed him to identify S. bideni as a cephalopod. â€œOnce youâ€™ve gotten that, then you can start to focus on interpreting the different structures under that framework.â€Â Â
Prioritizing one piece of evidence over another can become somewhat subjective. â€œEven with well-preserved species, you can get terrific differences in interpretation,â€ says Kevin Padian, a vertebrate paleontologist at the University of California, Berkeley. Some scientists prefer not to stray from traditional means of classification. Some choose to emphasize certain parts of the anatomy over others. Some opt to lump specimens together into the same species, whereas others will differentiate them more readily.
Ultimately, the strength of the interpretation depends on how reasonable it is. â€œI usually use the phrase: What is consistent with the evidence that we have?â€ Plotnick says.
It might not sound like an exact science, but thatâ€™s the trick: Only the addition of evidence can increase certainty. In the case of S. bideni, the discovery of more specimens could help researchers home in on the correct interpretation. Sophisticated technologies could also help. In the last decade, new imaging techniques have been developed to look at the chemical makeup of fossils, allowing scientists to identify previously hidden details.
Still, â€œthere often isnâ€™t a definitive answer, because thereâ€™s just not enough evidence to decide for sure,â€ Padian says. â€œNobody speaks ex cathedra in science.â€