First evidence of fossilized ‘earstones’ in Cretaceous-aged cephalopods

First Cretaceous cephalopod statoliths fill the gap between Jurassic and Cenozoic forms

Summarized by: Gabrielle Scrogham completed her Bachelor’s of Marine Biology degree in 2020 and is currently a Geology Master’s student at the University of South Florida, Tampa. She is studying methods for tracking changes in diet of fish over time based on stable isotope and trace metal analysis. Their interests include marine ecology and biogeochemistry. Outside of academia, Gabrielle enjoys snorkeling, painting, and practicing martial arts.  

What was the hypothesis being tested? Cephalopods are a group of animals which include octopuses and squids. In this paper, researchers compared recently discovered fossils of cephalopod statoliths, also known as earstones, from the Cretaceous Period (145 – 66 million years ago) to older earstone fossils from the Jurassic Period (201 – 145 million years ago) and to earstones found in modern squid and cuttlefish to see how they change over time. Earstones are small, calcified parts found in the head of animals to help with navigation, movement, and balance. Earstones, or other structures for sensing balance, are found in most animals and even some plants. Statoliths in cephalopods are analogous to structures called otoliths in bony fish, as they perform the same function. The researchers hypothesized that differences in the shapes and abundances of earstone fossils would reflect evolutionary changes in cephalopod lineages. They also hypothesized these newly discovered fossils would support evidence from other paleontologists that a large-scale shift in animal dominance from cephalopods to fish occurred during the Cretaceous. 

What data were used? Five fossilized cephalopod earstones were collected from two sites in Poland and England. Several images from scanning electron microscopes were used in this study, including the newly collected earstones, a Jurassic earstone fossil, and earstones from modern squids and cuttlefish. This allowed for detailed comparisons of their shapes. The earstone structures were then compared to data compiled from previous studies on cephalopod lineages. The number of earstones found for both cephalopods and fish from each time period were also counted. 

Methods: Sediments were wet sieved using a fine (0.375 mm) mesh to separate fossils from the clay and silts in the rocks. Both collection sites were chosen due to being known Lower Cretaceous formations and for being uniquely accessible at the time of sampling. The collection site in Poland is now flooded, meaning additional samples from this area cannot be accessed. Once identified, the fossils were photographed using a scanning electron microscope. 

Results: The shape of the single earstone fossil from Poland differ than those found in England, which are more similar to earstones from Jurassic fossils. Some differences in traits include the shape and angle of the rostrum, the spur, and the lateral dome (see Figure 1 for what these features look like). Since the earstone from Poland has unique features not seen in other fossils, this likely represents a new lineage of cephalopod. There are some indications that the Poland specimen may have been a juvenile organism, so in addition to only having one fossil available, this data cannot be considered comprehensive. The earstones from England are more similar to fossils from the Jurassic, which means they may be more closely related. In addition to the analysis of the shape differences, the number of cephalopod and fish earstones were compared. Cephalopod earstones were more abundant than fish earstones in the Jurassic sediments, while fish earstones become more dominant in Cretaceous sediments. Thus, this is evidence for a shift in dominance from cephalopods to fish around 145 million years ago.

Four rows of earstone fossils pictured at different angles with significant features labelled. Figures are labelled by letter for specimen and by number for angle, with a total of six angles showing differences in the structures. Most are shaped loosely like ovals, but have thicker and thinner parts to them distorting the shape. Average size is just over 200 microns.
Images from scanning electron microscope comparing cephalopod earstones, also called statoliths, from different angles. Four samples are represented including A: modern pygmy squid; B: Cretaceous cephalopod fossil from Poland; C: Jurassic cephalopod fossil from Poland, and D: modern cuttlefish. Some distinguished features include the rostrum (r), spur (sp), and the lateral dome (ld). Scale bar = 200 microns.

Why is this study important?: Cephalopod statoliths have been found and recorded from both the Jurassic and the Cenozoic (modern), but not from the time period between those two, which is the Cretaceous (145 – 66 million years ago). This type of comparative data can reflect changes in animal lifestyle, such as shallow versus deep water environments, which can be used to reconstruct ancient habitats. Similarities between these fossils also allow researchers to look for connections in evolutionary lineages over time. Some of these fossils are also collected from sites that are no longer accessible, increasing their scientific value as it would be difficult to collect new evidence from the same regions. 

Broader Implications beyond this study: This study documents shapes of new cephalopod earstones, which could be used to examine ancient environments and reconstruct what types of animals lived in those habitats. The proposed shift in dominance of cephalopods during the Middle Jurassic to a dominance of bony fish in the Early Cretaceous would demonstrate a large-scale change in marine ecology. This dominance shift would also additionally explain the noticeable increase of fish abundance over cephalopods in the fossil record and modern oceans.

Citation: Pindakiewicz, M, K., Hyrniewicz, K., Janiszewska, K., & Kaim, A.  (2022). First Cretaceous cephalopod statoliths fill the gap between Jurassic and Cenozoic forms. Comptes Rendus Palevol, 21, 801–813. 

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