Earliest cephalopod fossil reveals ancient buoyancy control

An octopus can remain perfectly still in mid-water without drifting upwards or downwards. Scientists have now found the earliest known cephalopod fossil to show the emergence of buoyancy control.

The fossils are minute: their shells are only 1 millimetre long, or 0.04 inches, making them smaller than a grain of rice. They were collected from rocks in South China that are around 520 million years old.

Within these tiny remains sits a narrow tube extending along the shell’s length. The research team considers it the oldest siphuncle yet recorded - the tube cephalopods use to regulate buoyancy.

How a siphuncle works

Cephalopods are molluscs, a group that includes octopuses, squid, cuttlefish and nautiluses. Their ancient relatives produced either straight or coiled shells, separated into enclosed chambers.

A siphuncle is a fine tube running through these chambers. Moving fluid and gas in and out enables the animal to make its shell lighter or heavier, allowing it to maintain its position in the water.

Together with jet propulsion, this arrangement allowed cephalopods to move away from the seabed and pursue prey. It is among the characteristics distinguishing them from snails, clams and other molluscs.

However, the siphuncle’s origins have long been uncertain. The fossil record becomes sparse at the point where the structure first emerges, leaving an important phase of cephalopod evolution undocumented.

Earliest cephalopod fossil

The newly identified species has been named Eoceras shaanxiense. It was found in South China’s Shuijingtuo Formation, a rock unit assigned to Cambrian Stage 3 and dated to approximately 520 million years ago.

An international research group retrieved 32 specimens from this formation. Junfeng Guo of Chang’an University led the study, while Zuchen Song was the first author.

The co-authors were based in China, the United Kingdom and the United States. They included Bing Pan from the Nanjing Institute of Geology and Palaeontology, and Jakob Vinther from the University of Bristol.

Previously, the earliest cephalopod in the fossil record was Plectronoceras cambria, which dates to the late Cambrian. Eoceras shaanxiense is about 30 million years earlier.

This missing interval in the record has concerned researchers for many years. Molecular clocks, which use genetic data to estimate divergence dates, had instead suggested that cephalopods split from related groups during the early Cambrian.

Inside a 1 millimetre shell

The specimens were preserved by phosphatisation, in which phosphate minerals replace and coat the original shell material. This process can retain features only a few microns across.

Scientists examined the fossils using scanning electron microscopy and micro-computed tomography. Combined, these techniques allowed them to reconstruct both the shell’s exterior and its internal structure.

Eoceras shaanxiense possessed an orthoconic shell - a straight cone rather than a coiled form. Its aperture was angled, while a succession of internal walls, known as septa, divided the interior.

A segmented tube ran down one side of the shell and seems to have connected the chambers via tiny canals. The team interprets this tube as a primitive siphuncle.

A proposed route to buoyancy

The pattern retained in Eoceras enabled the authors to outline how the cephalopod shell may have developed. They propose three stages.

The first was a straight, chambered shell partitioned by septa. As the animal grew forwards towards the aperture, it sealed the space left behind.

The second stage involved a closed, segmented tube joining the chambers, which appears to be the condition represented by Eoceras. The third was a fully developed siphuncle with septal necks and connecting rings.

Later cephalopods used this final form to swim. Ammonites, belemnites and the nautilus that survives today all inherited this system.

Life on the seafloor

An early buoyancy mechanism would have provided little lift. The authors therefore suggest that Eoceras lived mainly on the seabed, rather than swimming in open water.

Its size supports that interpretation. At roughly 1 millimetre long, the animal was nearer the size of a grain of sand than that of an organism able to chase prey through the water column.

The active predatory way of life for which the group later became known emerged after its internal plumbing improved. Eoceras shaanxiense represents an earlier point, when that equipment was still rudimentary.

Limitations of the study

The paper is deliberately restrained in its conclusions.

The researchers refer to the tube as a possible primordial siphuncle and place Eoceras among stem cephalopods, rather than within the modern crown lineage of the group.

There is good reason for this caution. A number of past claims for early Cambrian cephalopods were subsequently reinterpreted as chimaeras - fossils combining parts from two unrelated animals.

Eoceras also does not show traits found in later cephalopods, and no soft tissue has survived. The specimens contain no evidence of arms, a funnel or muscles that could conclusively resolve the question.

The authors state that the issue remains unresolved. Further early Cambrian specimens will be required before the initial stages of cephalopod evolution can be clarified.

Why this discovery matters

If this interpretation is correct, the earliest cephalopod in the fossil record shifts back into the early Cambrian. That would place it close to the period already indicated by genetic estimates for the group’s origin.

It would also locate the group’s beginnings within the surge in animal diversification that generated most modern body plans. Under this view, cephalopods were part of that event rather than later arrivals.

The discovery also changes where scientists might search. Small shelly fossils - millimetre-scale fragments abundant in early Cambrian rocks around the world - may now be valuable places to look for cephalopod ancestors.

For a lineage that would eventually produce the octopus, this is an unassuming beginning. Its record may begin with a fossil small enough to lose in the palm of a hand.

Image credit: YANG Dinghua