The hunt for life on Mars: new findings on rock ‘chimneys’ could hold key to success

By Alexander Brasier, University of Aberdeen; David Wacey, University of Western Australia, and Mike Rogerson, University of Hull

The search for life on Mars has taken a step forward with the NASA Curiosity rover’s discovery of organic matter on the bottom of what was once a lake. It may once have been part of an alien life form or it might have a non-biological origin – either way this carbon would have provided a food source for any organic living thing in the vicinity.

The discovery adds extra intrigue to NASA’s search for extra-terrestrial life forms themselves. When hunting remotely with one car-sized machine, the question is where best to focus your efforts. It makes sense to look for the same types of places we expect to find fossilised microorganisms on Earth. This is complicated by the fact that these fossils are measured in microns – mere millionths of a metre.

The Curiosity rover looks for certain sedimentary rocks deposited near water, as it did for the latest discovery. This is based on the latest geological advice about the best prospects. Yet which rocks to prioritise is still a matter of some debate – and it’s a question that is just as relevant to geologists trying to unlock the secrets of our own ancient world. The Earth’s rocks and fossils are the nearest thing we have to time machines.

For a century or so, geologists focused on a type of rock called a stromatolite – devoting long hours to crawling around in awkward spaces trying to find them. Stromatolites occur mainly in shallow water and are layered on a millimetre scale. Many of them are undoubtedly built by slimy microbial “biofilms”, but to cut a long story short we now appreciate there is more than one way to make a stripy rock – and they don’t all involve microbes.

Stromatolite city.
Mike Beauregard, CC BY-SA

More recently geologists have become more interested in other types of rocks, including the “black smoker” tube-type deposits formed by hot hydrothermal water being squeezed out of the Earth’s crust in the deep sea. Slightly easier to examine are similar chimney-like formations found in certain alkaline lakes around the world.

Mono Lake

One place on Earth where these chimneys occur is Mono Lake in California, a vast and beautiful stretch of water several hundred miles north of Los Angeles on the eastern slope of the Sierra Nevada mountains. In October 2014, our team obtained permission from the California State Parks to examine and sample some of the calcium carbonate chimneys that have formed there.

The rocks, which are frequently between two and three metres tall, are very young in geological terms, usually only tens of thousands of years old. But since first being described by the famous American geologist Israel Russell in 1889 they have proven an excellent natural laboratory for groups of scientists trying to understand how these structures came about.

Exploration begins.
Alexander Brasier

Before our visit, geologists were essentially divided about these chimneys. A group we might call “pure geochemists” proposed they were nothing to do with microbes, but produced by calcium-rich spring waters coming into contact with the alkaline lake, with its abundance of carbonate ions.

A smaller opposing camp agreed it should be possible for these structures to emerge in the way that pure geochemists were suggesting. But they pointed out that, in the few recorded observations of carbonate rocks forming at the lake in the 19th and 20th centuries, some kind of biofilm did appear to have an influence. They also cited other studies that had shown that waterborne microbes called cyanobacteria did produce slimy substances that can accumulate calcium.

We went to Mono Lake to find out who was right. Our six-strong expedition divided into two factions: one looked for chimneys on the lake bottom using a research boat, while the other explored the famous “tufa towers” that rise up from the lake shore.

Tufa towers on the shoreline.
Alexander Brasier

The boat party toiled and cursed the astonishingly salty waters of the lake, while the shore party made steady progress with the invaluable assistance of local state park ranger, Dave Marquart. Their peace was interrupted only by a phone call from the stranded boaters requesting they urgently try to find someone with a four-wheel drive capable of pulling the boat back out of the water – luckily help was at hand.

One of the sites the shore party visited was in Marquart’s own back garden to the north-west of the lake. The rocks there were part of a set of ancient chimneys formed along a small tectonic fault. Their features suggested they had been built by microbes, but we needed to send them to a lab to be sure.

Microbial ‘threads’

Using an optical microscope, we were able to see dark thread-like structures entombed in slices of the rock. As we outline in our new study published in Geobiology, these “threads” are millions of fossilised photosynthesising cyanobacteria that once surrounded waters rising from a spring on the lake floor.

We sent the samples to Australia for further testing to establish whether the microbes played a key role in building the chimneys. This revealed surrounding patches of carbon and nitrogen, which we took to be fossilised cyanobacterial slime. This slime traps calcium and when it breaks down it creates calcium carbonate, entombing any living and dead cells in rock.

We found other ways in which this microbial slime had affected the fabric of the rock: grains of quartz and aluminosilicates that were clearly sand that had got stuck there, too.

Thread-like filaments in the Mono Lake rock.
Alexander Brasier

In short, we found evidence that cyanobacteria formed tubular mats around rising spring water in the ancient Mono Lake – probably producing the majority of the resulting chimneys there, though there may be examples of “pure geochemistry” chimneys as well. This suggests that these rock formations do indeed represent a promising and fairly large target for exploring ancient or extra-terrestrial life.

They have the added advantage that the calcite rocks in question are geologically quite stable. This means the fossils could potentially be preserved for a very long time – easily hundreds of millions, quite plausibly billions of years.

The ConversationTo our knowledge no chimneys have been found on Mars yet, but they are not common on Earth and there is every chance that they have a Martian equivalent. There, and on other planets and moons, we should be looking for areas with conditions as similar as possible to where these chimneys exist on Earth – volcanic rocks where spring waters might once have risen through the bedrock into an alkaline lake. Without any question, NASA’s hunt for suitable rocks on the red planet should make finding them a high priority.

Alexander Brasier, Lecturer in Geology, University of Aberdeen; David Wacey, Australian Research Council Future Fellow, University of Western Australia, and Mike Rogerson, Senior Lecturer in Earth System Science, University of Hull

This article was originally published on The Conversation. Read the original article.

Ancient ancestors of modern baleen whales were toothy not-so-gentle giants

File 20180509 34038 10d0yhp.jpg?ixlib=rb 1.1
A life-like reconstruction of Llanocetus denticrenatus, the second oldest “baleen” whale ever found.
Carl Buell, CC BY-SA

By Felix Georg Marx, Monash University and Robert Ewan Fordyce

The largest living whales – including the gigantic 30-metre blue whale – are fast predatory hunters that support their massive bodies by filtering large volumes of tiny prey from cool near-polar waters. They do this using baleen: plates of a tough substance hanging from their upper jaw.

Evidence of early evolution of baleen whales remains both sparse and controversial, with several ideas competing to explain the origin of baleen-based bulk feeding.

New evidence published today, based on our detailed analysis of a large, 34 million year old Antarctic fossil whale, Llanocetus denticrenatus (“yano-seetus” denticrenatus), shows that this whale was all gums and teeth, but had no baleen.

Read more:
How ‘Alfred’ the whale lost its teeth to become a giant filter feeder

Our findings suggest that large gums gradually became more complex over time and, ultimately, gave rise to baleen – and that these ancient whales became giants before they evolved their baleen feeding habits.

A snapshot of Llanocetus

The specimen is the second oldest “baleen” whale ever found. It is an ancestor of modern baleen whales, such as humpback and blue whales, except that it had no baleen. Instead, this whale had large gums and teeth, likely used to bite prey some 30 centimetres long.

The size of this whale is surprising, given its place in the evolution of whales. Its skull proportions indicate a body length of 8 metres, about the size of a modern minke whale.

Partially reconstructed cast of the skull of Llanocetus. Andrew Grebneff, who prepared much of the fossil, gives an idea of its size.
R Ewan Fordyce, CC BY-SA

The nearly complete skull, minus the tip, is more than 1.6 metres long, and in life was probably more than two metres. Other parts of the skeleton are similarly large and strongly built. There are other unexpected features: Llanocetus has huge openings for jaw muscles, implying a powerful bite, but its teeth are small relative to skull size.

Further, adjacent teeth are separated by wide gaps, and the bony palate has multiple grooves for soft tissues, reminiscent of baleen-related grooves of living species. Yet, we propose that Llanocetus was a predator that bit and sucked its prey, rather than filtering it from the surrounding water like modern baleen whales.

Serendipitous discovery

Field site on Seymour Island, Antarctica, where Llanocetus was discovered.
R Ewan Fordyce, CC BY-SA

Fossil discoveries in new territory are hoped for, and sometimes expected, but there is usually an element of serendipity. One of us, Ewan Fordyce, found the Llanocetus specimen at an inauspicious site while visiting Seymour Island, just east of the Antarctic Peninsula, with a US field party of paleontologists. Rocks on the island include shell-rich marine sediments – with reports of rare whale bone dating to about 34-35 million years.

Fordyce initially saw bone fragments scattered in an eroding gully, and followed them uphill to a mother-lode: hard cemented boulders containing obvious large skull bones and, finally, a distinctive tooth with finger-like projections. Our field party excavated more bones from their source in a layer of icy sandstone, and crated the material for eventual preparation and study in New Zealand.

Toothed “baleen” whales

A cheek tooth of Llanocetus.
R Ewan Fordyce, CC BY-SA

At 34-35 million years, Llanocetus is a little younger than a related whale, the roughly 36 million year old Mystacodon from Peru. These two whales are the oldest described for the lineage leading to modern baleen whales. They lived shortly before long-term global climates changed from warm greenhouse conditions to a cooler icehouse world.

Today’s baleen whales include the fast-swimming rorquals, such as the blue and minke whale, and the slower-moving right whales. These whales are toothless, but have hair-fringed flexible plates of baleen hanging from the upper jaw – hence their name.

Baleen plates have a distinct bony origin on the upper jaw bones, which in modern whales is often marked by a series of openings and grooves for associated blood vessels. We can trace that bony origin in fossil whales back to around 24-30 million years ago. Such fossils from New Zealand represent several groups in the early history of baleen-bearing whales.

For example, Mauicetus is near the start of the rorqual lineage, while Toipahautea is close to the common ancestor of modern rorquals and right whales. Tokarahia traces back to the very base of the baleen whale radiation.

If we go further down the evolutionary tree, we find smaller whales with ancient-looking skulls – the aetiocetids and mammalodontids. These animals lack clear evidence for baleen, but they do have functional teeth. Hence, we often use the formal name Mysticeti for the lineage of true baleen whales, modern and fossil, plus their toothed precursors such as Llanocetus.

Interpreting Llanocetus

Initially, the skull of Llanocetus looks like a hybrid: a flattened, triangular upper jaw like that of a minke whale, but with the teeth and the remaining skull reminiscent of a basal whale, such as Basilosaurus. Detailed comparison of structures amongst baleen and other whales, however, shows that Llanocetus indeed is in the lineage leading to modern baleen whales.

We used tooth form, wear and placement in the jaws to infer feeding. The cheek teeth are separated by wide gaps, but these gaps were not filled by alternating upper and lower teeth to form a mechanical sieve. Rather, polished wear patterns on the tooth crowns indicate that the upper and lower teeth sheared against one another to bite and slice food.

What, then, filled the large gaps between the adjacent teeth? Baleen is unlikely. The bony palate has multiple grooves probably for blood vessels and nerves, but the grooves run directly to the tooth sockets where baleen plates would be unlikely to function, given the shearing movement of the teeth. We propose that the palatal grooves supplied gum tissue both around and between the teeth.

Baleen origins

Earlier research proposed that several of the toothed ancestors of modern baleen whales sucked in small fish with a piston-like tongue. In some species, wear patterns on the teeth indicate that prey items were sheared apart. It seems, however, that no species fed using a combination of teeth and baleen, or fed using teeth to sieve prey from the water.

The ConversationLlanocetus confirms this pattern, and suggests that the earliest whales did not filter feed, but used raptorial and/or suction feeding. Baleen arose only later, probably from the enlarged gums that were already present in Llanocetus. Nevertheless, Llanocetus managed to grow large some 25 million years before our modern gentle giants. Long before orca and giant sperm whales, it was one of the largest predators of its time.

Felix Georg Marx, Post doctoral research fellow in evolutionary biology, Monash University and Robert Ewan Fordyce, Professor in paleontology

This article was originally published on The Conversation. Read the original article.