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A mysterious evolution of wonky whale skulls revealed by a new study

Some whales are winky. You may not know how to look at them, but their skulls are actually incredibly asymmetrical. This mysterious feature helps with echolocation, the way whales are worked out where things come in by making sounds and feeling how they are reflected back.

But this winkiness is not present in all whales. Colleagues and I recently conducted research to find out why and when wonky whales began to evolve differently for their symmetrical cousins. We now know wonky whale skulls that first appeared about 30 million years ago, and that they continued to become more asymmetrical as creatures evolved into the modern species we know today.

In order to understand how wonky whales got this way, we need to look at how they lived and adapted in the past. Fortunately for us, the whale fossil record is remarkably represented that scientists have even called whales a “posterchild of evolution”. Complete skulls and skeletons date back to the first whales of 50 million years ago, and more fossils are marked throughout whale history, to the living animals we know today.

3D scanning of a narwhal skull showing asymmetry
Asymmetrical narwhal skull – the red arrows highlight the scattered bone.
Ellen Coombs / UCL, Author provided

With this record, we could see that the whales ’nostrils moved from the tip of their chins to the top of their heads, an evolutionary tactic to make breathing easy at the surface of the water. And toothed whale skulls (which technically includes dolphins, as well as species such as whale sperm) have become more lopsided, with bones on one side in different positions for the same bone on the side. the other.

This is due to a mass of fatty tissue called “melon” that toothed whales use for echolocation. The melons and soft tissue needed for echolocation are placed upwards on the skull on the toothed whales, giving them a bulbous brain and also causing the bones in the skull beneath them to grow scattered towards the -left. As toothed whales evolved, their skulls got winkier.

But why don’t whales have this outrage? The first whales were called “archaeochetes” (literally meaning “ancient whales”). They evolved from walking on land to completely aquatic in a relatively short time of 8 million years.

We know that archaeocyte fossils have wonky rostrums (or snouts). This could be a fossil distortion or some feature that helps archaeocytes work out what sounds were coming from underwater.

A skeleton of a four-legged whale-like creature.
Ambulocetus natans, ancestor of the early whales.
Ghedoghedo, CC BY-SA

Then, about 39 million years ago, whales diverged into two groups: those with teeth in their mouths, known as the “odontocetes,” and those with bullets (rows of bristles that allow whales to filter water-based foods), known as ‘mysticetes’.

At some point, toothed whales evolved winky skulls and echolocation. However, mysticetes, which include large whales (such as blue whales), have diverged to a completely different evolutionary path. They have evolved the bullet and feed of filters and skulls that are more symmetrical than both archaeochetes and toothed whales.

We wanted to understand why, and exactly when it happened. So to trace the asymmetry in the evolution of the whale skull, they prepared 3D scans of 162 skulls, 78 of which were fossils. By mapping this change in the winning shape in the skull around the whale family tree, we can trace it precisely when in evolutionary history it first appeared and in which families it evolved.

Asymmetry appears

Based on an analysis of these skulls, nasofacial asymmetry (outrage) appears to have first evolved about 30 million years ago. This occurred after the transition from archaeocetes to modern whales, and after the split between odontocetes and mysticetes. Around the same time this appearance was seen, these early-toothed whales were evolving high-frequency hearing and complex echolocation.

We also confirmed that the early ancestors of live whales had little cranial asymmetry in the naso-facial area and were unlikely to be able to collocate. As such, it is likely that bullet whales were never able to remedy.

Most surprisingly, this asymmetry has reached its highest levels in some specific animals such as whale sperm and narwhals and other species living in deep or extreme environments.

This suggests that animals living in these complex environments, including frozen, embarrassed belugas and river dolphins living in shallow and missing rivers, have evolved a different ecolocation ability such as a more diverse or discreet sound repertoire to help them navigate and hunt, and with it the bones around the nose and face became more asymmetrical.

This evolving path of increasingly asymmetrical toothed whales suggests that their skulls and soft underlying tissues may continue to get even winkier as their echolocation techniques become more specialized.

These findings remind us not only of the complex evolutionary paths that cetaceans have taken to become the iconic inhabitants of the well-adapted oceans we know today, but also that despite living with some of the largest animals that have ever existed. , there is still much for us to learn about.

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