Scientists at the Natural History Museum in London used whale skulls to trace three waves of evolution among whales
Scientists from the Natural History Museum in London have used whale skulls to explain how their order Cetacea transformed from ground-dwelling creatures to the marine mammals of today with specific adaptations, in a first-of-its kind study.
The team used “high-dimensional landmark data to comprehensively represent the entire cranium for 201 living and fossil whales.”
The 201 whales included 88 living species of cetaceans, representing 95 per cent of the order as well as 113 fossil species.
The experts reconstructed as to what changes in ecology led to variations in the whales’ cranial shapes, disparity and evolution rates. Their study The tempo of cetacean cranial evolution was published May 9, 2022 in the journal Current Biology.
But why the cranium? “Because the cranium captures many of the most extreme shifts in feeding, respiration, and sensory structures, it is ideal for understanding this transition, but no previous study has reconstructed the evolution of the cetacean cranium through the full breadth of their extinct and living diversity,” the researchers noted.
A 50 million-year saga
Disparity across the skull in cetacean families
The researchers were able to discern that whale evolution took place in three distinct waves over a period of 50 million years. Archaeocetes or stem whales evolved 47.8-42 million years ago when four-legged, furry creatures such as Pakicetus went from living on land to inhabiting the water.
The cranium of the Archaeocetes evolved rapidly according to the study, “because of a lack of competition or high productivity allowing or promoting rapid change.”
Still, these ancient whales continued to exhibit ‘plesiomorphic’ or ancestral characteristics according to the study. These included heterodont dentition, meaning their teeth were of various different forms.
The transition from land-dwelling to marine-dwelling took place in eight million years, which the study authors termed ‘super quick’ in terms of evolution.
The next wave of changes took place 39 million years ago. That is when two suborders of Cetacea emerged: The odontocetes or toothed whales and the mysticetes or baleen whales which filter feed.
“In odontocetes, high rates and disparity are associated with the rapid reorganisation of the naso-facial region as they became increasingly specialised in their echolocation abilities. High disparity in the cranium in early mammalodontids suggests diversification and rapid occupation of their distinct niche with further adaptations to bulk (and later filter) feeding, consistent with patterns observed for mysticetes,” the scientists wrote.
The third and last wave of change occurred 18-10 million years ago when odontocetes such as sperm whales became even more specialised and became suction feeders.
The study noted that diet and echolocation “have the strongest effects on cetacean skull variation and evolution.” This, it said, was consistent “with previous work linking prey size, diet type, feeding method, and echolocation with skull shape and diversity in odontocetes.”
The scientists added that they had excluded encephalisation, or relative brain size, “despite its relevance for cranial morphology and other key factors,” given the limitations to estimate them in fossils.
The other factors included echolocation specifics, thermoregulation and complex social. These would be difficult to study in case of fossils, the experts said.
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