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  • Periodic Reporting for period 1 - MYSTICETI (The oldest and the rarest - combining insight from both hemispheres to gain a global picture of baleen whale origins and macroevolution)

MYSTICETI Report Summary

Project ID: 656010
Funded under: H2020-EU.1.3.2.

Periodic Reporting for period 1 - MYSTICETI (The oldest and the rarest - combining insight from both hemispheres to gain a global picture of baleen whale origins and macroevolution)

Reporting period: 2015-11-16 to 2017-11-15

Summary of the context and overall objectives of the project

Cetaceans (whales and dolphins) are a spectacular example of secondary adaptation of vertebrates to aquatic life. Whales, in particular, are the largest animals on Earth, and play key ecological roles as mass consumers and nutrient redistributors. They owe their success to their highly specialised and efficient way of feeding: unlike most other mammals, whales have no teeth, and instead use use baleen – an array of comb-like plates suspended from their upper jaws – to filter vast quantities of tiny prey directly from seawater. How filter feeding evolved, and what role it played in whale evolution, remains one of the key questions of marine mammal evolution.

This project combines cutting-edge techniques (e.g. Bayesian analysis, quantification of tooth sharpness, biogeochemistry) with functional, behavioural, molecular and morphological data to investigate how whales came to be the leviathans we know today. Specifically, the projects aims to address such questions as:

(i) When did baleen first evolve?
(ii) How did whales transition from biting prey with teeth to filter feeding?
(iii) Can the feeding behaviour of living marine mammals inform evolutionary history?
(iv) Have key innovations, such as baleen, driven whale diversification?
(v) When and how did whales become giants?
(vi) Was the evolution of filter feeding linked to environmental change?

Historically, the study of marine mammals has often focused on species and fossils from the Northern Hemisphere, reflecting its longer history of research and higher population density. To obtain a more global data set, this project specifically focuses on previously undersampled areas, animals and time periods. These include major Southern Hemisphere localities in Australia, New Zealand and Peru, all of which have yielded numerous, yet still largely undescribed fossils; the enigmatic pygmy right whale, Caperea marginata, which is the smallest of the living whales, and only occurs in southern seas; and the earliest phase of whale evolution, lasting from ca 35 to 25 million years.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

This project has involved field work in Peru and Australia, and led to the identification of at least ten new extinct species. Two of these have now been described: the rorqual Incakujira anillodefuego (in collaboration with the Japan Society for the Promotion of Science); and the cetotheriid Tiucetus rosae. Three further specimens have already been prepared, and a further two are being collected. Further results include the description of fossilised baleen in a new specimen of the cetotheriid Piscobalaena nana, and a reassessment of the auditory morphology and evolutionary relationships of the extinct pygmy right whale Miocaperea pulchra.

Two papers associated with this project focus on the pygmy right whale, whose origins remain highly controversial. The first uses 3D reconstructions of the inner ear of Caperea to determine its hearing abilities and morphological similarities to other whales. The second describes two unexpected Caperea fossils from the Pleistocene (ca 2–0.5 million years) of Japan and Italy. These fossils are the only occurrences of pygmy right whales north of the equator, and hint at a previously underappreciated role of glaciation in shaping the modern marine mammal fauna.

To test whether behavioural observations on living animals can be used to infer evolutionary patterns, we produced a comprehensive review of the feeding behaviours of living marine mammals. From this review emerged a novel framework suggesting that marine mammal feeding strategies form an evolutionary continuum, ranging from semi-aquatic to raptorial, suction and filter feeding. In a follow-up paper, we suggest both physical and biological reasons for why such an evolutionary continuum might exist, and show that behavioural observations are an independent source of evolutionary data that can be tested against the fossil record.

Fossil data provide direct evidence of feeding evolution, e.g. in the form of dental wear caused by ingested food items. We describe horizontal wear features in a member of an archaic whale family previously thought to possess both teeth and baleen. Horizontal wear indicates front-to-back transport of food items inside the mouth, which is more consistent with suction than raptorial or filter feeding. Similar wear occurs in specimens from Australia, and suggest that suction may have preceded the evolution of filter feeding in whales.

One prominent idea holds that early whales strained prey from water using complex teeth, as seen in living filtering feeding seals. To test this idea, we developed a way to measure tooth shape and sharpness. We found that the teeth of filter feeding seals are blunt, with wide notches that allow water flow. By contrast, those of early whales were sharp, with narrows notches that restrict water flow. We conclude that early whales did not filter with their teeth, and instead used them to grasp prey.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The new material collected as part of this project increases our knowledge of whale diversity, especially in the poorly known Southern Hemisphere. Prior to this project, only three extinct whales had been described from Peru, one of the richest fossil sites anywhere in the world. This number has already increased to five, and is set to triple by the end of this project.

Until recently, almost nothing was known about the pygmy right whale, Caperea. Our results demonstrate that Caperea has one of the highest low frequency hearing limits of any baleen whale, adding to the impression of a highly specialised species that is unlike any of its peers. We also show that the inner earl resembles that the inner ear of Caperea resembles that of rorquals and grey whales, rather than right whales. This pattern contradicts traditional morphology-based analyses of evolutionary relationships, but is consistent with studies based on molecular data.

The finding of pygmy right whale fossils north of the equator is highly unexpected, and provides insights into ocean dynamics during the ‘age ages’. During cold periods associated with northern glaciation, equatorial waters cooled and allowed normally restricted species to spread across the tropics. This process affected even highly localised taxa, with Caperea in the north being as out of place as a polar bear would be in Antarctica. New fossil material of Caperea from the Southern Hemisphere is currently being described, and will further constrain the time when pygmy right whales crossed the equator into the north.

Our behavioural framework provides a novel hypothesis for the evolution of filter feeding in whales (suction first, filtering second) that is consistent with the fossil record. Our follow-up paper makes several advances in the theory underlying this transition, and demonstrates how aquatic mammals have repeatedly followed similar evolutionary paths. Additional work currently in review focuses on the transition from semi-aquatic to raptorial feeding, and will provide insights into how mammals adapted ‘terrestrial’ behaviours to enable aquatic feeding.

The new aetiocetid fossil showing evidence for suction preserved rare direct evidence of aquatic foraging, and helps to test alternative ideas that envision a direct transition from raptorial to filter feeding. By the end of the project, the fossil will be described as a new species, and it internal anatomy investigated using CT scanning and 3D reconstruction.

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