marine mammal evolution 1: cetaceans Flashcards
Two groups of cetaceans exist
Odontecetes = toothed whale
Mystices = moustache wale (baileen whales)
closest ancestor believed to be hippo
Artiodactyla and Perissodactyla
Artiodactyla includes cattle, sheep, pigs, deer, giraffes, camels, etc. All have even no. of toes – 4 toes
Perissodactyla includes zebra, horse, tapir and rhinoceros
Have an odd no. of toes (1 in horse or 3 in animals such as tapir)
Evolution leading to cetaceans
See diagram from Gatesy et al. (2013)
A: a raoellid from the Eocene (43-56 Mya) – an extinct hoofed mammal from the artyodactyl lineage (even-toed ungulate)
B: Early ‘archaeocete’ called Pakicetus from ~48-56 Mya
C: Ambulocetus – ‘the walking whale’, ~50 Mya
D: Remingtonocetus – a foot-powered swimmer from 43-45 Mya
E: Georgiacetus – also middle Eocene, discovered in Georgia, USA
^ limbs still present do not become residual until F
F: Dorudon – hind limbs almost lost, 34-40MYa
G&H: Janjucetus & Aetiocetus – early whales of the modern form (~25 MYa)
the ‘archaeocetes’ can be represented as ‘stem’ cetaceans, and the modern Odontocete and Mysticete lineages represented as ‘crown’ lineages
Pakicetus attocki
Thought to be an aquatic deer this fossils ankle bones closely resemble modern pig
Pakicetus attocki was an important find because it shows a direct link with the Artiodactyls for the first time
(see Thewissen et al. 2001. Nature 413, 277-281).
The ankle bone of Phenacodus (an early Perissodactyl) looks quite like Pachyaena (a Mesonychid), but Pakicetus looks more like Diacodexis (an early Artiodactyl) and Sus (the modern pig)
(see diagram in notes: u.p. = upper pulley, connecting to the tibia, while l.p. = lower pulley, articulates to distal ankle)
Skull changes associated with evolving to become aquatic
nostrils move to the top of the skull above the eyes – this adaptation started developing ~36mya
‘Within the cetacean lineage, modification of the skull was important, such as the migration of the nasal bones, allowing whales to breath from a horizontal position.’
Tooth (dentition) evolution
Most lineages had teeth, and some of the earliest forms had ‘lobed’ teeth used for straining (filtering prey from water) and heterodont dentition.
Modern mysticetes all have baleen instead of teeth – a keratinaceous material with fringed edges used for ‘bulk’ feeding – sieving food out of the water.Baleen is keratin – same as what our hair and nails are made of. Different types of prey require different baleen.
Another key adaptation along the mysticete lineage is the shape of the lower mandibles, narrow in the ancestor and progressively broad until becoming ‘bowed’ to facilitate bulk feeding in the modern lineage (Fitzgerald (2012) Biol. Lett. 8, 94-96).
Similar feeding methods in closely related species of Balaenoptera :
Phylogeny of baleen whales groups whales of common ancestry that share foraging strategies, ‘gulpers’, ‘skimmer’ and benthic foragers (grey whale).
‘Gulpers’ (balaenopterids) can expand the capacity of their throats using ‘gular pleats’, taking in vast quantities of water to sieve through.
Grey whale a ‘Benthic forager’ scours the sea floor
Right Whales are ‘skimmer’ feeders collecting food at the surface
Odontocetes
Odontocete – the toothed whales. More species than in the baleen whale lineage, and many have evolved quite recently (phylogeny based on mtDNA genomes from Cunha et al. 2011 PLoS One, 6, e28297).
Radiation is still happening, a lot of speciation particularly in dolphins is very recent.
Sperm whale (Physeter macrocephalus,) the largest of the Odontocetes, highly social, and displays the homodont dentition typical of Odontocete cetaceans. The forehead houses a massive wax ester filled cavity called the ‘spermacete organ’. Sperm whales prey almost exclusively (> 80%) on cephalopods (squid).Teeth only on lower mandible, thought to stun prey and suck them in.
From whole genome analysis we can see the node is deep within the tree for sperm whales although they do fit most closely to odontocetes they diverged very early on.
Beaked whales
The beaked whales (Ziphiidae) also prey mostly on cephalopods (squid in particular) and are deep divers. Many species are poorly known, some from solitary strandings. They have few teeth – typically 2, and in the ‘strapped tooth whale’, these wrap around the upper mandible preventing it from opening fully –perhaps providing a filtering/sucking function.
They are hard to research preferring open sea and deep diving.
River dolphins
Evolved independently in different parts of the world and show strong convergent evolution:
eyes nearly vestidual not used much, long jaws to catch prey at a distance, both inhabit riverways
River dolphins (Platanistidae, Iniidae, and Pontoporiidae) are mostly fresh water, sometime estuarine (or even along coasts near estuaries), and adapted to exploiting fish prey in murky water.
Arctic whales
Arctic whales (Monodontidae) includes two species, the beluga and the narwhale. The lineage is named after the narwhal which has only one (or sometimes two) teeth, erupting from the upper lip as a tusk.
Porpoises
smallest of all the cetaceans
Vaquita porpoises often get stuck in fishing nets in Northern California, their total population is less than 50 individuals at present.
Porpoises (Phocoena and Neophocoena) characterised by small body size, a blunt forehead, short beak, and ‘spatulate’ teeth. Note that even though a specialised shape, the dentition is ‘homodont’ (all the same). Solitary lifestyle, though often found in aggregations.
Dolphins
Dolphins (Delphinidae) quite a broad range of body sizes (killer whale the biggest), most species social or gregarious, most prey on fish, some on squid, killer whale preys on other marine mammals (including other dolphins), most have a pronounced beak.
Broad range – largest is killer whale
Gregarious – mostly prey on fish, some on squid killer whale on other marine mammals
Most have a long nose with killer whale as an exception (snub nosed)
Dolphin dentition: Dolphins have homodont, conical teeth, such as the bottlenose dolphin (Tursiops truncatus) shown below, and the killer whale (Orcinus orca) shown on the right. Picture on the lower right gives a sense of scale – most dolphins intermediate in size
Note the distinction between the Pacific white-sided dolphin (Lagenorhynchus obliquidens) on the lower left, and a river dolphin – the Ganges river dolphin (Platanista gangetica) on the lower right. Both have conical teeth – good for gripping fish prey, but the teeth at the front of the river dolphin’s mandibles are exaggerated. Also note the bony flanges on the river dolphin skull.
Echolocation in dolphins
Echolocation: An adaptation of the odontocete cetaceans, especially well developed in dolphins. It allows odontocetes to effectively ‘see’ with sound at quite high resolution. Melon refracts and focuses sound into a beam which reflects of prey back to the fat in the jaw which directs sound to the ear for echolocation
The amazon river dolphin has a very pronounced melon as do most other river dolphins. As they can see very little in turbid muddy water they rely on echolocation instead – clicks start at 120000 hertz beyond our field of hearing – locate fish extremely well
Echolocation convergent evolution
the characteristics of prestin gene (used for echolocation) are more similar between dolphins and bats than dolphins and other cetaceans
Evolutionary convergence for echolocation: This phylogeny is for just one gene, Prestin – a motor protein expressed in mammalian outer hair cells thought to confer high frequency sensitivity in the mammalian auditory system. Convergent amino acid substitutions in bats and dolphins suggest shared function during echolocation (Liu et al. 2010 Current Biol. 20, R53-R54)
Summary
1)Marine mammals evolved independently from several distinct lineages, with the cetacean and sirenian lineages radiating the earliest (in the Eocene)
2)Cetaceans evolved from the even-toed ungulates (the Artiodactyls), and many taxonomists now refer to the broader group as the Superorder Cetartiodactyla (though some propose just including whales in the order Artiodactyla – see https://doi.org/10.1007/s10914-021-09572-7).
3)The earliest known fossil showing the clear link between ungulates and cetaceans was Pakicetus attocki, and in particular, the shape of its ankle bones.
4)Mysticete lineages divide in part by foraging strategy (associated with morphology)
5)Delphinid lineages are loosely associated with biogeography, phenotype and foraging behaviour, and all cetaceans have evolved specialisations adapted to the marine environment (e.g. echolocation among the odontocete species).
