#delphinoidea

Albireonids were an early branch of the delphinoid whales, with their closest living relatives being modern oceanic dolphins, narwhals and belugas, and porpoises. Known from temperate latitudes of the North Pacific Ocean between the late Miocene and the late Pliocene, about 9-2.5 million years ago, their fossil remains are very rare in coastal deposits and they seem to have primarily been offshore open ocean animals.

Albireo whistleri is the best known member of this family, represented by a near-complete skeleton from what is now Isla de Cedros in Baja California, Mexico, dating to the late Miocene between about 8 and 6 million years ago. It was a rather small dolphin, around 2.5m long (~8'2"), with a stocky body, fairly broad flippers, and skull anatomy with some convergent similarities with the modern Dall's porpoise.

Interestingly these dolphins also seem to have frequently had pathological neck vertebrae, with both Albireo whisteri and the younger species Albireo savagei from California, USA, showing unusually asymmetrical atlas bones – but on opposite sides to each other. This might be due to illness or injury earlier in life, or possibly be evidence of some sort of "handedness" with individuals preferring to perform some actions more with one side of their body than the other.

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Harbor porpoise X

Atlantic spotted dolphin (Stenella frontalis)

A Risso’s dolphin caught up in fishing line. Dolphin numbers in the Indian Ocean may have dropped by more than 80% in recent decades, with an estimated 4 million small cetaceans caught as “by-catch” in commercial tuna fishing nets since 1950, according to a study

Photograph: Andrew Sutton/Central Studio

Mammalian tusks usually grow in symmetrical pairs with only minor developmental asymmetry, but a few species have evolved much more uneven arrangements.

Odobenocetops peruvianus was a small toothed whale that lived during the Miocene, about 7-3 million years ago, in shallow coastal waters around what is now Peru. Around 3m long (~10'), it was a highly unusual cetacean with binocular vision, a vestigial melon, muscular lips, and a pair of tusks – features convergent with walruses that suggest it had a similar lifestyle suction-feeding on seafloor molluscs and crustaceans.

In males the right tusk was much more elongated than the left, measuring around 50cm long (~1'8") in this species and up to 1.35m (4'5") in the closely related Odobenocetops leptodon. Since these teeth were quite fragile they probably weren't used for any sort of combat, and they may have instead served more of a visual display function.

And despite being closer related to modern narwhals and belugas than to other toothed whales, Odobenocetops' long right-sided asymmetric tusks actually seem to have evolved completely independently from the iconic left-sided asymmetric spiral tusks of narwhals.

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The woolly mammoth (Mammuthus primigenius) lived across Eurasia and North America during the last ice age, mostly from the Pleistocene about 400,000 years ago to the early Holocene about 10,000 years ago – altohugh a few relict populations survived until around 4,000 years ago in isolated areas of Alaska, Siberia, and eastern Russia.

Around 3m tall at the shoulder (~10ft), these hairy proboscideans had very long curving tusks that were used for digging out vegetation from under snow and ice, scraping bark from trees, and for fighting.

The tusks showed a lot of variation in their curvature, and were often rather asymmetrical, a condition also seen in the closely related Columbian mammoth. Like modern elephants mammoths may have also favored using one side over the other for certain tasks, which over their lifetimes could result in uneven wear exaggerating the natural asymmetry even more.

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Modern beluga whales and narwhals are the only living representatives of the monodontid lineage, found only in cold Arctic and sub-Arctic waters. But this whale family actually first evolved in much warmer climates – and some of them were downright tropical.

Casatia thermophila lived about 5 million years ago during the early Pliocene, in the Mediterranean Sea around Tuscany, Italy. Although known only from a couple of partial skulls and a few vertebrae it was probably similar in size to its modern relatives, around 5m long (16'4").

It seems to have had a larger number of functional teeth than modern monodontids, and probably didn't suction feed like its modern close relatives. Instead it may have fed more like most porpoises and dolphins, relying more on speed and snapping jaws to capture prey.

It inhabited the Mediterranean at a time not long after the sea there had mostly dried up and then been rapidly refilled. The presence of warm-water marine species such as bull sharks, tiger sharks, and dugongs in the same fossil beds as Casatia indicates the local climate at the time was hotter than it is today, with tropical temperatures – and suggests that this whale's ancestors must have originally moved into the replenishing Mediterranean from lower latitudes alongside these other warmth-adapted animals.

This tropical monodontid was also much closer related to modern belugas than modern narwhals are, which raises the possibility that the two living monodontid species actually specialized for colder conditions completely independently of each other rather than descending from a cold-adapted common ancestor. Instead modern belugas and narwhals may have originated from separate warm-water monodontid ancestors who evolved similar cold-tolerant adaptations in parallel as the climate cooled during the onset of the Quaternary ice age, while the rest of their relatives all went extinct.

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Atlantic spotted dolphin (Stenella frontalis)

Atlantic white-sided dolphin (Lagenorhynchus acutus)