Filter feeding: convergent evolution

[lpetrich]

Several large aquatic predators have become filter feeders, straining out their prey from the water. Filter feeding has evolved several times. I will list them, their approximate body length, their dates of occurrence, and their closest relatives

Aegirocassis benmoulai - 2 m - Early Ordovician, 480 Mya - other anomalocaridids, like Anomalocaris itself

Titanichthys spp. - 6 to 10 m - Famennian (end Devonian), 365 Mya - large predatory placoderm fish, like Dunkleosteus

Pachycormidae - closest relatives not very clear, closest living relative likely the bowfin, Amia calva
- Leedsichthys problematicus - 10 to 15 m - Callovian-Kimmeridgian (late Jurassic) - 165 to 155 Mya
- Bonnerichthys gladius - 6 m - Late Cretaceous - 87 to 81 Mya
- Rhinconichthys spp. - 2 to 4 m - Cretaceous - 145 to 66 Mya

Whale shark, Rhincodon typus - 10 to 12 m - present day - Orectolobiformes, carpet sharks

Basking shark, Cetorhinus maximus - 6 to 8 m - present day - Lamniformes, mackerel sharks

Megamouth shark, Megachasma pelagios - 4 to 6 m - present day - Lamniformes, mackerel sharks

The Lamniformes are mostly predatory species, like the great white shark, Carcharodon carcharias - 3 to 5 m - present day - and basking sharks and megamouth sharks likely evolved filter feeding separately

Manta ray, Manta spp. - 5 to 7 m - present day - Myliobatidae, eagle rays

Baleen whales, Mysticeti - 6 to 112 m - late Eocene, 35 Mya to present day - Odontoceti, toothed whales and dolphins and porpoises

That means at least 8 instances of large predators becoming filter feeders.

 

[rousseau]

The thing that's even more surprising than convergent evolution, is the surprise people have over convergent evolution.

Right now it seems like we're in an era where evolution is looked at like some kind of dark magic that most people don't really understand, and convergent evolution looks weirdly improbable. But put two similar species in the same environment and it's extremely unlikely that their evolution won't be convergent.

This is also why I've held that where life exists on other planets, it probably looks a lot like life on this planet. Similar conditions give rise to life, and given the laws of physics life should have a fairly similar orientation regardless of where it is in the universe.

 

[lpetrich]

Then there is the question of how one goes from being a predator to being a filter feeder. There is a simple intermediate strategy: swimming into a shoal of prey, opening one's mouth, and swallowing the collected prey. I say "prey" because it need not be fish.

There is not much of a fossil record of that transition for most filter feeders. Sharks do not fossilize very well -- we find lots of shark teeth and not much else. But there is an exception.

Baleen whales: a lovely transitional form « Why Evolution Is True

The paper includes two kinds of evidence for a tooth-baleen transition. The first is paleontological, based on a fossil whale, Aetiocetus weltoni, found in Oregon. It dates from 24-28 mya, close to the time when we see the first truly toothless mysticetes. The whale is clearly on the mysticete side of the toothed/toothless branching based on several skeletal features, but has teeth; it's an early toothed mysticete. But when the authors reexamined they jaw they found previously overlooked grooves ('nutrient foramina') in the lateral parts of the palate. In modern baleen whales, these slits allow passage of both nerves and blood vessels that feed the baleen-forming parts of the epithelium. (Baleen grows continuously throughout the whale's life.)

These slits aren't found in modern toothed whales.

So here is a perfect transitional form: a whale with both teeth and baleen.

Present-day baleen whales grow tooth buds as fetuses, but later resorb them. These buds have dentine but not enamel. That article mentioned baleen whales having pseudogenes for proteins involved in making enamel. So they have ancestors that had made complete, enameled teeth like what most toothed whales have.

 

[James Brown]

Speaking of convergent evolution, this popped into my feed today. A marsupial that looks and acts like a canine.

[YOUTUBE]https://youtu.be/fHs_lniTNvI[/YOUTUBE]