Surviving a mass extinction: being small, proliferating rapidly afterward

[lpetrich]

Now to fish. I first wish to mention that we don't see a taxon Pisces anymore. That is because it is Vertebrata - Tetrapoda, making it "paraphyletic", including its ancestor but not all of its descendants, and thus cladistically illegitimate. But we use "fish" as a convenience taxon, like "reptile" and "invertebrate": Reptilia = Amniota - Aves - Mammalia, Invertebrata = Metazoa - Vertebrata.

Vertebrata - 518 Mya

• Jawless fish: lamprey, hagfish, conodont animal, some early armored fish
• Gnathostomata: jawed vertebrates - 439 Mya
  • Some early armored fish
  • Chondrichthyes: cartilage fish: sharks, rays, ... - 439 Mya
  • Eutelestomi / Osteichthyes: mouth at front end / bony fish - 425 Mya
    • Sarcopterygii: lobe-finned fish: lungfish, coelacanth, tetrapods: land vertebrates - 425 Mya
    • Actinopterygii: ray-finned fish - 425 Mya
      • Non-teleosts like sturgeon
      • Teleostei: teleost fish ("complete bone") - 250 Mya

Most present-day fish species are teleost species. Teleosts have a distinctive tail structure: the spinal cord ends before the tail fins ("homocercal"). Other fish have spinal cords extending into their tails with a fin extending downward ("heterocercal") - sharks and sturgeon, for instance. Sharks also have multiple gill slits, while sturgeon and teleosts have one gill slit on each side.

 

[lpetrich]

Explosive morphological diversification of spiny-finned teleost fishes in the aftermath of the end-Cretaceous extinction | Proceedings of the Royal Society B: Biological Sciences

The anatomical diversity (disparity) of acanthomorphs is low throughout the Cretaceous, increases sharply and significantly in the wake of the Cretaceous–Palaeogene (K–P) extinction, and shows little change throughout subsequent Cenozoic intervals.

 Acanthomorpha - 1/3 of present-day vertebrate species

Some otoliths, calcium carbonate structures that form the ears of fishes, have been found from the Jurassic Period that may belong to acanthomorphs, but the oldest body fossils from this taxon are only known from the middle of the Cretaceous Period, about 100 million years ago. Acanthomorphs from the early Late Cretaceous were small, typically about 4 centimeters long, and fairly rare. Toward the beginning of the Cenozoic era, they exploded in an adaptive radiation, so by the time their fossils begin appear more frequently in Eocene-aged strata, they had reached their modern diversity of 300 families.

Another case of the K-Pg disaster having small survivors that then had lots of descendants.

How the author did it.

Here, I reconstruct the trajectory of morphological diversification in this major radiation from its first appearance in the Late Cretaceous to the Miocene using a geometric morphometric database comprising more than 600 extinct species known from complete body fossils.

I recall from somewhere that the large majority of vertebrate fossils are fish fossils. "Complete body fossils" - that's as good as it gets in fossil preservation. It also means that one does not have to infer any of the fish's body form: it's all there.

 

[lpetrich]

Two waves of colonization straddling the K–Pg boundary formed the modern reef fish fauna | Proceedings of the Royal Society B: Biological Sciences

Two waves of invasion were accompanied by increasing morphological convergence: one in the Late Cretaceous from 90 to 72 Ma and the other immediately following the end-Cretaceous mass extinction. The surge in reef invasions after the Cretaceous–Palaeogene boundary continued for 10 Myr, after which the pace of transitions to reef habitats slowed. Combined, these patterns match a classic niche-filling scenario: early transitions to reefs were made rapidly by morphologically distinct lineages and were followed by a decrease in the rate of invasions and eventual saturation of morphospace.

Paleogene? Is that like Paleocene? How was geological time named? | Internet Infidels Discussion Board
A very quick summary:

• Phanerozoic
  • Cenozoic: Paleogene, Neogene, Quaternary
  • Mesozoic: Triassic, Jurassic, Cretaceous
  • Paleozoic: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
• Proterozoic
• Archean
• Hadean

Everything before the Phanerozoic is often lumped together as the Precambrian. The Cenozoic:

• Quaternary: Pleistocene, Holocene
• Neogene: Miocene, Pliocene
• Paleogene: Paleocene, Eocene, Oligocene

The Tertiary is the former name of the Paleogene and Neogene together. That's why an older name for the Cretaceous-Paleogene disaster was the Cretaceous-Tertiary one.

 

[lpetrich]

New Age of Fishes initiated by the Cretaceous−Paleogene mass extinction | PNAS
The original one was the Devonian Period (419-359 Mya) of the Paleozoic Era, with its proliferation of early fish.

Here we use microfossil teeth and shark dermal scales (ichthyoliths) preserved in deep-sea sediments to study the changes in the pelagic fish community in the latest Cretaceous and early Paleogene. We find that the Cretaceous−Paleogene (K/Pg) extinction event marked a profound change in the structure of ichthyolith communities around the globe: Whereas shark denticles outnumber ray-finned fish teeth in Cretaceous deep-sea sediments around the world, there is a dramatic increase in the proportion of ray-finned fish teeth to shark denticles in the Paleocene. There is also an increase in size and numerical abundance of ray-finned fish teeth at the boundary. These changes are sustained through at least the first 24 million years of the Cenozoic.

So ray-finned fish recovered much more from the K-Pg disaster than sharks.

Two pulses of morphological diversification in Pacific pelagic fishes following the Cretaceous–Palaeogene mass extinction | Proceedings of the Royal Society B: Biological Sciences

Our record does not show significant turnover of fish tooth morphotypes at the K/Pg boundary: only two of 48 Cretaceous tooth morphotypes disappear at the event in the South Pacific, a rate no different from background extinction.

There wasn't much of an extinction of fish.

The first pulse, at approximately 64 Ma, included short-lived teeth, as well as forms that contribute to an expansion into novel morphospace.

then

A second pulse, centred at approximately 58 Ma, produced morphotype novelty in a different region of morphospace from the first pulse, and contributed significantly to Eocene tooth morphospace occupation.

But after that,

There was no significant increase in origination rates or expansion into novel morphospace during the early or middle Eocene, despite a near 10-fold increase in tooth abundance during that interval.

The Eocene: 56.0 - 33.9 Mya.

So what allowed fish to expand so much? I have a theory: the extinction of shelled cephalopods that had previously been very abundant: ammonites  Ammonoidea

 

[lpetrich]

The diversity of teleost fishes during the terminal Cretaceous and the consequences of the K/Pg boundary extinction event | Netherlands Journal of Geosciences | Cambridge Core

The Late Cretaceous was a time of blossoming teleost diversification that came to a sudden restriction and partial termination during the extinction event at the Cretaceous–Paleogene (K/Pg) boundary.

...
It is interesting to recognise that several of the surviving lineages are represented by groups that probably originated during the Late Cretaceous but were not particularly common up to the K/Pg boundary and began to expand rapidly and diversify during the early Paleogene. Such lineages probably took advantage to populate void ecospace that may have opened following the extirpation of previously dominant lineages

 Cretaceous–Paleogene extinction event
Turtles were not much affected.

More than 80% of Cretaceous turtle species passed through the K–Pg boundary. All six turtle families in existence at the end of the Cretaceous survived into the Paleogene and are represented by living species.[99] Analysis of turtle survivorship in the Hell Creek Formation shows a minimum of 75% of turtle species survived.[100] Following the extinction event, turtle diversity exceeded pre-extinction levels in the Danian of North America, although in South America it remained diminished.[101] European turtles likewise recovered rapidly following the mass extinction.[102]

Crocodilians?

"Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to the K–Pg boundary."

"Crocodyliform survivability across the boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary." and "Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments as do modern marine crocodile juveniles, which would have helped them survive where other marine reptiles became extinct; freshwater environments were not so strongly affected by the K–Pg extinction event as marine environments were."

Alligator vs. crocodile: alligators have broad snouts and crocodiles have narrow ones, something connected to their feeding strategies.

Crown crocodilians diverged in the Late Cretaceous, some 100 million years ago, and as far as I can tell, three or four crocodilians survived the K-Pg disaster. The three or four is from whether the ancestors alligators and caimans diverged before or after that disaster. However, crocodile ancestors and gharial ancestors survived separately.

Crown vs. stem taxa:  Crown group has present-day members, their most recent common ancestor, and all other descendants of that ancestor.

A total group is one that includes the crown group and all organisms closer to the crown group's members than to other present-day organisms.

A stem group is the total group without the crown group.