Benton and Wu 2022 considered several factors
affecting the return of life following the Permian – Triassic extinction event, (Fig 2) universally considered the worst extinction event of all time. Pertinent abbreviations include:
PTME – Permian-Triassic Mass Extinction – 251 mya (= Siberian traps)
CPE – Carnian Pluvial Episode – 231 mya (= widespread arid to rainfall)
ETME – End Triassic Mass Extinction – 200 mya (= Mid-Atlantic volcanoes)
MMR – Mesozoic Marine Revolution, (= the increase in shell-crushing and boring predation starting in the Early Triassic.
One factor Benton and Wu omitted
was the near complete absence of Permian marine vertebrates in the fossil record (Figs 1, 2). That’s 47 million years of virtually absent taxa.
According to Benton and Wu 2022,
“Marine predatory vertebrates show spectacular and rapid diversifications in the Early and Middle Triassic, and new discoveries from China have confirmed their early start in the Triassic, but not in the Late Permian.”
Or was this a geological illusion? There was a new dawn for animals following the PTME. We know which clades survived. Where did the survivors find refuge? Unfortunately, this topic is rarely to never covered, perhaps because those rare refugia have never been discovered or described. Extinction events garner headlines and take the spotlight.
From another point of view
that ‘spectacular and rapid diversification‘ in the Triassic was at least partly due to the current paucity of marine fossils in the Permian (Figs 1, 2) and a relative trove of marine fossils in the Early Triassic. Blank spaces (Fig 2) can still provide data based on phylogenetic bracketing with a large gamut cladogram, like the large reptile tree (LRT, 2119 taxa, subset Fig 3).
As an example:
here (Fig 3) the currently unknown pre-Triassic ancestors of ichthyosaurs and thalattosaurs would have to have been coeval with known and related Early Permian mesosaurs. Unfortunately all we have are two Middle Triassic late-surviving basal pachypleurosaurs and pre-pachpleurosaurs, Honghesaurus and Anarosaurus. That’s too few. Way too few for 47 million years. Where are all the Early to Late Permian marine fossils?
One answer: The late-surviving representatives of missing Permian taxa are present in the Early and Middle Triassic according to the LRT. That’s a clue that primitive relatives of Early Triassic taxa were present in the Permian.
There are no large phylogenetic gaps in the LRT.
It documents a continuous gradation phyogenetically, but not chronologically due to the patchy fossil record in general. That also means Benton’s and Wu’s “spectacular and rapid diversificaiton” might actually represent a chronological illusion. In other words, with phylogenetic bracketing the wealth of Early Triassic ichthyosaurs and pre-ichthyosaurs likely arose sometime during the Early Permian. So far we’ve only found late survivors in the geologically richer Early and Middle Triassic strata. If so marine predators, including ichthyosaurs, had a slower, steadier, more typical radiation and evolution.
We haven’t found many Permian marine vertebrate fossils yet.
Perhaps they weren’t fossilized. Perhaps those strata are not present. Perhaps those strata await discovery buried under other sediments. At times like this it’s smart to remember, in general fossils from all eras are extremely rare and often restricted to tiny pin points on the Earth’s surface skipping large swathes of geological time. That’s just the way it is.
Overlooked by Wikipedia, Benton and Wu and saved ’til now,
there are several Permian marine and semi-marine younginiforms known from the late Permian. These include Claudiosaurus. Adelosaurus (Fig 4) and related late survivors of a Carboniferous radiation. They likewise had larger, more derived Triassic ancestors, like Atopodentatus (Fig 4).
Benton and Wu 2022 cited
Sanders et al. 2021 who reported on a giant ichthyosaur, Cymbospondylus youngorum (Fig 5). The authors reported, “The animal existed at most 8 million years after the emergence of the first ichthyosaurs, suggesting a much more rapid size expansion that may have been fueled by processes after the Permian mass extinction.” Benton and Wu considered this “a prime example of an ‘early burst’ radiation.“
That’s only 3 million years past the Early Triassic or 35 milllion years past the Early Permian, when pre-ichthyosaurs had their roots. How fast ichthyosaurs evolved is left up to the vagaries of the poor Permian marine fossil record vs the rich Triassic marine fossil record known to geology at present.
On land, Benton and Wu remind us,
“the Triassic was marked by a posture shift from sprawling to erect, and a shift in physiology to warm-bloodedness, with insulating skin coverings of hair and feathers.”
The authors consider the origin of hair in Permian synapsids as “an insulating pelage”. Before hair can act as insulation it must be thick enough covering a broad enough area. So the genesis of sparse and individual hairs (Fig 6) must have had some other use that enhanced survival. Perhaps the burrowing naked mole rat, Heterocephalus, can give us some insight into the origin of individual hairs first appearing in burrowing cynodonts, retained in mole rats by either a deep time reversal or neotony from ‘hairless’ newborns, or both as ontogeny recapitulates phylogeny. Yes, this is what some cynodont skin looked like.
Benton and Wu report,
“Pterosaurs, the sister group of dinosaurs, also have dermal insulating structures commonly
Pterosaurs are not the sister group of dinosaurs. The omitted ancestors and non-volant cousins of pterosaurs, like Sharovipteryx (Fig 7), also have dermal insulating, heat-radiating and decorative structures.
Benton and Wu continue,
“even if researchers balk at calling pycnofibres feathers, it does not change the fact that insulating dermal structures appeared in the first dinosaurs and the first pterosaurs, and the shared ancestry of these two clades is dated to the Early or early Middle Triassic.”
Fact? Insulating dermal structures appeared in the first dinos and pteros? Yes, for pterosaur ancestors like Cosesaurus in the Middle Triassic. No, for dinosaurs like Herrerasaurus, also in the Middle Triassic. Benton has a long history of omitting taxa related to pterosaurs recovered by Peters 2000a, b. Paleo academics at large (any exceptions?) continue to do the same.
Benton MJ and Wu F 2022. Triassic Revolution, Frontiers in Earth Science (2022). DOI: 10.3389/feart.2022.899541
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Sanders et al. (6 co-authors) 2021. Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans. Science374 (6575): DOI: 10.1126/science.abf5787