A new and bigger ‘Cymbospondylus’ enters the LRT

Sanders et al. 2021 bring us
a new ‘Cymbospondylus‘ (Fig. 1), a Middle Triassic ichthyosaur with a 2m skull and and estimated length of 17m. The authors also focused on the rapidity (18 million years max) for this “size evolution” (= attainment of giant size) to take place in ichthyosaurs following the end Permian extinction event. The authors compared the “faster evolution of large body size” of Triassic ichthyosaurs to that of ‘whales‘ as they convergently evolved large size in a marine niche following the Cretaceous extinction event some 200 million years later.

Figure 1. Skull of Cymbospondylus youngorum from Sanders et al. 2021. Colors and DGS reconstruction added here. It is time to stop using pen-and-ink to identify skull bones. Colors work so much better.

From the Sanders et al. abstract
“Body sizes of marine amniotes span six orders of magnitude, yet the factors that governed the evolution of this diversity are largely unknown. High primary production of modern oceans is considered a prerequisite for the emergence of cetacean giants, but that condition cannot explain gigantism in Triassic ichthyosaurs.”

According to Wikipedia, “primary production… principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis. Almost all life on Earth relies directly or indirectly on primary production.”

“We describe the new giant ichthyosaur Cymbospondylus youngorum sp. nov. with a 2-meter-long skull from the Middle Triassic Fossil Hill Fauna of Nevada, USA, underscoring rapid size evolution despite the absence of many modern primary producers. “

Sanders et al. seem to indicate many blue green algae (= primary producers) were absent during the Early and Middle Triassic after existing for 3.25 billion years. The ‘absence of many modern primary producers’ at any time seems untenable by definition.

“Surprisingly, the Fossil Hill Fauna rivaled the composition of modern marine mammal faunas in terms of size range, and energy-flux models suggest that Middle Triassic marine food webs were able to support several large-bodied ichthyosaurs at high trophic levels, shortly after ichthyosaur origins.”

The Middle Triassic ended 18 million years after the end Permian extinction event. So that’s the window of time Sanders et al. consider “shortly after ichthyosaur origins” (Figs. 2,3).

Figure 2. Select early ichthyosaurs to scale demonstrating the rapid (18 million year) acquisition of giant size prior to the Late Triassic. Wumengosaurus, with fingers, is a late survivor in the Middle Triassic, but phylogenetically preceded Early Triassic Chaohusaurus, with flippers (see figure 2). So that stretches ichthyosaur origins deeper into the Permian, a time period known for only a few marine tetrapod fossils.

Figure 3. Subset of the LRT focusing on Ichthyopterygia and their proximal ancestors. Thalattosaurs are only known from Middle and Late Triassic strata. Compare to Sanders et al. 2021 in figure 4. Here a long list of ichthyosaurs phylogenetically precede Early Triassic Chaohusaurus in the middle of this cladogram.

The proximal outgroup for Ichthyopterygia
in the large reptile tree (LRT, 2028 taxa, subset Fig. 3) is the Mesosauria, known from Early Permian strata. Phylogenetically preceding mesosaurs is Claudiosaurus along with other basal marine younginiformes, all rare late survivors known only from the Late Permian. So there’s a long Permian gap in basal ichthyosaur evolution represented presently only by late survivors in the Triassic. Sanders et al. did not consider this gap due to taxon exclusion.

Figure 4. Portion of the same figure in Sanders et al. 2021, tones added here. Ichthyosaur mimics are basal pachypleurosaurs when more taxa are added.

Sanders et al. 2021 compared Triassic ichthyosaur size increase
to that of whales, again without a valid phylogenetic context. Since Sanders chose a 2m long skull in Cymbospondylus youngorum as his finish line, we will do about the same.

Unfortunately
Sanders et al. continue the myth of a monophyletic clade ‘Cetacea‘, still taught in university textbooks. Simply by adding more taxa to analysis:

The starting point for right whale mysticetes is Desmostylus in the Late Oligocene to Late Miocene, a range of 28 to 7 million years assuming the finishing point is the extant right whale, Eubalaena (5m skull). We don’t yet know of any fossil right whale skulls in the 2m range. Late Miocene Miocaperea had a 40cm skull. Right whale skeletons eventually sink to the sea floor and have done so for tens of millions of years. Fossils are hard to come by.

The starting point for rorqual mysticetes is Behemotops in the Late Oligocene, a range of 34 to 23 million years assuming the finishing point is the extant blue whale, Balaenoptera (9m skull). The Early Miocene rorqual, Isanacetus, had a 1 m skull. The Early Miocene immediately followed the Late Oligocene.

The starting point for odontocentes is the ‘whale with legs’, Rhodocetus in the Early Eocene, a range of 50 million years assuming the finishing point is the extant sperm whale, Physeter (5m skull).

Instead, if the odontocete finish line is Livyatan (2.5m skull) in the Late Miocene (9 mya), then the time range is reduced to 25 to 14 million years.

However, if the odontocete finish line is Basilosaurus (1.5m skull length, 21 m total length) from the Late Eocene (40 to 33 mya), then the time range is 10 to 17 million years, about the same as the new ‘Cymbospondylus’ (Figs. 1, 6) in Sanders et al. 2018.

Figure 5, Portion of figure from Sanders et al. 2021 dcoumenting their traditional but invalid vision of whale evolution. Tones added here. The traditional clade Cetacea is no longer monophyletic. Desmostylians, anthracobunids and other mysticete ancestors are missing here. Pakicetids are basal to odontocetes, not mysticetes.

When Sanders et al. claim,
“faster evolution of large body size in ichthyosaurs than in cetaceans” that depends on the phylogenetic and chronologic starting and ending points. With a patchy fossil record as their dataset, Sanders et al. expressed a risky hypothesis here falsified by simply adding taxa.

FIgure 6. ‘Cymbospondylus’ youngorum compared to Shastasaurus pacificus. The eyeball of C. youngorum is relatively smalller, but overall larger than in the smaller relative, Shastasaurus.

In the LRT the new ‘Cymbospondylus youngorum‘
(Sander et al. 2021; Fig. 6) nests with Shastasaurus pacificus, not with Cymbospondylus petrinus (Figs. 2, 3) which nests with other Middle Triassic crested ichthyosaurs, Mixosaurus and Phalarodon.The orbit is relatively small in C. youngorum, but actually larger than in big-eyed Shastasaurus (Fig. 6). Distinct from most ichthyosaurs, the temporal length is longer as it anchors relatively larger jaw muscles. The teeth remained were relatively small.

Figure 7. Graphic of ichthyosaur evolution as told by skull shapes. Click to enlarge.

It’s been awhile since an ichthyosaur was added to the LRT.
Adding the new taxon described by Sanders et al. provided an opportunity to review and update all scores and DGS colors within this clade. The gradual accumulation of traits in this clade is shown above (Fig. 7).

References
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

wiki/Cymbospondylus
esearchgate.net/ The_triple_origin_of_whales

Publicity
https://www.cnn.com/2021/12/23/world/giant-marine-reptile-skull-intl-scn/index.html