u/New_Boysenberry_9250

Along with Homotherium, Smilodon was the last of the machairodontines, an early off-shoot of the cat family that were top predators across most of the world during the Late Miocene to Early Pleistocene. Existing for the entirety of the Pleistocene (2.5 million to 10,000 ya), the genus is generally divided into three species which you could call the lesser sabretooth, the common sabretooth and the giant sabretooth.

Like the scimitar cat, Smilodon is the most derived of the machairodonts, a powerfully built ambush predator with with sabers up to a foot long in the largest species. Along with Homotherium, smilodontins first entered North America around 5 million years ago, following the local extinction of Miocene genera like Nimravides and Amphimachairodus (both relatives of Homotherium) and many other native animals like American rhinos and the last nimravids (Barbourofelis). The early evolution of the American smilodontins across the Pliocene is poorly understood, having long been represented by fragmentary and scattered material attributed to the dubious Megantereon hesperus, while fragmentary fossils and one well-preserved skull from the Palmetto Fauna of Florida (dating to the Early Pliocene) have been renamed as Rhizosmilodon fiteae in 2013 (said taxon also coexisted with the last of Amphimachairodus).

We know that the cougar-sized Smilodon gracilis appears in the earliest Pleistocene (2.5 to 1. 8 mya, the end of the Blancan age), with many of its remains (also far from complete) hailing from Florida, though the type material comes from the Port Kennedy Cave in Pennsylvania, and some teeth from the El Breal de Orocual tar pits in Venezuela are the only purported record of it from South America. During its time, S. gracilis was beholden to the larger, lion-sized Homotherium ischyrus and the robust, jaguar-sized scimitar cat Xenosmilus, which mirrored the situation in Eurasia and Africa, where Megantereon was also playing second fiddle to Homotherium. But in the Old World, the balance of power was shifting among the big cats, with the panthers (lions, leopards, jaguars and tigers) gradually replacing machairodonts as top cats, with Megantereon vanishing from most of its range by one million years ago (sans a relic population in China lasting until half a million years ago) and Homotherium becoming rare by the Late Pleistocene (the youngest record dating to roughly 30,000 years ago), with cave lions becoming the dominant felids across Eurasia.

By contrast, panthers remained absent from North America until 850,000 years ago (when jaguars colonized the continent, possibly descendended from P. gombaszogensis), and as such, machairodonts continued to reign supreme, dwarfing the local pumas (Miracinonyx and the still extant Puma). While the spotty fossil record leaves Smilodon's transformation into an apex predator during the Irvingtonian/Ensenadan murky, by about one million years ago, Smilodon had grown much larger (possibly due to the extinction of the similar Xenosmilus), with the famous Smilodon fatalis (at around 400-600 lb) being widespread south of the Laurentide ice sheet, from Alberta to Central America, with even some records from South America, while the gigantic Smilodon populator (the type species) was unique to South America, being found as far south as the southern tip of Patagonia (like the Milodon Cave site) and at its largest, it could have been pushing 1,000 lb (600-800 lb would have been the average), making it one of the largest cats ever to have lived. The three Smilodon species are actually hard to distinguish aside from size, geography and stratigraphy, especially when working with incomplete material, hence why the full fossil ranges of each species is unclear (and their evolution likely wasn't a case of anagenesis).

Scimitar cats seemingly didn't find much success in South America (only found at El Breal de Orocual and one fragmentary jawbone from Uruguay, both seemingly representing the Ensenadan), and although jaguar also colonized South America and grew as big as lionesses during the Pleistocene, they were still dwarfed by S. populator, while the local short-faced bear (Arctotherium) lost its largest species (A. angustidens) around the time S. populator showed up (the latest Ensenadan-Lujanian), so the giant sabretooth was the undisputed top predator of Lujanian South America. By contrast, S. fatalis faced a more competitive environment during the late Irvingtonian-Rancholabrean of North America, having to contend with Homotherium serum, the dire wolves (Aenocyon), giant jaguars and the giant short-faced bear (Arctodus simus), the last of which might have frequently stolen it kills like a grizzly usurping a cougar's kill. For hundreds of thousands of years, S. fatalis hunted the native large ungulates like horses, llamas, camels, tapirs, peccaries, smaller ground sloths and probably juvenile proboscideans, with S. populator doing the same down south, along with preying on the native South American ungulates, the toxodontids and macraucheniids (the latter converging with the camelids that sabretooths coevolved with in the north). Around 130,000 years ago (the start of the Rancholabrean), bison finally colonized North America and became another prey item, but with them came a new rival, the giant American lion (descended from Beringian cave lions), which was approaching sizes similar to S. populator, though lions seemingly never reached South America (with possible finds being reinterpreted as large jaguars). After 30,000 ya, the Americas might have been the last place in the world to house saber-toothed cats and they vanished as part of the Quaternary extinction, ending a legacy of apex predators that lasted for at least 10 million years.

Known from a fairly complete holotype skeleton from the upper Oldman Formation of Alberta (previously attributed to Daspletosaurus) and two additional specimens from the lower Dinosaur Park (previously attributed to Gorgosaurus), dating to 76.7 to 76.3 mya, Tromerosaurus overtly resembles an albertosaurine but also shows morphological characteristics of tyrannosaurines. As such, it's been recovered as either a basal albertosaurine or a basal tyrannosaurine (a transitional form between albertosaurines and the robust tyrannosaurines). This find could rewrite what we know about the evolution of the Late Cretaceous tyrannosaurids of Laramidia. If it is indeed an albertosaurine, it could suggest that the gracile tyrants aren't a ghost lineage (given that some purported tyrannosaurines like Lythronax date to as far back as 82 mya, near the Santonian-Campanian boundary) but a more recent split from the robust forms during the lower Campanian.

And yes, DNA evidence suggests that some mammoth specimens from the Late Pleistocene of North America are indeed woolly-Columbian hybrids. Genetic admixture seems to have contributed quite a bit to elephantid evolution during the Pleistocene across the globe.

Both Glyptotherium and capybaras (both the larger, extinct Neochoerus and the surviving Hydrochoerus) migrated into the southern corners of North America during the Great American Interchange, often being found together in marshy, forested environments like Florida and (during certain geological periods) Arizona. While capybaras would be fair game, sticking to a one-ton glyptodont might have been one way of discouraging predators like terror birds or sabretooths from attacking, or provide an armored shield while they escape to the water.

These guys are often called "American gomphotheres", but gomphotheres are now understood to be a paraphyletic grouping and an evolutionary grade leading to elephantids. Recent cladistics do indicate that the American branch represents a monophyletic grouping, having likely evolved from "Gomphotherium" productum, which colonized North America along with the ancestors of American mastodons around 16.5 million years ago. "Cuvieronidae" would be an apt name for this endemic clade of New World proboscideans, who are also the only ones to have colonized South America during the Great American Interchange. They, mastodons and mammoths all evolved from Gomphotherium-esque ancestors, with long faces, shovel mouths and more pronounced lower tusks, but they all converged on the more modern elephantine appearance; long, flexible trunk and curvy, extravagant upper tusks.

At least four genera are generally recognized, three of which survived into the Pleistocene, though the taxonomy within this clade has been messy, with various species jumping between genera or getting synonymized with other species. Right now, we have Stegomastodon (known only from North America), Notiomastodon (known only from South America) and the Pan-American Cuvieronius. These animals generally weren't very big by proboscidean standards, generally falling within the size range of the modern Asian elephant, compared to the towering mammoths and exceptionally robust and rotund mastodons. They coexisted with mastodons in North America for millions of years, generally being mixed feeders or even leaning towards a grazing diet (like Stegomastodon), while mastodons were browsers (folivores).

Like with most North American colonists, when "cuvieronids" reached South American is unclear, with the oldest definitive evidence of them dating to about 760,000 years ago, though fragmentary vertebrae from the Uquia Formation suggests they might have been roaming Argentina as far back as 2.5 mya (alongside horses, llamas and peccaries), and some suggest that Cuvieronius was the only taxon to colonize South America via the Andes, and the southern population gave rise to Notiomastodon, but that remains disputed. Stegomastodon vanishes from the fossil record around 1.2 million years ago, which corresponds with the arrival of mammoths to North America, and the range of Cuvieronius in the north also seemed to subside, with Late Pleistocene fossils being mainly known from Mexico and Central America. This has been suggested to be the result of competitive exclusion from Columbian mammoths, though Cuvieronius still survived up until the end of the Pleistocene and might have been hunted by humans, like one specimen from the El Fin del Mundo in Sonora, which was found in association with Clovis spear points. Note that the size difference between Cuvieronius and the Columbian mammoth was so great, that the latter could have given the former the rhino treatment when in a foul mood.

Much like extant big cats, machairodonts started off as tiny balls of fluff who were wholly dependent on their mothers for survival. In recent years, no less than five Pleistocene big cat cubs have been found in the Siberian permafrost, four of them being familiar cave lions (which were common across Eurasia in the Late Pleistocene) but also the surprising discovery of a machairodont cub, a Homotherium, the first permafrost mummy of an animal with no close relations alive today (even woolly rhinos are fairly closely related to the Sumatran rhino).

Titanotylopus nebraskensis stood up to 12 feet at the hump and weighed an estimated 2.5 tons, and it was the last of the giant camels that roamed North America since the Miocene. It vanished during the Mid Pleistocene (middle Irvingtonian), after which Camelops was the only "true camel" left in North America (the other ones being llamas).

Megatherium and Glyptodon are found in the southern half of South America, while these ones inhabited the more tropical biomes in northern South America and crossed into Central America, Mexico and the southern United States. The some glyptodont scutes from Mexico indicate that Glyptotherium had already colonized the north around 3.9 mya.

Seeing Cotylorhynchus be brough to live in Surviving Earth really hammers in how absurd caseids really look.

Yeah, I'm certain they didn't call the pliosaur Megacephalosaurus or Brachauchenius in the episode (despite what some posts by Impossible Pictures claim) because they just wanted a super-giant brachauchenine akin to Sachicasaurus or Monquirasaurus, but those guys are from the Early Cretaceous.

The next episode of Surviving Earth will be about the Cenomanian-Turonian extinction, an important event that shaped the familiar Late Cretaceous fauna, though there is only so much they can cover with a limited runtime (so no tyrannosaurs and megaraptors replacing carcharodontosaurs). This one might shake things up by having two main creatures, Spinosaurus and a brachauchenine, and presumably taking place around the Tethys Sea (hence the Croatian mosasaur Portunatasaurus), while the flashforward epilogue takes place in the Western Interior Seaway during the Coniacian, with Pteranodon, the Ginsu shark and Niobrarasaurus, as well a new mosasaur (presumably Tylosaurus).

The penultimate episode of Surviving Earth will be a about the Great American Interchange, and the plot will evidently be "Mammoth Journey" in the setting of "Saber Tooth", except in Panama rather than Paraguay, and focus on a herd of Cuvieronius crossing the Isthmus of Panama and being the first proboscideans to colonize South America, 2.5 million years ago, followed by the ancestral sabertooth Smilodon gracilis. And of course there's the native South American wildlife, Eremotherium, Glyptotherium, Falcontoxodon and phorusrhacine terror birds. And there will be an epilogue showing how things have changed in South America by the Mid Pleistocene, with Smilodon populator as the new apex predator.

I doubt the episode will explore all the nuances of the GAI, given time and budget restraints, like how terror birds, Glyptotherium and ground sloths colonized North America before the formation of the Isthmus of Panama (megalonychids actually appearing in North America 10 million years ago), and suffice to say, the spotty fossils record of South America from the Pliocene-Early Pleistocene leaves a lot of uncertainty, with the oldest fossil material of gomphotheres and Smilodon in South America suggests they colonized it closer to one million years ago (the episode being set in Panama is likely meant to mitigate that).

Of course, people want to see the familiar animals (like elephants and cats) running into the exotic oddities of South America, as it's the most narratively palatable, but the fossil record actually suggests it was the other way around. In that sense, perhaps ground sloths would be a more appropriate protagonist (including seeing them swimming between islands), but seeing elephants going on an epic odyssey is never boring.

Tylosaurus is a mosasaurid genus with a messy taxonomic history and will likely see furthe revisions, but regardless, tylosaurines have a rich fossil record in the Western Interior Seaway, and SGM-M1, a skull a little over 2 feet long, is one of the oldest specimens found yet, stemming from the middle Ojinaga Formation in Chihuahua, Mexico, dating to the upper Turonian, around the time mosasaurs were first taking over as the new apex predators of Earth's oceans. This specimen was probably only around 17 feet long, but tylosaurines in the Western Interior Seaway grew bigger over time, and by the earliest Campanian, we find 40-43 foot long giants like T. rex.

With the second episode of Surviving Earth out, we can see the plot structure the rest of the series will likely follow. Essentially, it's one longer story followed by a flash-forward epilogue, showing how things have changed. "When the Climate Broke" isn't really the tale of how dinosaurs took over in the Triassic, it's actually more about the non-dinosaur fauna. Frankly, it's a nice change of pace (now dinosaurs know what it's like to be sidelined)

The whole thing is based on the Agua de la Peña Group in Argentina, with the main focus being the Carnian Ischigualasto Formation, following the stahleckeriid Ischigualastia as the main creature, and featuring the prestosuchid Saurosuchus, the early dinosaur Panphagia, and an uncredited Hyperodapedon sanjuanensis (or Scaphonyx, as Hyperodapedon might be a wastebasket taxon). The epilogue is set in the overlying, Norian Los Colorados Formation, and the focal animal is the little cynodont Chaliminia, while Zupaysaurus and Lessemsaurus. This episode really fixes a lot of the issues in WWD's "New Blood", other than not showing us these Triassic creatures and large dinosaurs coexisting, which we know they did (see Herrarasaurus and Fasolasuchus).