The Lujanian Land Mammal Age, the South American Equivalent of the Rancho La Brean Land Mammal Age

(Note: I accidentally published this article earlier today before I added the text.  I deleted that mistake. Here is the text.)

The Rancho La Brea Fossil site in California produced so many spectacular fossils that it gives the name to the Rancho La Brean Land Mammal Age, a period of time including the last 300,000 years of the Pleistocene.  All the fauna from this age in North America is referred to as Rancho La Brean.   In South America this age is known as the Lujanian Land Mammal Age and is named after a former site in Lujan, Argentina where fossil hunters found in quality and quantity specimens that are at the very least the equal of those found at Rancho La Brea.   People began collecting fossils here as early as the late 18th century and continued to do so well into the 20th century until the site was swallowed up by urban development.  The Lujanian Land Mammal Age was originally considered to have begun when horses of the equus genus first entered South America, however, paleontologists have since determined equus horses colonized South America much earlier, perhaps over 1 million years ago.  (Horses from the hippidion genus lived in North America even earlier.)  In any case land mammal ages are an artificial construct invented by men to define the composition of fauna that lived during certain periods of time.

Location of Lujan, Argentina highlighted in white.

At least 31 species of mammals weighing over 40 pounds lived in and around what is now known as Lujan during the late Pleistocene.  This list includes an astonishing 14 species of xenarthans: 2 large species of armadillos, 7 species of glyptodont, and 5 species of giant ground sloth.  By comparison Rancho La Brea was home to just 3 species of xenarthans.  Ecologists puzzle over how the environment could support such a wide variety of closely related species.  The different species of animals likely ate different species of plants.  The ground sloths Glossotherium and Lestodon were bulk feeders of grass, Mylodon and Scelidotherium were mixed selective feeders, and the huge Megatherium was the most selective feeder of all the sloths.  Similar niche partitioning likely occurred among the large armadillos and glyptodonts.

The glyptodont, Doedicurus clavicaudatus.

3 species of primitive ungulates occurred during the Lujanian Age–2 species of toxodon and the bizarre ancient litoptern.  Toxodons were hippo-like in build and may have been semi-aquatic.  Litopterns (Macruachenia patachonica) diverged from the ancestors of horse, tapir, and rhino before the dinosaurs became extinct, yet those odd-toed ungulates are their closest living relatives.

Toxodon platensis.

The extinct litoptern, Macrauchenia patachonica.

More modern ungulates ranging near Lujan during the late Pleistocene were 4 species of llamas (3 now extinct), collared peccaries, an extinct species of horse, and a large extinct species of deer (Morenelephus lujanensis.  A mastodon-like gompothere (Stegomastodon platensis) roamed the land with them.

5 large species of carnivores preyed upon the plant-eating beasts.  Smilodon populator, a 750 pound saber-toothed cat, took on and took down some of the megaherbivores.  Jaguars and cougars attacked smaller prey than Smilodon’s victims.  The little known small wolf (Dusicyon avus) may or may not have hunted in packs but was probably more a scavenger, like a coyote.  An extinct bear (Arctotherium tarijensis) opportunistically ate meat whenever it had a chance.

Reference:

Farina, Richard; Sergio Vizcaino and Gerry De Juliis

Megafauna: Giant Beasts of Pleistocene South America

Indiana University Press 2013

 

1 Thing I Knew and 2 I Didn’t

I’ve learned a couple things this past week in my search for blog fodder.  A new study presented evidence saber-toothed cats fought each other.  This is not at all surprising–I always just assumed this was true.  All extant large mammals battle each other in intraspecific conflict over mates or territory.  Of course, saber-toothed tigers were no different.  However, scientists found actual evidence–2 Smilodon populator skulls found in Argentina (1 by an amateur and 1 by a professional paleontologist) have punctures in them that exactly match the canine of another saber-tooth cat.  Smilodon populator was a huge species of saber-tooth, reaching 750 pounds in weight, that lived in eastern South America until about 11,000 years ago.  It was closely related to the more widespread Smilodon fatalis which ranged throughout most of North America and western South America until the end of the Pleistocene.  Previous studies have suggested Smilodon biting pressure was weak compared to most other species of cats, but apparently it wasn’t that weak…they were capable of puncturing bone.  The same kinds of injuries occur in extant species of cats.  Undoubtedly, most of these wounds are fatal, and the canine of a Smilodon was so long it definitely caused a fatal bite because it penetrated well into the brain.

Saber-toothed cats sometimes died during intraspecific fights.

I was watching a nature show on National Geographic wild entitled Wild Portugal and learned there were wolves still living in Portugal.  This, I did not know.  There is a population of 2000 wolves living in northeastern Spain and northern Portugal, and they are protected, though farmers try to kill them when they can get away with it.  The wolves take an heavy toll on the local sheep.  Some consider them beneficial because they control populations of wild boar.  They also hunt feral horses and deer.  Genetic evidence suggests the Iberian wolf has been isolated from other European wolves since before the Last Glacial Maximum when the populations were separated by a glacier.

Iberian wolf.

I also didn’t know there was a feral population of cats in Madagascar that already evolved to twice the size of a regular house cat.  Arab traders brought cats to Madagascar about 1000 years ago, and they went wild.  The evolution in size is an adaptation for hunting lemurs–a regular part of their diet along with rodents, snakes, and birds.  They outcompete a native predator, the fossa–a distant relative of the mongoose.

Feral cat in Madagascar.  They are twice the size of a regular house cat.

Reference:

Chimento, N; et. al.

“Evidence of Intraspecific Agonistic Interaction in Smilodon populator (Carnivora, Felidae)”

Comptes Rendus Palenal 18 (4) June 2019

The Largest Saber-toothed Cat, Smilodon populator

Smilodon populator may have been the largest cat ever to have hunted in the wild.  Richard Farina, a South American paleontologist, estimated this species reached a weight of over 800 pounds.  This is more than twice as big as the more famous Smilodon fatalis, a species that lived all across North America during the late Pleistocene.  Smilodon fatalis also ranged into South America west of the Andes Mountains, while S. populator occurred east of this mountain chain.  Apparently, the Andes Mountain chain served as a geographical barrier, resulting in the evolutionary divergence of the 2 species of Smilodon.

Smilodon populator reached an astonishing 800 pounds in weight.

S. populator was an ambush predator that wrestled its prey to the ground and bit its throat, slicing through windpipe and jugular with its long fangs.  Scientists assumed it lived in forested environments where it could more easily surprise its prey from behind cover.  However, a recent study, using isotopic analysis of bone collagen from sub-fossil specimens, determined S. populator and its favorite prey lived in an open dry environment.  According to this study of specimens from 6 different sites in Argentina, S. populator’s favorite prey was macrauchenia, a now extinct primitive ungulate.  Macrauchenia occupied an ecological niche similar to that of a modern day giraffe’s.  With its long neck it could reach tree leaves unavailable to horses and deer, but it also grazed on grass.  S. populator also often preyed upon 2 species of ground sloths–the enormous megatherium and lestodon.  Prey species other than macrauchenia, megatherium, and lestodon made up less than 5% of S. populator’s diet, if this isotopic analysis accurately reflects the big cat’s diet.  Giant ground sloths moved so slowly that S. populator would likely have had no problem seizing an individual.  I suspect they mostly attacked weaned sub-adults, no longer protected by their mothers.  Full grown sloths would have been difficult to subdue because they were large, powerful,  armored under thick skin, and armed with big claws.  Some scientists believe ground sloths dug burrows as a kind of strategy to defend themselves from S. populator.  These burrows still exist.  (See: https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/ )  A ground sloth in a burrow with its back protected would have been impossible for a Smilodon to kill.

I hypothesize S. populator had a black or dark gray coat that made it difficult to see in the dark.  This allowed them to ambush sleeping macrauchenia in the open.  It explains how a large relatively slow predator could catch a better runner, like macrauchenia. I believe S. populator hunted ground sloths during the day when they left their burrows to feed and sleeping macrauchenia at night when the latter were less alert.

An isotopic study suggests macrauchenia was Smilodon populator’s favorite prey.

An extinct canid, Protocyon trogl0dytes, may have competed with S. populator.  The isotopic values of 1 specimen of this species closely matches those of the many specimens of S. populator analyzed in the study referenced below.  This suggests Protocyon ate the same prey species as S. populator, however, the sample size is too small to reach a definite conclusion about this species diet.  It’s not known whether this species was an active hunter, a scavenger, or both.  Isotopic evidence from a single specimen of a jaguar from the late Pleistocene of South America indicates it preyed on different species–horse, deer, and rodent.

S. populator lived in South America for at least 750,000 years.  It ecologically replaced a saber-toothed relative of the marsupials known as Thylacosmilus atrox.  Smilodon may have outcompeted Thylacosmilus and driven it to extinction.  However, there is a 1 million year gap in the fossil record between the most recent specimen of T. atrox and the earliest specimen of S. populator.  It’s not known for certain whether the 2 species ever overlapped in time.

In the conclusion of the below referenced paper the authors suggest a change from dry open environments to more humid forested conditions may have contributed to the extinction of S. populator.  I strongly disagree with this notion because a) S. populator survived as a species for over 750,000 years of climatic changes and b) dry open environments are still widespread in South America, most notably in the Gran Chaco region.

Smilodon populator hunted in a dry open environment, not dissimilar from this palm savannah in the Gran Chaco region of Argentina.

I’m convinced man is entirely responsible for the extinction of S. populator.  Humans overhunted Smilodon’s favorite prey, depriving the big cat of food, and probably directly hunted the big cats.  Its great size became a disadvantage after man colonized the region because it was an easy target to hit with spears thrown from a great distance.

Reference:

Bocherens, Herve; et. al.

“Paleobiology of Sabretooth Cat Smilodon populator in the Pampean Region (Buenos Aires Province, Argentina) around the Last Glacial Maximum: Insights from Carbon and Nitrogen Stable Isotopes in Bone Collagen”

Paleogeography, Paleoclimatology, and Paleoecology March 2016

New Species of Early Pliocene Saber-tooth (Rhizosmilodon fiteae) Discovered in Florida

Phosphate mining requires the removal of 50 feet of soil, resulting in severe environmental degradation.  By law, mining companies must restore the land following completion of mineral extraction, but it will be centuries before anyone can honestly say the eyesore has been returned to a pristine condition.  Strip mining operations are bad for the environment, but they often expose fossiliferous strata, much to the delight of paleontologists.  Fossil hunters prospecting the spoil piles of the Fort Meade phosphate mine in Polk County, Florida found the remains of 70 species of vertebrates (from thousands of specimens)  dating to the early Pliocene (~4-5 million years BP).

Polk County, Florida–the location of the Fort Meade Mine where many early Pliocene fossils were found.

Fort Meade Mine in Florida.  Scientists discovered an early Pliocene fauna of fossils here in the excavated spoil piles.  

Periodic sea level transgressions explain why marine fossils were found mixed with fossils of land mammals at this site.  Shark’s teeth, including those of the extinct Hemipristis, along with gar and barracuda remains, turtle shells, and fossils of alligators, seals, and whales were found next to the bones of many species of mammals known to have lived in North America during the early Pliocene, just before Ice Ages began occurring.  This was the final age of the hippo-like rhino (Teleoceras), a beast that was about to become extinct after being the most common large mammal here for millions of years.  Several species of 3-toed horses, as well as long-necked camels and llamas browsed the subtropical vegetation.  Eocoileus, the earliest known ancestor to the modern white tail deer and probably the first deer species to live in North America, was part of the fauna then.  Two elephant-like animals–a gompothere and Matthew’s Mastodon–trudged through the forests.  There were 2 types of short-faced bears, a flat-headed peccary, a ground sloth, and a bobcat (Lynx rexroadensis) that were all probable ancestors to the more recent and better known Pleistocene forms.  However, it was the last stand for the extinct bone-eating dog (Borophagus).  This carnivore left no descendents.  Altogether, this list of species is known as the Whidden Creek Local Fauna which corresponds to the regional Palmetto Fauna.

Perhaps the most interesting fossils from the Whidden Creek Local Fauna were the 2 species of saber-tooth cat.  Machairodus colorodensis was a lion-sized saber-tooth that may have been top predator here then.  Scientists at first misidentified the smaller jaguar-sized saber-tooth.  All they found was 1 jaw and based on that specimen it was thought to belong to a Megantereon hesperus, a species known to have ranged well to the north and west of Florida.  However, scientists later found a few more jaws and leg bones assumed to originate from the same species.  Based on this material, they determined the fossils were from a previously unknown species.  They named this new species Rhizosmilodon fiteae.  They believe this new species was ancestral the the early Pleistocene species Smilodon gracilis which was in turn ancestral to 2 late Pleistocene species–Smilodon fatalis and Smilodon populator. The former ranged across North America, while the latter lived in South America.

Jaw and leg bones of Rhizosmilodon fiteae–a probable ancestor to Pleistocene species of Smilodon.

There are several anatomical characteristics that differentiate Rhizosmilodon from its later descendents: It has more minor serrations on its canines, a less developed mandibular flange, larger lower canines, and a lower premolar that is absent or reduced in Pleistocene species of Smilodons.  The mandibular flange is the recess in the lower jaw that acted as a space for the large upper canines when the jaw was shut.

The scientists who identified Rhizosmilodon believe it is a related sister species to Megantereon hesperus.  Rhizosmilodon and Megantereon likely co-occurred temporally but in different geographical ranges.  Rhizosmilodon’s ancestors spread throughout North America, while Megantereon’s ancestors dispersed to Asia, Europe, and Africa.  Rhizosmilodon is the oldest known ancestor to Smilodon and suggests a North American origin of the genus.

Refererence:

Wallace, Steven; and Richard Hulbert

“A New Machairodont from the Palmetto Fauna (Early Pliocene) of Florida, with comments on the origin of the Smilodontii (Mammalia, Carnivora, Felidae)

Plos One March 2013

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0056173