Scientists Recognize New Species of Late Pleistocene Horse (Haringtonhippus francisi)

Scientists recently recognized a new species and genera of extinct Pleistocene horse from fossil specimens already in museums.  Some of these specimens were collected over 100 years ago and were wrongly assumed to represent previously known species or genera.  During the Pleistocene there were 3 lineages of horses in the Americas–the caballine horses, the New World stilt-legged horses, and the hippidion horses.  The caballine horses belong to the Equus genus which includes all living species of horses, donkeys, and zebras.  The species of caballine horses that lived in North and South America likely included the predecessor of the modern day domesticated horse.  It was probably the same species.  The New World stilt-legged horses so anatomically resembled Asiatic wild asses and donkeys that paleontologists mistakenly thought they were closely related.  In recent years paleontologists began to reject this assumed affinity, and the genetic study cited in this blog entry supports their re-assessment.  The hippidion horses were robust species restricted to South America.  A new genetic study determined the New World stilt-legged horses, previously classified as belonging to the equus genus, were different enough to deserve their own genus.  Scientists gave this species the scientific name Haringtonhippus francisi. The species was named after the renowned Canadian paleontologist, Richard Harington.  The type specimen anatomically described in the paper was originally discovered in Wharton County, Texas.

Artists’s representation of Haringtonhippus francisi.  The coat color is the artist’s fanciful guess.

The genetic evidence suggests haringtonhippus  horses diverged from equus horses between 4-5 million years ago.  The hippidion horses diverged from the equus/haringtonhippus genera between 5-7 million years ago.  Convergent evolution explains why haringtonhippus horses anatomically resembled Old World asses.  Both evolved long slender limbs as an adaptation to arid environments.

Fossil remains of Haringtonhippus francisi  have been found in east Texas, eastern Mexico, Kansas, Nevada, California, the Yukon, and Alaska.  Stilt-legged horse fossils are known from sites thought to be 3 million years old, and they occurred until as recently as 12,000 years ago about the time man became prevalent on the continent.

If scientists are able to extract DNA from even more ancient extinct genera of horses, they may be able to straighten out horse evolution.  Many biology textbooks use the fossil record of horses and their ancestors as an example of evolution, but these family trees are based on anatomical analyses that can be misleading.  DNA evidence would produce more reliable family trees.

Reference:

Heinztman, P; et. al.

“A New Genus of Horse from Pleistocene North America”

Genomics and Evolutionary Biology Nov. 2017

Raccoon (Procyon lotor) and Swamp Rabbit (Sylvilagus aquaticus) Latrines

Many animals defecate to mark their territory, but raccoons share communal latrines where all the individuals in an area deposit their feces. Communal raccoon latrines impact the ecosystem. The raccoon roundworm (Baylisascaris procyonis) is a parasite that spends part of its lifecycle in a raccoon’s intestine. Raccoons can live with this parasite, but it can kill mice, birds, and humans. Therefore, mice and birds avoid raccoon latrines, despite the nutritional value found in undigested seeds embedded in raccoon feces. Somehow, they evolved the ability to sense the danger of a parasite playground. However, raccoon roundworm is not dangerous to rats, so raccoon latrines actually attract rats seeking edible seeds. Seeds that survive transport through a raccoon’s digestive system and are overlooked by rats may then germinate. Raccoon’s play a role in the dispersal of some plant species.

Raccoon latrine.  This batch is full of blackberry seeds.

Unlike raccoons, swamp rabbits don’t defecate in communal latrines, but oddly enough they often crap on moss-covered stumps or fallen logs. Perhaps the moss disguises the odor, preventing predators from triangulating their scent. It is also elevated, so a predator following its nose might miss it.  Researchers surveying swamp rabbit populations use these latrines to record their presence because this nocturnal species is difficult to find in the thick swamps and wetlands where they range.

Swamp rabbit latrine. Note the moss.

Swamp rabbits are a species of cottontail that inhabits aquatic habitats from the Mississippi River Valley east to western Georgia and northwestern South Carolina. Another species of semi-aquatic cottontail–the marsh rabbit (S. palustris)–inhabits wetlands from western Georgia to the Atlantic Ocean and throughout all of Florida. There is little overlap between the 2 species, though they occupy the same ecological niche. How curious?

Swamp rabbit genetics has rarely been studied. A 20 year old genetic study determined swamp rabbits and marsh rabbits are closely related sister species, but this doesn’t explain why their ranges don’t overlap. So far, no geneticist has employed a molecular clock to estimate when swamp rabbits and marsh rabbits diverged or when they diverged from eastern cottontails (S. floridanus). The latter is an habitat generalist with an extensive Pleistocene fossil record. Specimens of Pleistocene eastern cottontails have been found all over North America. By contrast marsh rabbit remains dating to the Pleistocene are restricted to 8 sites in Florida and 1 site near the Georgia coast. Pleistocene swamp rabbit remains are even less common, having been discovered at 1 site in Missouri, and 1 site in Tennessee where Pleistocene remains were mixed with Holocene material, so this specimen might not even be from the Pleistocene. Some marsh rabbit remains date to the Sangamonian Interglacial (132,000 years BP-118,000 years BP). Swamp rabbits, as a species, are probably at least that old too. Both species thrived during wetter stages of climate when wetland habitat expanded. I hypothesize the common ancestor of both was a semi-aquatic species that was isolated into 2 separate founder populations during arid Ice Ages when unsuitable desert grassland habitat expanded between refuges along the Mississippi River Valley and Florida. But I still can’t figure out why they haven’t invaded each other’s ranges since then.

Genetic studies may be the best way to resolve this mystery because the fossil evidence is scant. I hope a geneticist takes an interest in this unresolved secret of nature.

References:

Fantz, Debbie; et. al.

“Swamp Rabbit Distribution on the Northern Edge of their Range in Missouri”

Southeastern Naturalist  16 (4) 2017

Halanych, K.; T. Robingon

“Phylogenetic Relationships of Cottontail (Sylvilagus, Lagamorpha) Congruence of 125r DNA and Cytogenetic Data”

Molecular Phylogenetics and Evolution 7 (3) June 1997

Weinstein, Sara; et. al.

“Fear of Feces? Trade-offs between Disease Risk and Foraging Drive Animal Activity around Raccoon Latrines”

Oikos  Jan 2018

The Ancient Rivalry Between Cats and Dogs

The PBS documentary series, Nature, recently featured a 2 part episode about cats.  During the first episode the narrator claimed cats caused the extinction of at least 40 species of dogs after the felines colonized North America.  I knew there had to be a journal article behind this claim, so I googled “cats caused extinction of 40 dog species.”  I found the paper (referenced below) and also discovered 90% of media outlets misreported the conclusions of the study.  Cats did contribute to the extinction of many dog species, but competition with other dog species and the extinct carnivores known as Barbourofelidae were also responsible for the extinctions.

Dogs originally evolved in North America, while cats originated in Asia.  About 40 million years ago a land bridge began to periodically emerge across the Bering Strait, allowing cats and dogs to colonize each other’s continent of origin.  When cats first colonized North America there were 3 subfamilies of dogs–the Hesperocyoninae, the Borophaginae (or bone-eating dogs), and the Caninae.  The Hesperocyoninae and the Borophaginae are extinct.  All living species of dogs, jackals, wolves, and foxes belong to the Caninae subfamily.

The scientists who authored the below referenced paper collected data about climate change, and the fossil occurrences of predators including Felidae (cats), Amphicyonidae (the extinct bear-dogs), Barbourofelidae, Nimravidae (false saber-tooths), and the Ursidae (true bears).  They used statistics to determine whether climate change or competition with other carnivores caused the extinction of some species of dogs.  They concluded the extinction of 1 subfamily of dogs, the Hesperocyoninae, was caused by competition with another subfamily of dogs, the Borophaginae.  Cooler climate may have contributed to the extinction of some Borophaginae species 15 million years ago.  Finally, competition with Barbourofelidae, cats, and the surviving subfamily of dogs (the caninae) drove the remaining species of Borophaginae into extinction.

The Barbourofelidae are an extinct group of carnivores distantly related to cats but different enough to be classified as a separate family.

The species of dogs that did become extinct were often cat-like in build and probably occupied ecological niches preferred by cats.  So cats were just better than these cat-like canids at surviving in these niches.  But the Caninae were also better adapted to survive in the constantly evolving environment.  The cat and dog species that emerged from this age-old competition have achieved a kind of stalemate.  Representatives of both naturally occur on every continent but Antarctica and Australia.  (Dingos were brought to Australia by man.)  1 species of each–Canis familiaris and Felis catus —live in our homes and compete for our affections today.

References:

Silvestri, D.; A. Antonelli, N. Salamin, and T. Quentas

“The Role of Clade Competition in the Diversification of North American Canids”

PNAS 112 (28) 2015

Pterosaurs may have Cared for their Young

Some imagine the Cretaceous and Jurassic Ages as a time when the earth was strange and full of terrifying monsters.    The earth was a vast wilderness then, dangerous perhaps for most creatures, but it was no more strange or terrifying than the world we live in today–the Anthropocene with its genocides, terrorism, potential nuclear war, and extensive environmental destruction caused by a single dominant species.  The dinosaur world hosted species different from those of modern day earth, but these organisms were part of ecosystems recognizably comparable to those of today.  For example fish-eating pterosaurs nested in communal colonies, not unlike present day heron and egret rookeries.  Pterosaurs were not dinosaurs but instead were flying reptiles–the only vertebrates besides birds and bats to evolve the ability to fly. After their initial evolution the early Jurassic pterosaurs radiated into many species and occupied different ecological niches.  From the middle of the Jurassic until their extinction at the end of the Cretaceous 66 million years ago, there were probably about as many species of pterosaurs living in the world as there are birds today.  Evidence from 1 site in northwest China suggests pterosaurs, like so many modern day vertebrates, cared for their young.

Paleontologists found 215 fossilized eggs of a species of pterosaur known as Hamipterus tianshanensis, a fish-eating species that nested communally.  The fossils from this site date to about 120 million years BP, and they are from many generations. The nests were located next to a lake at the time of deposition.  Apparently, pterosaurs used this site annually.  Perhaps it was difficult for predators to access.  Some of the eggs contain visible embryos.  The embryos show well developed legs but underdeveloped wing bones.  This suggests the hatchlings couldn’t fly and depended upon parental care for food until their wings developed.  However this conclusion isn’t certain.  The fossils are of an embryonic stage, not actual hatchlings.  The wing bones may have developed at a later embryonic stage.

Artist’s representations of pterosaurs have changed over the years.  In this old issue of Green Lantern from the early 1970s the pterosaur is larger than a man, featherless, and conveniently yellow.  Green Lantern’s power ring doesn’t work against yellow objects.  The wingspans of some species of pterosaurs were longer than the length of a man, but they could not have seized and carried a man away.  They were able to leap straight up and fly though, unlike large modern bird species which must take a running start.

This more modern representation of pterosaurs by Masato Hattori, a depiction of Hamipterus tianshanensis, shows the reptile covered with hair-like feather structures.  It also had teeth.

Cretaceous-aged outcroppings occur near Columbus, Georgia and the Chattahoochee River.  These are the only regions in the state where Cretaceous fossils have been found.  David Schwimmer, a professor at Columbus State, excavated 3 pterosaur wing bones from an outcropping here–the only evidence pterosaurs formerly existed in the state.

References:

Deeming, Charles

“How Pterosaurs Bred”

Science 358 (6367) December 2017

Wang, Xi; et. al.

“Egg Accumulation with 3-D Embryos Provides Insights into the Life History of a Pterosaur”

Science 358 (6367) December 2017

Pleistocene Wood Ducks (Aix sponsa)

Wood ducks differ from most other species of ducks because they nest in hollow trees, rather than in thick wetland vegetation.  Unlike migratory species of ducks that prefer to fly over open water or high in the sky, wood ducks comfortably fly between trees.  However, wood ducks do share a love of water with their kin.   Shortly after wood ducklings hatch, they jump out of their nest and follow their parent to water.  Oftentimes, their den tree is located in flooded terrain and the water guarantees a safe landing.  But the ducklings are so light they can land on solid ground without sustaining injury, though they are not yet able to fly.

Male wood ducks are much more colorful than females.  I’ve only seen wood ducks on 1 occasion, while I was visiting Phinizy Swamp Park in Augusta, Georgia.

Wood ducks probably first speciated during the early Pliocene when Ice Ages began occurring, and glaciers caused a divergence in the Holarctic ancestral population that also gave rise to their closest living relative, the mandarin duck (Aix galericulata) of east Asia–the only other species of duck in the Aix genus.  Fossil evidence of wood ducks dating to the late Pliocene and Pleistocene has been found at 6 sites in Florida and 1 each in Oregon, New Mexico, and Georgia; suggesting the species has been widespread for millions of years.  (Pleistocene wood duck remains in Georgia were excavated from Kingston Saltpeter Cave, Bartow County.)  Wood ducks were likely most common during interglacials and interstadials when their favored habitat–beaver ponds and woodlands with abundant streams–expanded.  Wood ducks eat acorns, seeds, berries, and insects.  Oaks increased in abundance during wetter climate phases, therefore providing more acorns for wood ducks to eat.

There are eastern and western populations of wood ducks.

Wood duck range map.

Genetic evidence suggests these populations diverged ~34,000 years ago.  This is consistent with the record of climate change.  The stage 2 stadial that included the Last Glacial Maximum started about 29,000 years ago and before this climate frequently fluctuated between stadial and interstadial. Any 1 of the previous stadials preceding stage 2 or stage 2 itself could have caused the ecological changes isolating the 2 populations.  Dry grassland habitat expanded and streams dried up, so that eastern and western populations were separated into different refugia.  They still haven’t reconnected, even though the 2 populations come so close to each other in the midwest.

Reference:

Peters, J.L.; W. Gretes, and K. Omland

“Late Pleistocene Divergence between Eastern and Western Populations of Wood Ducks (Aix sponsa) inferred by the ‘Isolation with Migration’ Coalescent Method”

Molecular Ecology (11) October 2005