Wednesday, February 12, 2014

Fairmead Landfill, California

The Fairmead Landfill fossil site was discovered in May 1993 during excavation of an expansion cell at the Madera County Fairmead Landfill. The site has since yielded the first diverse Irvingtonian-age (mid Pleistocene) fauna from north-central California. The animal remains found here have been dated to around 780,000 years ago and include mammoths, horses, camels, deer, pronghorns, ground sloths, wolves, and big cats.
The Pleistocene fossil replica room inside the Fossil Discovery Center of
Madera County, across the street from Fairmead Landfill.
There are three mid to late Pleistocene formations in the eastern San Joaquin Valley, all of which occur at Fairmead Landfill; Turlock Lake Formation, Riverbank Formation, and Modesto Formation. All fossils from the site have been unearthed from the Turlock Lake Formation.

Site History and Discovery
In May 1993, while excavating for a new five acre expansion cell at the Madera County Fairmead Landfill, north of Madera County and just southwest of the junction of state highways 99 and 152, a Madera Disposal Systems crew encountered vertebrate fossils.

Because the land is public property owned by Madera County, the California Environmental Quality Act (CEQA) requires that scientifically significant fossils be protected. Shortly after the initial discovery and at the invitation of the Madera County engineering department, J. Howard Hutchison and Robert Dundas of the University of California Museum of Paleontology (UCMP) at Berkeley travelled to Fairmead Landfill to assess the findings and recommend an appropriate course of action in order that Madera County adhere to state regulations. The UCMP recommended that fossils be salvaged and preserved as they are uncovered by landfill excavation activities. Paleontological monitoring has been ongoing ever since.

Under the direction of curator/collections manager Dr. Robert Dundas, California State University, Fresno oversees the management of the Madera County Paleontology Collection, which houses the fossil specimens found at the mid-Pleistocene Fairmead Landfill locality. The San Joaquin Valley Paleontology Foundation manages the Fossil Discovery Center of Madera County, which displays the specimens found at the Fairmead Landfill fossil site. The foundation also oversees the paleontology mitigation program at Fairmead Landfill, monitoring the site to collect any fossil specimens that may be uncovered during excavation by heavy equipment operators.

Local Fauna and Paleoenvironment
The Pleistocene is divided into two land mammal ages; the Irvingtonian (1.8mya to 240,000ya) and the Rancholabrean (240,000 to 11,000ya), both ages are named for California fossil sites. In general, Irvingtonian-age sites and fossils are rather sparse compared to those of the younger Rancholabrean. The Fairmead Landfill site is therefore important in enhancing our understanding of vertebrate faunas from that time. The fossils unearthed at the Fairmead Landfill were found in alluvial fan, fan channel, and marsh/lacustrine sediments. The presence of the Western Pond Turtle (Clemmys marmorata) and waterfowl of the family Anatidae further indicates the presence of a perennially wet environment which had been surrounded by semi-arid grassland.
A scene depicting the habitat that would have been present at the
Fairmead Landfill site 780,000 years ago. Artwork by David Douglas.
As with any fossil site, the smaller animals are the greatest indicators of the paleoenvironment. At Fairmead Landfill, these included smoothed-toothed pocket gophers (Thomomys), kangaroo rats (Dipodomys), and hares (Lepus). Pocket gophers prefer areas with high primary productivity and nitrogen soil concentrations. Their presence at the site reveals that the habitat was very fertile with a diverse plant community. The pocket gophers themselves likely played a valuable role in aerating the soil. Kangaroo rats live in arid to semi-arid habitats with sandy or soft soils suitable for burrowing. Known as desert animals, kangaroo rats have adaptations to conserve moisture and rarely visit standing water, although they will drink occasionally when this is available. The presence of kangaroo rats at the site reveals that the land surrounding the marshy area was a semi-desert habitat similar to that of southern California today.  Hares are grazing animals that favor open spaces with abundant food in the form of grasses, shrubs, and forbs, and their remains reveal that such vegetation was dominant at the Fairmead site. Other small vertebrates recovered here include Desert Tortoise (Gopherus agassizii), salamanders, frogs, and Colubrid snakes.
The small mammal community around the Fairmead Landill site included
pocket gophers (Thomomys), kangaroo rats (Dipodomys), and hares
(Lepus), all of which can still be found in the region today. 
Three species of ground sloth are known from this site; Jefferson’s Ground Sloth (Megalonyx jeffersoni), Shasta Ground Sloth (Nothrotheriops shastensis), and the Harlan’s Ground Sloth (Paramylodon harlani), representing the families Megalonychidae, Megatheriidae, and Mylodontidae respectively. Of the three, Harlan’s Ground Sloth was the most common sloth found at the site. This species grazed in small herds and its remains are never found far from a permanent water source like that found at Fairmead. Like the kangaroo rats, Shasta Ground Sloths are associated with drier environments and were likely infrequent visitors to Fairmead Landfill.
Jefferson's Ground Sloths, Shasta Ground Sloths, and Harlan's Ground Sloths
were a few of the major herbivores of the area.
The ungulates found at the site were all adapted for life on open grasslands and either grazers or mixed-feeders. These include Mule Deer (Odocoileus hemionus), the pronghorns Capromeryx and Tetrameryx, the camels Hemiauchenia and Camelops, and several species of the horse Equus. These herbivores would have found ample grazing in the surrounding grasslands and would have visited the marsh regularly to drink.
Mule Deer inhabited the semi-arid plains surrounding Fairmead Landfill
together with horses, camels, and pronghorns. Columbian Mammoths were
the largest animals represented here.
The Columbian Mammoth (Mammuthus columbi) is well represented here. These giant grazers inhabited the plains, savannas, and open woodlands of North America and required regular drinking water like today's elephants. In contrast, the American Mastodon (Mummut americanum) is absent from this site. This indicates that there was a lack of tree cover in the area, which these browsers would have required to exist here long-term.

The most common predators at Fairmead Landfill were Saber-toothed Cats (Smilodon fatalis) and Dire Wolves (Canis dirus). Elsewhere in the Americas, these two social predators are found in association with ancient marshland, wetland,floodplain, and riparian habitats. They would have ambushed medium-sized to large prey animals as they came to the water. The other large predator here, the Scimitar Cat (Homotherium serum), was much rarer at the site. A hunter of the open plains, this cat would have spent most of its time running down large herbivores on the grasslands.
Known predators that lived in Madera County were Coyotes, Dire Wolves,
Saber-toothed Cats, and Scimitar Cats. The former of the four is still
abundant today.
Coyotes (Canis latrans) are still alive today, occupying the arid to semi-arid lands of western North America east of the Mississippi River. Opportunistic hunters of small game, the Coyotes found at the Fairmead Landfill would have found plentiful prey in the local rodents, hares, and birds. The small pronghorn Capromeryx would have been a common year-round prey item, while the fawns of medium-sized ungulates such as the Mule Deer and the larger pronghorn Tetrameryx were taken opportunistically mostly during the rainy season. The Coyotes at Fairmead Landfill would have also readily scavenged from kills made by the larger carnivores at the site.

References
Dundas RG, Smith RB, Verosub KL. “The Fairmead Landfill Locality (Pleistocene, Irvingtonian), Madera County, California: preliminary report and significance”. PaleoBios, Vol 17 pp 50-58 (September 1996) < http://dundaspaleolab.com/images/Dundas%20et%20al%201996.pdf>

Dundas, Robert (2013). “Dundas Paleontology Lab: Fairmead Landfill” Retrieved Febuary 9, 2014 from http://www.dundaspaleolab.com/index-4.html

Artwork
"Water" by David March Douglas (2012). sorenskuld.com

Sunday, February 9, 2014

Oxyaenidae: Cat-like Predators of the Paleogene

The Oxyaenidae is a family of creodonts first described by renowned American paleontologist Edward Drinker Cope in 1877. While hyaenodonts are often described as dog- or hyena-like, oxyaenids are considered to be more cat-like or weasel-like in their general morphology with rather long bodies set on short, robust limbs. They were ambush predators with robust limbs, muscular bodies, shortened faces, and powerful jaws. They possessed enlarged, reinforced canines with blade-like molars, and some species even developed blunt premolars for cracking bones.

Mounted skeleton of Patriofelis ferox. Wiki
Evolutionary History
The earliest known oxyaenid was a domestic cat-sized animal called Tytthaena parrisi, known from the middle Paleocene (Tiffanian) of what is now the state of Wyoming. Throughout the rest of the Paleocene and Eocene, a number of other oxyaenid species existed across the Northern Hemisphere ranging from fox-sized to bear-sized. From the middle Eocene onward, however, oxyaenids would face mounting competition from another group of cat-like predators known as the Nimravidae, close relatives of true cats (Felidae) in the modern order Carnivora*. By the late Eocene, oxyaenids are absent from most faunal assemblages while nimravids are relatively common. The last known member of this group, Sarkastodon mongoliensis, became extinct early in the Oligocene epoch. Unlike the hyaenodonts, oxyaenids were restricted to North America and Eurasia but never appear to have invaded Africa. 


Stance & Locomotion
When Dr. Jacob L. Wortman first examined the fossils Oxyaena and Patriofelis in the late 1800s, he concluded that the animals' spreading paws were evidence that they once supported a web-like structure like that found in many semi-aquatic animals such as otters. He remarked “the broad, flat, plantigrade feet with their spreading toes suggest at first glance their use for swimming”. This in mind, he also theorized that oxyaenids were the likely ancestors of today’s pinnipeds (seals, sea lions, and walruses). This proposal led artists to reconstruct Patriofelis as an otter-like animal with webbed toes as shown in this 1896 reconstruction by Charles R. Knight.

Charles R. Knight's outdated and inaccurate illustration of
Patriofelis ferox (1896), depicted here as an otter-like animal
with webbed toes and plantigrade feet. Wiki
However, large paws with widely spaced toes are not unique to aquatic mammals: many of today's land predators like cats, bears, mustelids, and others have them as well. In fact, this characteristic has three functional advantages befitting a terrestrial carnivore;
  1. They enable silent movement through the animal’s environment by muffling any sounds made as the animal walks.
  2. Though not suitable for sustained running, they are well suited for short bursts of speed necessary for an ambush predator, which is a more conservative hunting method.
  3. Their wide surface area enhances grip, enabling greater manual dexterity for climbing and seizing prey.
A 1900 study comparing the feet of Oxyaena and Patriofelis to those of several modern opossums, raccoons, and cats revealed that the proportions of oxyaenid paws were comparable to those extant animals. The structure of the metapodials* also show that some oxyaenids may not have been fully plantigrade* as previously thought, but rather semi-digitigrade*. This would have enabled more efficient and agile movement on dry land. In life, oxyaenids would have possessed cushioning paw pads that would muffle their footsteps to further enable silent movement through their respective habitats.


Mounted skeleton of Patriofelis ferox, shown in a more accurate stance. Wiki
Oxyaenids were not the slow-moving, clumsy animals depicted in past writings and artwork. Rather, they were agile terrestrial predators adapted for long-distance walking, short bursts of speed, and perhaps some climbing ability.



Teeth & Jaws
Oxyaenids are described as being cat-like, while hyaenodonts are said to be dog-like. Indeed, the skulls do exhibit some “cat-like” characteristics designed to kill prey with a single bite. As in felids, shortening of the rostrum is achieved by the loss of the first premolars and the last molars. This reduction helps direct more of the animals’ bite force to the canine teeth, which in oxyaenids were extremely robust with deep roots for durability. The molars are highly specialized for shearing with the primary cutting teeth (the carnassials) comprising the first upper and second lower molars. Interestingly, among oxyaenids there seems to be a trend toward the reduction of the incisors, possibly an adaptation to enhance the puncturing aspect of the dentition.

Skull of Patriofelis ferox at Museum national d'Histoire
naturelle, Paris. Wiki
Oxyaenid skulls are exceptionally broad, strong, and appear quite large compared to the rest of the skeleton. For example; the middle Eocene predator Patriofelis ferox was the size of a Jaguar in terms of its head-and-body length. However, it had a skull that was just as long and considerably wider than that of a Lion, an animal nearly twice its size. High sagittal crests*, flared zygomatic arches*, and huge depressions in the mandible were the attachment sites of powerful jaw muscles which resulted in a tremendous bite force. In these aspects, the skull morphology of oxyaenids is more analogous to the extinct marsupial lions (Thylacoleonidae) of Australia, which were themselves superficially cat-like animals with heavily-built skulls and teeth adapted for piercing. 

Partial dentary and skull of Palaeonictis occidentalis.
Specimen on display at the Museum fur Naturkunde, Berlin.
Wiki
Members of the genera Machaeroides and Apataelurus are special in that they are the earliest known mammals to have developed saberteeth; specialized upper canines that have become elongated and laterally-flattened. As with nimravid and felid sabertooths, their skulls are designed to increase their maximum gape and employ a downward stabbing bite with the upper canines. It should be noted that the placement of these genera within the Oxyaenidae is still a subject of debate; some authors place them within the Hyaenodontidae or Limnocyonidae.

Skull of the sabertoothed creodont, Machaeroides eothen, on
display at the American Museum of Natural History,
New York. Wiki
Feeding Ecology
Based on their cat-like morphology, we can assume that oxyaenids were mostly solitary predators that did not require the assistance of a pack to bring down relatively large prey items. Although they lacked the ability to retract their claws like nimravids and felids, their broad forepaws were clearly capable of grasping and could have easily have been involved in the seizing of prey in the initial stages of the kill. Other predators like mustelids, ursids, and dasyurids* do this as well. Their powerful skulls, meanwhile, could then deliver precise and devastating bites to captured prey. The most likely killing method would have been a piercing bite to the head or neck. In addition to sectorial* dentition, some oxyaenids also possessed blunt, heavy premolars similar to those of modern bone-cracking hyenas. Many oxyaenids were most likely hypercarnivores which fed almost exclusively on animal matter. However, at least some, like the genus Palaeonictis, seem to have been more mesocarnivorous: having a diet mostly comprised of animal matter with small doses of plant matter.


Head sketches of three Oxyaenids from the early, middle, and late Eocene respectively.
From left-to-right: Oxyaena lupina, Patriofelis ferox, and Sarkastodon mongoliensis.
Genera & Species
Tytthaena (Gingerich, 1980)
The genus Tytthaena contains a single species T. parrisi, the earliest and smallest known oxyaenid. This species from the middle Paleocene of Wyoming was about the size of a modern domestic cat and was similar in morphology to later oxyaenids such as Oxyaena and Dipsalodon.

Oxyaena (1874)
This animal’s genus name is a combination of the Greek words oxys and hyaena, literally translating to “Sharp Hyena”. Oxyaena lived from the late Paleocene to early Eocene of North America with most of its fossils being known from the state of Colorado. The last upper premolar and the second lower molar formed the carnassials. In contrast to others of its family, O. lupina was a nimble and lightly-built predator that may have been able to hunt both on the ground and in the trees. It had a dental formula of I3/3, C1/1, P4/4, M2/2 x 2 = 40 teeth.

Palaeonictis (De Blainville, 1842) 
The genus name is derived from the Greek words palaio, meaning “old” or “ancient”, and ictis or ictidis, a type of weasel. Palaeonictis lived from the Paleocene to the early Eocene and are known to have inhabited North America and Europe. The genus is characterized by their relatively blunt premolars that were apparently adapted for crushing bones. Four species have been described ranging in size from that of a Wolverine to a Black Bear; P. peloraP. occidentalis, and P. wingi from North America, and P. gigantea from Europe.

Patriofelis (Leidy, 1872) 
The genus name translates literally to “Father Cat”, derived from the Latin words patrius, “father”, and feles, “cat”. Compared to Oxyaena the legs are stockier and the lumbar vertebrae are much larger and more robust. The caudal (tail) vertebrae are also larger and bear pronounced chevrons on their ventral surfaces. This suggests that Patriofelis was adapted to grapple with large prey animals by using its tail prop itself up while it stood up on its hind legs. The dental formula was I2/2, C1/1, P3/3, M1/2 x 2 = 30 teeth. Two species have been described based on body size, possibly representing different sexes of the same animal. P. ferox is the larger of the two, the size of a large Jaguar or a female Lion. P. ulta is smaller, at about one-third the size and may simply represent the female morph of P. ferox rather than its own species.

Sarkastodon (Granger, 1938) 
The name Sarkastodon is derived from the Greek words sarx, “flesh”, and odous, “tooth”, translating to “Flesh-tearing Tooth”. The last and largest of the oxyaenids Sarkastodon mongoliensis lived during the middle to late Eocene of Asia. This animal is known only from a single skull with a dental formula of I2/1, C1/1, P3/3, M1/2 x 2 = 22 teeth. However, if its body proportions are similar to its close relative Patriofelis, Sarkastodon would have been an exceptionally heavily-built, powerful predator comparable to the largest felids such Smilodon populator and Xenosmilus hodsonae. Among contemporary predators, only Andrewsarchus mongoliensis was larger. 

Other oxyaenid genera include: Ambloctonus, Dipsalodon, Dipsalodictis, Dormaalodon

Glossary*
Carnivora: the mammalian order which contains dogs, cats, bears, hyenas, etc.
Dasyurid: any member of a family of small, predatory marsupials native to Australia.
Digitigrade: a type of locomotion in which only the toes touch the ground.
Metapodial: the bones which comprise the palms of our hands and the soles of our feet.
Plantigrade: a type of locomotion in which the whole foot is planted on the ground.
Sagittal crest: the ridge of bone that runs down the midline of the skull in many mammals.
Sectorial: adapted for cutting.
Zygomatic arch: commonly known as the “cheek bones”.

References & Further Reading
Chester SGB, Bloch JI, Secord R, Boyer DM (2010). “A new small-bodied species of Palaeonictis (Creodonta, Oxyaenidae) from the Paleocene-Eocene Thermal Maximum”. Papers in the Earth and Atmospheric Sciences 298: 227-243 <Full Article>

Gingerich PD (1980). “Tytthaena parrisi, oldest known oxyaenid (Mammalia, Creodonta) from the late Paleocene of western North America”. Journal of Paleontology 54(3): 570-576 <Full Article>

Gazin LC (1946). “Machaeroides eothen Matthew, the saber-tooth creodont of the Bridger Eocene”. Proceedings of the United States National Museum <Full Article>

Granger W (1938). “A giant oxyaenid from upper Eocene of Mongolia”. American Museum Novitates 969: 1-5 <Full Article>

Osborn HF (1900). “Oxyaena and Patriofelis restudied as terrestrial creodonts”. Bulletin American Museum of Natural History 13(20): 269-279 <Full Article>

Wortman JL (1899). “Restoration of Oxyaena lupina Cope, with descriptions of certain new species of Eocene creodonts”. Bulletin American Museum of Natural History 12(6): 139-149 <Full Article>

Wortman JL (1894). “Osteology of Patriofelis, a middle Eocene creodont”. Bulletin American Museum of Natural History 6(5): 129-165 <Full Article>