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The Rise and Domination of Mammals - Millions of Years Ago (MYA)

Paleocene 67 to 54 MYA -  Eocene 54 to 34 MYA - Oligocene 34 to 24 MYA

Miocene 24 to 5.3 MYA - Pliocene 5.3 to 1.8 MYA - Pleistocene 1.8 to 0.01 MYA





(Lower Eocene, Oligocene

Upper Miocene)


Sometimes called "The Hog From Hell," it is only hog-like in appearance.  Big and with a nasty disposition, Entelodont tooth marks have been found on other members of this species.  This would indicate that they even fought with each other.  This small brained carnivore/scavenger, with an unfriendly disposition, was among the top in the food chain.  The largest lasted into the Miocene.  It was called "Dinohyus" and stood almost six feet in height and had tusks rather than fangs.  Biggest of the Eocene/Oligocene fanged Entelodont was Archaeotherium Mortoni, which was the size of a large cow.

 Rhino jaws and other mammal bones have been found with bite marks on them that match the large canines of Archaeotherium mortoni. In leaner times, it is hypothesized that Archaeotherium dug for roots and tubers, as with other pig-like mammals.

Interestingly, juveniles have been found with serrations on canines, although these seem to disappear as the Entelodont matures.



Entelodont Upper Skull

Brule Formation, near Chadron Nebraska

Entelodont Front Lower Jaw, Archaeotherium Mortoni

White River Formation, Chadron Deposits, SW SD

Entelodont Juvenile Canines with Serrations

White River Formation, Douglas Wyoming

Entelodont Shoulder Vertebra

White River Formation, Douglas, Wyoming

Entelodont Snout "A" View 01

Entelodont Snout "A" View 02

Entelodont Maxilla Teeth Section

White River Group, Pennington County, SD

Entelodont Canine Old Archaeotherium

Entelodont Canines, Molars, Pre-Molars

White River Group, Brule Formation SD

Entelodont Hoof from Juvenile

White River Group, S.W. SD

Entelodont Juvenile Jaw Pieces

Juvenile-Lusk, WY...Adult-Douglas WY

Entelodont Vertebra Spinal Column

White River Group, S.W. SD

Entelodont Atlas Vertebra

20 Miles NE Lusk, WY

Entelodont Snout with full Canines

White River Group, S.W. SD

Entelodont Tusks Dinohyus (Largest of Group)

North West, Nebraska

Entelodont Rib

Bouldner Formation, Isle of Wight, UK









One of the most numerous herbivores, it's size varied widely, usually between a sheep and a cow.  They were most likely woodland and grassland browsers although they might have been omnivorous when the opportunity presented itself. The Eporeodont was about as large as a modern cow.  The Oreodont may have been a herbivore, but it was not necessarily an easy mark for the predators of the day.  Variations were either equipped with sharp canines or large, tearing tusks.  They all had claws rather than hooves. Oreodonts probably congregated in herds and like some of today's herd mammals, might have worked together for mutual protection.  They were certainly a very successful species. Existing through the Oligocene and well into the Miocene, they outlasted most of their predator adversaries.



Oreodont Skeleton in Matrix

Skeleton View 1

Skeleton View 2

Skeleton View 3

Skeleton View 4

Brule Formation, Pennington County SD

Oreodont Juvenile Skull with Bite Marks

Pennington County, SD

Oreodont Paw

White River Chadron, S.E. WY

Oreodont Pelvis

20 Miles North East Lusk, WY

Oreodont Skull, Jaws, Maxilla, Vertebra

White River Group, Chadron Lusk WY

Oreodont Skull & Vertebra Juvenile

White River Group, Chadron Lusk WY

Eporeodont Skull with Tusks

Eporeodont Skull with Canines

Oreodont Brain Case & Skull Piece

All from White River Group

Brule Formation, Pennington County, SD





(Early Oligocene)


An early American Rhino and probably the first in North America that can be considered a "true" rhino.  American Rhinos were hornless.  A herbivore somewhat larger than a cow at 8 feet in length and around 1,000 pounds, it was the largest prevalent American mammal of its day with the exception of Titanothere (Brontothere).  Fossils have been found near what were once woodland streams rather than swampy plains. 


Subhyracodon Full Skull

White River Brule, Pennington County, SD

Subhyracodon Jaw Segment

Subhyracodon Toe Bones

Subhyracodon Rear Right Jaw Section & matching Canines

All from White River Group

Chadron Formation, Pennington County, SD

Subhyracodon Left Jaw

White River Group, North West Nebraska

Subhyracodon Right Jaw

20 Miles N.E. Lusk, Wyoming




BEAR DOG (Amphicyon)

(Oligocene, Upper-Middle Miocene)


One of the Creodont group of carnivorous predators in the Oligocene.  Called "Bear Dog," it was in fact, not directly related to the bear or the dog.  The bear size Amphicyon longiramus was the largest, dominant, land predator in the mid-Miocene until slowly going extinct. Typical of early mammal predators, it was noted for a large head and small brain. The very successful Daphoenus was the size of a small female wolf or more likely, that of a coyote.  Interestingly, the most imposing sub-species (Longiramus) bore more of a skeletal resemblance to a large bear than a canid.  But size can be a hindrance in a slowly changing environment that favors nimbleness and speed.  It probably couldn't compete in conditions more favorable to more efficient predators and increasingly fleety prey.


Amphicyon Longiramus Femur, largest of species

Amphicyon Longiramus Tibia

Amphicyon Longiramus Ulna

Amphicyon Longiramus Teeth (Largest)

Daphoenus Teeth (small coyote size)

"Miller Site" Lower Suwannee River

Dixie County, FL

Amphicyon Jaw Pieces, Skull, Vert, all size species

Jaw Piece (Longiramus) - Suwannee River, FL

Cervical Vertebra (Daphoenus)

Skull and Jaw (Daphoenus) - N.W. Nebraska

Daphoenus Skull & Jaw

Upper Brule Formation, NW Nebraska

Daphoenus & Longiramus Teeth

Daphoenus (Oligocene) Longiramus (Miocene)

Daphoenus Partial Skull

White River Brule Formation, SD





(Eocene, Upper Oligocene)


One of the few large mammals of the Eocene, Coryphodon (meaning peaked tooth) is an extinct genus of mammal. It was widespread in North America between the early/mid Eocene.  Fossils have been found near Greenland, when the climate was much warmer and similar to the modern American South.

Possibly the dumbest large mammal to ever live if you use brain weight to total weight as a benchmark.  The Coryphodon weighed in at some 1,200 pounds, but its brain was only three ounces.  None the less, it survived over twelve million years, so it must have had something going for it.  We hypothesize that it was mostly vegetarian and lived in swampy areas.

Coryphodon Jaw with Teeth

White River Group, Chadron Formation

Pennington County SD

Coryphodon Teeth With Canines

San Juan Formation near Lindrith, NM

(In smaller Box) Willwood Formation WY

Coryphodon Calcaneum (Heel Bone) + Partial Toe Bone

Willwood Formation WY

Coryphodon Femur Ball Joint Proximal (upper end)

Coryphodon Molar Large with Root

Worland WY, S. Hwy 16 Washakie County

Coryphodon Pre-molars in Jaw Section

Coryphodon Tibia Leg Bone Section

Big Horn Basin, Willwood Formation, Worland WY

Coryphodon Molar in Jaw

Southeast of Worland, Banjo Flats near Rattlesnake Ridge Road, Big Horn Basin Willwood Formation Wyoming







Horridus, Cruentus, Mustelinus

(Oligocene, Upper Miocene)


Another nasty member of the Creodont family of predators, the Hyaenodon probably went head to head with Entelodont (above) in the killing fields of the Oligocene.  In later periods, Hyaenodon may have been unable to compete with developmental refinement of other predators such as canids and cats, leading to gradual extinction over millions of years.  But in the late Eocene and entire Oligocene, pure brute force was more than adequate for elevation to the top of the food chain.  Another theory is that a slowly changing environment and effects on natural selection might have favored their previously slower, smaller prey.  None the less, this impressive predator had a successful run of some fifteen million years, which is nothing to sneeze at.  Sub-species size ranged from that of a small a coyote to a large Black Bear.

Hyaenodon Horridus (Juvenile) Skull

Brule Formation near Chadron, NE

Hyaenodon Horridus Adult Skull

South West. South Dakota

Hyaenodon Horridus (Juvenile) Complete Lower Jaw

White River Brule Badlands, S.W. SD

Hyaenodon Cruentus (Adult) Complete Lower Jaw

White River Brule Badlands, S.W. SD

Hyaenodon Skull (Composite View)

Upper - Chadron NE, Lower - S.W., SD

Hyaenodon Mustelinus Jaw, Maxilla, Canine

Sioux County, Nebraska

Hyaenodon Mustelinus Lower Jaws

White River Brule Badlands, S.W. SD

Hyaenodon Horridus Canines & Molars

White River Group Brule Formation, SD

Hyaenodon Braincase & Exposed Brain Cast

White River, Pennington County, SD

Hyaenodon Cruentus Paw, Leg Bones, Others

White River, Pennington County, SD

Hyaenodon Horridus leg Bones - Femur, Tibia

White River, N.W. Nebraska

Hyaenodon Horridus Teeth Large Adult

White River Badlands, S.W. SD







Most people don't realize that the camel originated in the Americas and not Asia.  It's migration was westward, not eastward.  During this period, the Camel was barely three feet high.  Over millions of years, evolutionary natural selection favored larger, taller size on the drier, cooler, plains.  The late North American fossil record is replete with large size camels who disappeared with the arrival of Man.  This earliest of camels was barely three feet high which worked well in it's mixed forested environment of 30 million years ago.



Poebrotherium Skull (Upper)

Badlands, S.W. South Dakota

Poebrotherium Skull with Partial Jaw

20 miles N.E. Lusk, WY

Poebrotherium Upper Partial Sull & Teeth

20 Miles N.E. Lusk, WY

Poebrotherium Jaws & Partial Skull from Juvenile

White River Badlands, SD

Poebrotherium Femur with Patella (Knee Cap)

20 miles N.E. Lusk, WY

Poebrotherium Complete Leg with Toes

20 Miles N.E. Lusk Wyoming

Poebrotherium Jaws, Leg Bones, Ribs, Vertebra

20 Miles N.E. Lusk Wyoming

Poebrotherium Maxilla, Verts, Toe Bones Same Animal

White River Group, Douglas WY

Poebrotherium Jaw, Brain Cast and Partial Skull in Matrix

Pennington County SD







(Late Eocene/Oligocene)




 "Hesperocyon is an extinct genus of canid or dog family and earliest true dog found to date.  Earliest fossils go back about 40 million years. Most were just under three feet in length and weighted about 3.5 pounds, but some were rather larger.  Smaller examples looked more like a modern civet or a small raccoon than a canine. Its body and tail were long and flexible, although as with all fossils, the amount of fur on body and tail is purely speculative.  Its limbs appear to have been relatively weak and short. Still, the build of its ossicles (inner ear bones) and distribution of its teeth showed it was a canid. Although it was definitely a carnivore, it may also have been an omnivore.

The subfamily Borophaginae is an extinct group of canids in North America, surviving from roughly 35-2.5 million years.  Probably an offshoot of Hesperocyon they evolved to become considerably larger than their predecessors before extinction.  Size varied from small omnivores to powerful, bear-sized "bone crushing" carnivores.


Hesperocyon Skull with exposed Brain Cast

S.W. South Dakota

Hesperocyon Partial Skull

White River Group, Douglas, WY

Hesperocyon Teeth and Jaw segments

Hesperocyon Jaw, Right and Left Sides

(All) White River Chadron, S.W. SD

Hesperocyon Scapula (Shoulder Blade) & Vertebra

S.W. South Dakota

Borophaginae Early Jaw

White River Brule Formation, Lusk, WY

Borophaginae Canine & Atlas Vertebra

Suwannee River, Dixie County FL




DINICTIS ("False" Saber Cat)



A member of what is called "The false Cats," due to several differences in among other things, the foot and inner ear bones.  Canines were long,  but not quite saber toothed in length. 

Dinictis was about the size of a small Puma, approximately 3.5 feet in length and 2 feet in height.  As with it's relative Hoplophoneus (see below), it was in the Nimravid family. 

Reduction in number of teeth was only beginning and it still had the capacity to chew.

 We can speculate links to later cats, but the fossil record has many gaps.  This is very common between the limited fossil record of the past and what we see today.

There are many variant fossils of "false saber cats."  Some seem to contain qualities of both cats and canids.  Several subspecies have been found in Asia (China), right through the Miocene.  As with all too many Chinese fossils, you must be wary of forgeries.


Dinictis Front Leg Bones Radius & Ulna

Pennington County, South Dakota

Dinictis Skull (Frontal)

Pennington County South Dakota

Dinictis Squalidens Jaw With Teeth

White River Brule Formation, Dawes County Nebraska

Dinictis Jaw

20 Miles N.E. Lusk, WY

Dinictis Partial Pelvis

20 Miles N.E. Lusk, WY

Dinictis Teeth Incisors and Molars in jaw

Dinictis Patella (Knee Cap)

S.W. South Dakota

Dinictis Large Molar in Jaw

20 Miles N.E. Lusk, WY

Dinictis Scapula (Shoulder Blade)

N.W. Nebraska

Dinictis Femur & Tibia (Leg Bones)

N.W. Nebraska

Dinictis Vertebra from Juvenile

N.W. Nebraska

Dinictis Metacarpal distal end (Foot Bone)

N.W. Nebraska




HOPLOPHONEUS ("False" Saber Cat)



Another member of what is called "The false Cats," due to several differences in among other things, the foot bones and inner ear bones.  A saber toothed predator with large canines, about the size of a small Leopard and in the Nimravid family.  Hoplophoneus was outwardly structured more like the true but much later Pleistocene cat Smilodon, including the saber-like teeth.

There are many variant fossils of "false saber cats."  Some seem to contain qualities of both cats and canids.  Several subspecies have been found in Asia (China), right through the Miocene.  As with all too many Chinese fossils, you must be wary of forgeries.

Hoplophoneus Toe Bone, Jaw, Teeth + Fang segments

White River Group, N.W. Nebraska

Pelvis Saber Cat + Crystal Deposits in Bone

Species Undetermined, Gansu Province, China Miocene Deposits



ICTOPS (Insectavore)


ICTOPS (Insectavore)



During the dinosaur age, small insect eating mammals spent their time, consuming insects and hiding from reptiles. This is one of their descendants from the subsequent age of early mammals.  Descendants are still alive today, such as the Shrews and share many similarities.  Like other smaller animals, this group has survived several mass extinctions of larger mammals.  There have been many climate changes and mass extinctions, with most land animals over 20 pounds in weight disappearing forever.

Ictops Skull & Vertebra

Skull - Brule Formation, White River Badlands Pennington County, SD

Jaw Pieces and Vertebra - Sioux County, N.W. Nebraska









Called "Running Rhino," it is more horse-like than rhino-like.  It shared the plains with Oreodonts and others.  The early fossil record proves out few lineages (the horse being a possible exception) and we can only speculate.  It was a lightly built, pony-like mammal of about five feet in length. Hyracodon's skull was large in comparison to the rest of the body. Its teeth resembled that of later rhinoceroses, but it was a much smaller animal. It had a short, broad snout and its long, slender limbs had three digits.  Physiologically, it bore some resemblance to the very largest of all land mammals, the huge Indricotherium of the late Oligocene and early Miocene, which was 25 feet long, 17 feet tall, and weighed nearly 18 tons .

Hyracodon Complete Rear Leg with Pelvis

North West, Nebraska

Hyracodon Skull Full

Pennington County, South Dakota 

Hyracodon Skull Partial

Pennington County, South Dakota

Hyracodon Upper Teeth in Partial Skull (Maxilla)

Pennington County, South Dakota

Hyracodon Jaw in Matrix

Pennington County, South Dakota

Hyracodon Hoof & Toe Bone

N.W. Nebraska

Hyracodon Pelvis Femur Joint

20 Miles N.E. Lusk WY


CYNODICTIS ("In Between Dog")



CYNODICTIS ("In Between Dog")



A very interesting smaller but elegant predator, about the size of a female coyote.  Its name was taken because it shared physical qualities of the dog and cat.   This is not unique as can be witnessed by the modern Hyaena. Some feel Cynodictis had the ability to climb trees.  Cynodictis had a long muzzle and a low-slung body. It had carnassial scissor teeth for slicing chunks of meat off carcasses. Cynodictis probably used its speed to chase down rabbits and small rodents, but may also have been able to dig them out of their burrows. Cynodictis lived on open, semi-arid plains that were crisscrossed by rivers.


Cynodictis Skull View 01

Cynodictis Skull View 02

White River Brule Formation, Pennington County, SD













(Lower Paleocene/Mid Eocene)


Phenacodus was a long extinct sheep size mammal from the late Paleocene through middle Eocene, about 55-40 million years ago. It is one of the earliest and most primitive of the ungulate (hoofed) mammals found to date.  A multiple toe/hoofed mammal with five toes terminating with a small hoof on each.  Its substantial canines, with rear teeth more suited for eating vegetation, lead one to presume that it was omnivorous.


Paleocene fossil records are relatively scant.  It was the Epoch after the dinosaur extinction.  It allowed early mammals to begin a more natural development without encumbrance of most reptilian predators.  During this Epoch, we witness what might be called the "prototypes" of later mammals.  Of these, we have Simpsonodus Chacensis, which was a small Antelope-like adaptation and Hyopsodus, which was a smaller Weasel-like creature.  Eohippus (Hyracotherium) "Dawn Horse" appears below in the pictorial and discussion of horses.


Phenacodus Tooth and Partial Upper

Eocene Deposits, Willwood Formation, WY

Phenacodus Jaw Right Side

Eocene Deposits, Willwood Formation, WY

Phenacodus Jaw Left Side M2 & M3 Molars

Eocene Deposits, Willwood Formation Big Horn Basin near Worland WY

Phenacodus Molars

Willwood Formation Big Horn Basin, Worland WY

Simpsonodus Chacensis Jaw

W. of U.S. Hwy 20 in Washakie County, Worland WY

Hyopsodus Jaw

Southeast of Worland, Banjo Flats near Rattlesnake Ridge Road, Big Horn Basin Willwood Form. Wyoming



Hyracotherium (Eohippus or "Dawn Horse")

First Horse


HORSE (Eohippus to Pleistocene)



Hyracotherium/Eohippus through Equus

(Eocene through Pleistocene)


Horse evolution is probably the best example of slow change over tens of millions of years due to Natural Selection pressures of changing environment.  A small one foot tall forest dweller of the Eocene called Hyracotherium a.k.a. Eohippus (Dawn Horse), had four front hoofs on each limb and three in the back.  It was the size of a cat.  This was followed by the significantly larger Oligocene Mesohippus, with three toe hoofs and weighing in at just under 100 pounds.  As the forests receded over tens of millions of years and the environment dried to plains, larger and taller animals tended to survive and breed with each other.

On the other hand, there can be anomalies.  Nannippus, three toed horse of the mid-Miocene/Late Pliocene is estimated to have been about 160 pounds which is somewhat smaller than its predecessor, Merychippus.

In any event, growth was the norm, with the modern 1,100 pound Equus.  This is the graceful, fleety, single hoof modern horse, tall and quick enough to escape predators of the plains.  The multi toed legacy is a mere vestige on either side of the lower leg.

Horse identification from the mid Micene to the early Pleistocene can be a bit confusing.  The "catch all" phrase Hipparion is a genus with many species.  They had three hooves, although only the large center one touched the ground.  Adults are considered to have weighed between 200-400 pounds, but we can only estimate.  We can only identify by the fossil record which can be somewhat sketchy.  It can even be inconsistent at times.

The horse is native to America but was hunted to extinction by the arrival of the American Indian some 10,000 years ago.  It returned to this continent by the Spanish Conquistadors, who arrived on our shores some five hundred years ago, with descendants of American horses that migrated to Asia and Europe.

Mesohippus Kill Site View #1

Mesohippus Kill Site View #2

Mesohippus Kill Site View #3

Mesohippus Kill Site View #4

Mesohippus Kill Site View #5

White River Group, Pennington County, SD

Mesohippus Jaw Complete

Brule Formation, White River Group SD

Mesohippus Femur, Scapula, Atlas Vertebra, Teeth, Hooves, etc., from same animal

White River Brule Formation, SD

Mesohippus Skull (Oligocene)

Sioux County, Nebraska

Mesohippus Hoof (Oligocene)

Mesohippus Upper Jaws

White River, Brule Formation S.W., SD

Hyracotherium (Eohippus) Upper & Lower Teeth

Hyracotherium (Eohippus) Upper Teeth

Hyracotherium (Eohippus) Lower Teeth

Willwood Formation, WY

Hyracotherium (Eohippus) Jaw Section

Eocene San Jose Formation, N.W. New Mexico

Horse Teeth 40 Million Years to Today

Hyracotherium - (aka Eohippus) teeth from Willwood Formation, WY

Mesohippus - White River Chadron, S.W. SD

Nannippus - Florida phosphate mine

Parahippus - Wuwannee River Florida

Pleistocene Horse - North Central FL

Horse Vertebra Pleistocene

Northern Florida

Horse Teeth Hipperion and Others

China and Florida

Horse Hipparion Hoof & Phalanx 1

Charlotte County, SW Florida Pliocene

Horse Hipparion Nannippus Maxilla Section

South Dakota Pliocene

Horse Anchippus (Early Miocene 3 toe) Juvenile Jaw

Suwannee River, Dixie County, Florida






(Late Eocene/Oligocene)


Palaeolagus, meaning "ancient hare," is an extinct genus of lagomorph in the family Leporidae. While closely related to modern rabbits, its shorter hind legs indicate it ran more like rodents, to which it is more distantly related, than hopping like rabbits of today. Palaeolagus chewed differently than rodents due to having TWO pairs of incisors in the upper jaw as opposed to a single pair in rodents. Jaws are adapted for nibbling grass and plant material. While Palaeolagus was possibly highly prevalent in the environment of Oligocene North America, the size and fragility of their fossils make them extremely rare. Only two almost complete fossil bodies have been found to date.

Palaeolagus Skull

Palaeolagus Partial Pelvis, Vertebra & Assorted bones

Early Oligocene (Chadron) White River, SD







A deer-like ruminant of small stature, delicacy and a severely limited habitat. They seem somewhat related to the contemporary Mouse Deer which are not true deer either. If you want to picture one, think of Bambi who never grew up. They averaged around 17 pounds, somewhere between the size of a rabbit and a medium sized dog. Leptomeryx first appeared in the middle Eocene around 41 million years ago and became extinct in the early-mid Miocene. We donít know why they went extinct.

Leptomeryx Skull

Leptomeryx Foot & Leg Bones

Leptomeryx Jaw Segments

Leptomeryx Sacrum (Bone at Base of Spine)

(All) White River Badlands, Interior SD





(Early Oligocene)


This 2 foot long creature is one of the oldest rodents known.  Somewhat resembling a mouse in configuration, it's rear legs were proportionately long.  Some feel it was a tree dweller like the contemporary squirrel, but we really don't know. 

Ischyromys seems to be unique to North America and went extinct about 25 million years ago.


Ischyromys Brain Cast & Partial Skull

Pennington County, SD

Ischyromys Jaw Segment with Teeth

Pennington County, SD





TITANOTHERE (Brontothere)

(Late Eocene/Early Oligocene)


Largest of the day and looking similar to the Rhinoceros, the Titanothere (or Brontothere) is not really an early relative.  In the Badlands of South Dakota and Nebraska, the Sioux Indian tribes revered the long extinct fossils that they found.  Brontothere means "Thunder Beast."  They believed that when their spirits galloped across the sky, thunder was created.  Herbivores, the carnivorous predators of the day probably tread carefully around these giants, who stood over eight feet at the shoulders.  Picture an ancient animal looking like a giant Rhino, as large as a female Elephant, but related to neither.  Brontotheres had four toes on their front feet and three in the back.  Their teeth were adapted to soft vegetation.  This probably contributed to their eventual extinction in the increasingly drier conditions of the Oligocene, with the resulting tougher vegetation.

Titanothere Brontothere Atlas Vertebra

Pennington County, SD

Titanothere Brontothere Axis Vertebra

Pennington County, SD

Titanothere Brontothere Atlas & Axis Vertebra

Pennington County, SD

Titanothere Brontothere Juvenile Leg Bone With

Predator Predation Marks

White River Badlands, S.W. South Dakota

Titanothere Brontothere Rear Jaw Section with Tooth

White River Badlands, S.W. SD

Titanothere Brontothere Upper & Lower Molars

White River Badlands, S.W. SD

Titanothere Brontothere Molar in Jaw

White River Badlands, S.W. SD

Titanothere Brontothere Molar with Roots

White River Badlands, S.W. SD

Titanothere Brontothere Teeth

White River Badlands, S.W. SD

Titanothere Brontothere  Patella (Knee Caps)

Pennington County, SD

Titanothere Vertebra

Titanothere Brontothere Teeth Including Incisors & Foot Bones

Titanothere Brontothere Complete Radius Leg Bone in Matrix

Titanothere Brontothere Radius Leg Bone

Titanothere Brontothere Toe Bones

All from White River Badlands, S.W. SD

Titanothere Brontothere Horn Top

20 Miles NE Lusk, Wyoming

Titanothere Brontothere Scapula "Shoulder Blade" Distal (Lower) End

White River Badlands, S.W. SD

Titanothere Brontothere Pelvis Hip Socket

White River Badlands, S.W. SD








Megalodon means "big tooth."  It is obviously not a mammal, although it did eat many, including the large Sperm whale.  We've included Megalodon in this section because it fits into the time line.

Megalodon was a giant shark that lived between 18 million to 1.5 million years ago and was the premier marine predator of its time.  This magnificent terror was over fifty feet in length.  When fully opened, its mouth was so large that a small automobile could drive through.  Weight of Megalodon is estimated to be around 47 metric tons with a bite force some 10 times greater than the Great White.

The surviving Great White shark of then and today, was dwarfed by this massive creature, who probably became extinct due to changing near-shore hunting grounds.  Megalodon was powerful but relatively clumsy, again showing that as Darwin said, size and strength is not the primary determining factor in long term species survival. It's the ability to adapt to change.  That the Great White survived and Megalodon went extinct, is testament to natural selection through adaptation. 

Megalodon Teeth Compared to Great White

South Caroline Coast

Megalodon Tooth & Large Sperm Whale Vert.

South Carolina Coast

Megalodon Tooth & Small Sperm Whale Vert.

South Carolina Coast.



Ictitherium Viverrinum




Adcrocuta eximia



Ictitherium Viverrinum

& Adcrocuta eximia

(Mid-Late Miocene)


Fifteen million years ago, dog-like hyenas flourished, with 30 different species being identified. They were not canids although they did appear canid-like. Unlike some of their modern descendants, these hyenas were not specialized bone-crushers, but were more nimble, Jackal-like  animals, only larger. The dog-like hyenas had canid-like molars, allowing them to supplement their carnivorous diet with vegetation and invertebrates.  The best known of the Jackal-like hyenas was Ictitherium Viverrinum although others were very similar.  They lived between 11 million and 6 million years ago throughout Europe and Asia. Five to seven million years ago (Late Miocene), most dog-like hyenas were out-competed by true canids traveling from North America to Eurasia via the Bering land bridge. The first bone-cracking hyena, Adcrocuta eximia, does not appear in the fossil record until the late Miocene. The skull of Adcrocuta bears a close resemblance to that of modern spotted, brown and striped hyenas. However, Adcrocuta had a very stocky build, with short, robust limbs and fuller, shorter snout skull.

Ictitherium Viverrinum Hyena (or similar) Skull

Guanhe Beds, Gansu Province China

Adcrocuta eximia Hyena Jaw

Hipparion Fauna Gansu Province  China

Adcrocuta eximia Hyena Front Skull

Hipparion Fauna Gansu Province China






(Mid Miocene/Early Pliocene)


First identified as being in the the Mid-Miocene, this rather odd member of the rhino family thrived until the very early stage of the Pliocene. At first glance, it had a rhino-like head with a hippopotamus-like body. Its body was long and stout, with short, stumpy legs.  Males had a single, small cone-like nasal horn.  Over all physical qualities would indicate a semi-aquatic life, with teeth suggesting a grazing diet.  Carbon isotope analysis of teeth seem to support this theory.  Teleoceras was about 11 feet in length and weighted some 2.5 tons.


Teleoceras Adult Tooth and Baby Teeth

Bone Valley, Polk County Florida

Teleoceras Tooth

Ogalala Formation, Optima OK

Teleoceras Caudal (Rear) Vertebra

Dixie County, Florida






Chilotherium is a Rhinoceros that lived in China and southern Eurasia.  Estimates are that the largest subspecies was almost five feet in height and weighed almost 2 tons.  It's body was massive but its legs were short.  Chilotherium was hornless, but its lower canines were large and curved upwardly into what might be called tusks.

Chilotherium Jaw Segment with Tooth

Gansu Province, China

Chilotherium Incisor "Tusk" Juvenile

Hipparion Beds, Gansu Province, China






(Very Late Oligocene/Late Miocene)


An extremely rare, large, exotic, aquatic mammal that roamed the coastal areas of Asia and Western North America, these beings have what might be the oddest teeth ever seen.  It was between 6-6.5 feet in length and weight is estimated at between 450-500 pounds.  In the United States, most fossils are found around Fresno, California in matrix.  Their demise after millions of years was most likely caused by slowly changing coastlines and available specialized food supply.

In today's world, Fresno is in the middle of California and no longer coastline.  This is another result of natural climate change that we can do little to alter.  Some have argued it may have fed on seaweed during low tide. However, recent isotope work indicates that Demostylus more likely lived in freshwater or estuary ecosystems and ate aquatic freshwater plants.  Only two reasonably in tact skeletons have been discovered in the United States and Japan. 

Sporadic remains have been found along the northern Pacific Rim from Baja, Mexico northward along the coast of California, Oregon, Washington and west to Sakhalin Island, Hokkaido, Japan, and south to the Shimane Prefecture, Japan. Despite some similarities to manatees and elephants, desmostylians were entirely unlike any other living creatures. Perhaps the most prized fossil is the complete tusk, the size of which receded in later fossil remains.  These deeply rooted tusks are owned by a handful of museums and few collectors.


Desmostylus Teeth in Aquatic Matrix

Temblor Formation, Fresno County, CA

Desmostylus Front Lower Tusk Complete

Temblor Formation North of Coalinga CA







(Middle Miocene/Pliocene)


Speculated to be an early relative of later elephantine's and modern elephant or possibly a progenitor, Gomphotherium looked superficially similar to Platybelodon, but had two more distinctly separate lower tusks.  Both lived during the Miocene Epoch, about 15-5 million years ago and ranged over Africa, Europe, Asia and North America. Although they thrived during this time, a viable fossil record doesn't exist much past the Miocene.  Gophotherium was somewhat over 8 feet in height and weighed between 4-5 tons.  Platybelodon was significantly smaller, weighing in at approximately 2 short (USA) tons.

Typical of how theories change with the gaining of additional information, Platybelodon was previously believed to have used its lower teeth to "shovel up" semi-aquatic vegetation in swampy areas.  However, wear patterns on the teeth subsequently suggest that it used its lower, flat tusks to strip bark from trees, and may have used the sharp incisors that formed the edge of the "shovel" more like a modern-day scythe.  It is hypothesized to have grasped branches with it's trunk and use its lower incisors as cutters.


Platybelodon Teeth from Juvenile to Adult

Linxia Basin - Gansu, Northwestern China

Gomphotherium Radius from very Young Animal

Aucilla River, Taylor County Florida

Gomphotherium Sternum Section

Lee County, Florida

Gomphotherium Molar

Bone Valley Formation, Polk County Florida

Gomphotherium Tooth Young Juvenile

River/Spring Taylor County, Florida

Gomphotherium Molar Partial

Beaufort River, South Carolina

Gomphotherium or Mastodon Baby Lower Tusk #1

Suwannee River, Florida

Gomphotherium or Mastodon Baby Lower Tusk #2

Suwannee River, Florida

Gomphotherium (Juvenile) Tusk Tip & Tooth

Lee County, Florida











(Eocene & Early Oligocene)


Leptictidium or "Graceful Weasel" is one of the few mammals who ever walked or hopped on two legs.  Whether it walked or hopped is open to speculation.  Perhaps it did both.  The closest resemblance we have is the unrelated Elephant Shrew.  Based on physiology, we can conclude that it was omnivorous although most of it's diet was probably insects, small reptiles or whatever small mammals it was capable of catching and killing.  The ability to hop would also imply the ability to leap, which meant that it would probably jump up to catch low flying insects.

It is believed that they split off from the smaller Eutherians of the Dinosaur Era based on similar primitive aspects of bone structure.

Size including its lengthy tail was somewhere between 2-3 feet.  But environments change and the moist, warm, wooded climate of the Eocene gradually transformed into the dry, colder, open plains of the Oligocene.  Perhaps they were not able to adapt because their prey disappeared.  Possibly, they became easy prey to open plains predators.  In any event, they became extinct some 35 million years ago and have no descendants.



Leptictidium Brain Cast

Eocene Deposits South West South Dakota

Leptictidium Ribs & Bones

Eocene Deposits South West South Dakota










Perhaps the most insecure of mammals, these creatures survived the Dinosaur Era, paving the way for the rise of mammals after the reptilian demise.  They were probably omnivores, consuming whatever sustenance they could find.  Some speculate that when opportunity arose, they would raid reptilian nests for eggs.

Mostly small in size, little exists other than incomplete skeletal impressions and structures, skull fragments, jaws and numerous teeth.  None the less, it was obvious that they were plentiful, if rarely seen during daylight hours, when the Dinosaurs ruled the earth.  


Eutherians are thought to be Placental mammals rather than Marsupials.  Metatherians are thought to be Marsupials.  Multituberculates varied in structure, from mouse size to significantly larger.  Rhodent-like, they existed in the Jurassic, survived the Dinosaur extinction and even to the early Oligocene.  It is thought that in the evolutionary development, they were eventually outcompeted by the rodents and thus became extinct.


It is speculated that some early mammals in areas of the world with limited Dinosaurs were significantly larger, but evidence is sketchy.

Eutherian, Metatherian & Mulituberculate Mammal Teeth

Hell Creek Formation, Upper Cretaceous

Garfield County Montana

Claws and Hand Bones Almost Complete

Hell Creek Formation, Upper Cretaceous

Harding County, SD

Claws and Hand Bones


Hell Creek Formation, Upper Cretaceous

Harding County, SD





(Cretaceous 65-70 Million years ago)


Perhaps the most well known small mammal, nervously co-existing with the giant reptiles during the late dinosaur period.  Made popular in the BBC movie "Walking with Dinosaurs," the title  "hiding from dinosaurs" might be more appropriate.  The largest were Opossum size and based on their teeth, all are presumed to  be carnivorous.  Some like to think of it as being raider of dinosaur nests.  This is pure conjecture, but it's nice to think of mammals fighting back, even in a rather covert way.  Little more than teeth, some jaws and skull fragments have been found, which means that the possible shape is based on existing animals with similar characteristics, combined with the imagination of BBC artists.  It was probably nocturnal, hiding out from reptilian carnivores during the day.  It's teeth have similarities to the Sea Otter, leading some to speculate that it may have been somewhat semi-aquatic, but nobody really knows. 

Didelphodon Two Canines, One Double Didelphodon Rooted Molar

Powder River County, MT

Didelphodon Jaw & unrelated Tooth

Hell Creek Formation, MT

Didelphodon Full Rooted Incisor in Matrix

Hell Creek Formation, MT






(Triassic 200 Million years ago)


These small mammals co-existed in the world of dinosaurs,  albeit rather tenuously.  Their survival is a testament to what would mature into the larger dominant life forms, some one hundred fifty million years later. This mini-mammal lived during the Upper Triassic. It first appeared in the fossil record about 205 million years ago. Unlike many other very early mammals, Morganucodon is relatively well represented and preserved though skeletal fossil segments.  Most of this comes from Glamorgan in Wales UK (Morganucodon watsoni), but similar fossils have also been found in the Yunnan Province in China and in various parts of Europe and North America.  It is estimated to have weighed between 1 to 2 ounces with a body somewhat bigger than a large paper clip.


Morganucodon Molar the Size of a Grain of Sand

County of Glamorgan, Wales UK


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