Author: Ted Stankowich

Superorder Xenarthra

Post author By Ted Stankowich
Post date September 21, 2021

Brown-throated sloth (Bradypus variegatus) facing camera, partially occluded by green leaves — Brown-throated sloth (Bradypus variegatus)

Superorder Xenarthra includes the armadillos, sloths, and anteaters, which have a strange set of shared characteristics. Their relationship with Afrotheria and Boreoeutheria has long been debated but recent studies suggest they are sister to Afrotheria, diverging about 84 million years ago. Members of Xenarthra live primarily in South and Central America and underwent a great diversification there, resulting in the three very different orders we know today.

Jump down to:

Phylogeny
Order Cingulata
Order Pilosa

General Characters

The main diagnostic character of xenarthrans are two additional zygapophyses on the lateral surfaces of their lumbar and thoracic vertebrae. These xenarthrous articulations (“xenarthrales”) strengthen the spine and joints, but have been lost in extant tree sloths to increase flexibility (n.b., fossil ground sloths do have these articulations). The dorsal zygapophyses of xenarthrans are the typical mammalian form.

Xenarthrans are also unique in dentition, entirely lacking incisors and canine teeth. They either lack teeth entirely (anteaters) or have homodont cheekteeth lacking enamel. The lack of enamel exposes dentine, which is more prone to wear, creating flat grinding surfaces for processing insects (armadillos) or leaves (sloths).

Finally, xenarthrans have the lowest metabolic rates of all the therians. This makes most species slow moving and low-resource scrublands of South America could support larger numbers of big-bodied xenarthran species.

Evolution

For many years, the xenarthran taxa were grouped with other anteating mammals (pangolins and aardvarks) into a group called “Edentata”, meaning “toothless”. Now that we know that pangolins belong in Laurasiatheria and aardvarks belong in Afrotheria, the older name Xenarthra is now used to describe the current suborder. Xenarthrans evolved in South America 84 million years ago, becoming very diverse. The first fossils date to the Miocene, 59 million years ago, and only much later did they cross into Central and North America during the Great American Biotic Interchange (commonly abbreviated as GABI) about 2.7 million years ago.

The superorder consists of 6 families in 2 orders, but Order Pilosa is also divided into two suborders: Folivora (sloths: Choloepodidae and Bradypodidae) and Vermilingua (anteaters: Myrmecophagidae and Cyclopedidae). For a more detailed overview of this suborder and its evolutionary history, check out this video.

Phylogenetic tree of all extant Xenarthran Orders and Families, following Upham *et al.* 2019

Order Descriptions

Click on individual families to expand. You can also view a curated collection of 3D models of various xenarthrans on SketchFab. You may also download a spreadsheet of basic cranial and dental features of all of the xenarthran families:

XenarthraTraits Download

Order Cingulata

There are two families of living armadillos, which are very similar in morphology and only recently were split into separate taxa based on molecular evidence. They all have and armored carapace covering their head, body, and tail made of bony osteoderms. The osteoderms are derived from the dermis and the carapace is somewhat pliable. This armor protects them from predators when they burrow into the ground (leaving only the armor exposed) or when they curl up to protect their vulnerable ventrum (although only one species can completely roll into a ball). The carapace consists of a head plate; scapular, dorsal, and pelvic shields, and an armor wrapped tail. The dorsal region often consists of moveable bands that give the body flexibility. They have short powerful legs with strong claws to dig while foraging and a long, protrusible tongue to capture invertebrate prey.

Three-banded armadillo (Tolypeutes tricinctus)

Armadillos have no incisors or canines, and their homodont cheekteeth vary in number from 7-8 per quadrant to up to 100 total teeth in the giant armadillo (Priodontes maximus), more than two times as many as any other living land mammal. Their teeth are open rooted and continuously growing throughout life, they lack enamel and are cylindrical in shape and covered in dentin. The occlusal surfaces may be worn flat or beveled to form a chisel-like blade.

There are two extinct families of cingulates, the pampatheres and glyptodonts. Pamapatheres were very large with giant domed shells, hexagonal osteoderms, and three bands across the back to give them a bit of flexibility. They evolved larger body sizes over time, up to Holmesina, which lived in the Pleistocene of North America and was the size of a small car, weighed over 200kg, and was up to 2m long. They were too large to subsist on an insectivorous diet, so they were primarily grassland grazers. Glyptodonts also had large domed carapaces but were not flexible, and some had spiked club-like tails for battling opponents. Glyptdonts evolved in the early Miocene in South America. They too were grazers and could reach even larger sizes than pampatheres (Glyptodon: 2000kg and more than 3.3m long). Both families went extinct by about 10K years ago at the end of the last Ice Age.

Left: Pampathere (*Holmesina occidentalis*) and Right: Glyptodont (*Glyptodon clavipes*) fossils.
By Laslovarga, CC BY-SA 3.0, via Wikimedia Commons

Family Dasypodidae

Family Dasypodidae

Long-nosed armadillos
1 genus, 8 species
Nearctic, Panamanian, Neotropical Zones

Smallish body covered in armored carapace
Carapace more rounded at the ventral edges than chlamyphorids, banding more distinct
Tibia/fibula fused with expanded muscle insertions
Elongate skull, slightly more than chlamyphorids
Zygomatic arch complete
Teeth 0.0.[7-18]/0.0.[7-19]
Homodont cheek teeth

Long-nosed armadillos are small- to medium-sized, with a long snout and peglike teeth. They live in savannas, forests, and riparian areas in North and South America. The nine-banded armadillo (Dasypus novemcinctus) has the widest geographical range of all armadillos, from northern South America to the southeast United States, and is the only Xenarthran found in North America. They are somewhat less morphologically diverse than chlamyphorid armadillos with all species belonging to a single genus (Dasypus). The spines on the scapula, large processes on the humerus, and long olecranon process of the ulna all serve as attachment sites for muscles used in digging.

Nine-banded armadillo (*Dasypus novemcinctus*)
By Hans Stieglitz, CC BY-SA 3.0 , via Wikimedia Commons

Family Chlamyphoridae

Family Chlamyphoridae

Chlamyphorid armadillos
8 genera, 14 species
Panamanian, Neotropical Zones

Small to large bodies covered in armored carapace
Carapace flatter and less rounded at ventral edges with less distinct central banding than dasypodids
Tibia/fibula fused with expanded muscle insertions
Elongate skull, but more broad posteriorly than dasypodids
Zygomatic arch complete
Teeth 0.0.[7-50]/0.0.[7-50]
Homodont cheek teeth

Chlamyphorid armadillos are small- to large-sized, with a shorter snout than dasypodids and peglike or chisel-like teeth. They live in savannas, forests, pampas and arid deserts ranging from the humid tropics to the cold temperate zones in Central and South America. They can be insectivorous, omnivorous, or even carnivorous at times. They are much more morphologically diverse than dasypodids. The pink fairy armadillos (Chlamyphorous) are very small, more elongate in body shape, and their armor only covers their dorsum like a cape, with the lateral surfaces of the body being more exposed. On the other hand, the giant armadillos (Priodontes maximus) are large tropical species weight up to 45kg with a full coat of thick armor.

Three-banded armadillo (*Tolypeutes tricinctus*) rolled into a ball

Order Pilosa

Suborder Vermilingua

The anteaters have several characteristics that distinguish them from other xenarthrans. All species lack teeth, have a very long protrusible tongue (vermilingua = “wormlike”) for capturing prey, and have curved, elongated skulls with incomplete zygomatic arches. These traits allow them to easily extract termites and ants from nests after excavating them with large claws and powerful forelimbs.

The oldest anteater fossil (Protamandua) dates to the early Miocene in Argentina and shows the elongated skull and lack of teeth found in extant anteaters, but not the curved rostrum. The fossil record of anteaters is very poor and not much is known about their history.

Family Myrmecophagidae

Family Myrmecophagidae

Anteaters, tamanduas
2 genera, 3 species
Panamanian, Neotropical Zones

Mid-sized to large bodies with long tubular snout
Small eyes and large, strong claws
Larger in size than Cyclopedidae
Elongated downward sloping skull
Incomplete or absent zygomatic arch
Edentate (toothless)

Anteaters and tamanduas have lost all teeth, have an acute sense of smell, a long protrusible tongue attached to the sternum, a small mouth, large salivary glands to produce sticky saliva, and sharp powerful claws for tearing open termite and ant nests. Giant anteaters are very large (up to 39kg) with long, coarse hair, very large foreclaws, and powerful forelimbs to dig up ant and termite mounds, but also capable of killing a jaguar in defense. Their tongue can extend and retract up to 160 times per second and they can eat up to 35,000 ants per day. They range from Mexico down to Argentina, living in forests, savannas, steppes, and grasslands. Giant anteaters are more terrestrial while tamanduas are smaller and more scansorial with prehensile tails.

Family Cyclopedidae

Family Cyclopedidae

Silky Anteaters
1 genus, 7 species
Panamanian, Neotropical Zones

Mid-sized bodies with light silky fur
Smaller in size than other anteaters
Large eyes, large claws
Elongated downward sloping skull, but less so than other anteaters
Incomplete or absent zygomatic arch
Edentate (toothless)

Silky anteaters are arboreal, living in forests from Brazil to Mexico. They are much smaller than myrmecophagids (up to 400g), specializing on ants but also eating some termites and beetles. They have long tails with a partially hairless tip for climbing through trees. They are easily distinguished by their smaller size and light silky fur.

Silky anteater (Cyclopes didactylus) taxidermic mount — Silky anteater (*Cyclopes didactylus*) taxidermic mount

Suborder Folivora

Sloths, unlike other xenarthrans, have short, squarish rostra and very short tails. All extant species are tree sloths that move slowly through the canopy, using suspensorial locomotion with longer forelimbs than hindlimbs. They are folivores (i.e., leaf-eaters), climb down out of the trees to defecate, are surprisingly good swimmers, and have coarse fur that provides a home for green algae that helps to camouflage them in the canopy. While they only eat leaves, they have a very low metabolism and low body temperature, which reduces their energetic requirements.

In contrast to anteaters, sloths have a deep and hugely diverse fossil record. Ground sloths are not that closely related to the extant taxa. They were large in size with powerful forelimbs and claws that were probably used to pull tree limbs down so they could strip leaves off with their long tongues. There are four families of grounds sloths ranging from South America well up into the southern portions of the United States. Paramylodon (image below) stood nearly 4m tall and weighed around 1000kg; Thalassocnus was fully aquatic, showing dense bones, claws for clinging to rocks, and teeth adapted for eating sea grass and algae; Megatherium (image below) was an elephant-sized ground sloth standing 6m tall and weight up to 4000kg. All ground sloths were extinct by the end of the last Ice Age 10,000 years ago due to overhunting by humans and a rapidly changing climate.

Harlan’s ground sloth (*Paramylodon harlani*) from Rancho La Brea in Los Angeles, CA

Giant ground sloth (*Megatherium americanum*)

Family Choloepodidae

Family Choloepodidae

Two-toed sloths
1 genus, 2 species
Panamanian, Neotropical Zones

Mid-sized bodies with 2 digits on forefeet
Long, curved claws for suspensorial locomotion
Long coarse fur
Naked, fleshy nose
Incomplete zygomatic arch
Forked jugal
Shallow dentary with small angular process
Teeth 0.0.[5]/0.0.[4-5]
Caniniform premolar
Peg-like cheekteeth with deep basin and sharp ringed edge

Two-toed sloths have caniniform teeth that occlude on the anterior face of the lower teeth, allowing the teeth to self-sharpen as they wear down. The cheekteeth lack enamel, with a core of dentin and outer layer of harder orthodentin. As these wear down, they produce occlusal surfaces with a raised perimeter and deep basin in the center.
These sloths weigh 4-9kg, are larger than bradypodid sloths, and are mainly folivorous with only two clawed digits on the syndactylous forefeet (all sloths have three claws on the hindfeet). They live in tropical and montane forests.

Hoffmann’s two-toed sloth (*Choleopus hoffmanni*)

Hoffmann’s two-toed sloth (*Choleopus hoffmanni*) skeleton

Family Bradypodidae

Family Bradypodidae

Three-toed sloths
1 genus, 5 species
Panamanian, Neotropical Zones

Mid-sized bodies with 3 digits on forefeet
Long, curved claws for suspensorial locomotion
Long, coarse fur
Naked, fleshy nose
Incomplete zygomatic arch
Forked jugal
Deep dentary with enlarged angular process
Teeth 0.0.[5]/0.0.[4-5]
No caniniform premolar
Peg-like cheekteeth with deep basin and sharp ringed edge

Three-toed sloths have no caniniform teeth, a mandibular condyle that is taller than the toothrow, and a deep dentary, typical of herbivores. The anterior-most cheekteeth are more cone shaped than the caniniform teeth of megalonychids sloths. Though, like two-toed sloths, the cheekteeth lack enamel, with a core of dentin and outer layer of harder orthodentin. As these wear down, they produce occlusal surfaces with a raised perimeter and deep basin in the center. Their bodies are smaller than two-toed sloths (up to 6kg) with more limited diets (mainly leaves but occasionally twigs). They have three claws on their syndactylous forefeet with forelimbs much longer than their hindlimbs and live in tropical forests with dense canopies.

Brown-throated sloth (Bradypus variegatus) clinging to tree trunk — Brown-throated sloth (*Bradypus variegatus*)

Male Bradypus have bright orange, black, and white patches of fur (speculum) on their backs that may serve as a signal of quality to females. The color patch may be linked to testosterone and larger, stronger males have brighter patches.

Orange, black, and white fur of the speculum of a male three-toed sloth (Bradypus tridactylus) on a taxidermic mount. — Speculum of a male three-toed sloth (*Bradypus tridactylus*)

Specimen Images

California State University Long Beach (CSULB): 1587, 7139
Los Angeles County Museum of Natural History (LACM): 24827, 25990, 27345, 54547, 90859
Royal Ontario Museum (ROM): 32245, 46260

References

Feldhammer, G.A., Merritt, J.F., Krajewski, C., Rachlow, J.L., Stewart, K.M. 2020. Mammalogy: Adaptation, Diversity, Ecology. Johns Hopkins University Press, Baltimore.
Kelt, D.A. & Patton, J.L. 2020. A Manual of the Mammalia. University of Chicago Press, Chicago.
Lawlor, T.E. 1979. Handbook to the Orders and Families of Living Mammals. Mad River Press Inc., Eureka.
Nowak, R.M. 2018. Walker’s Mammals of the World. Monotremes, Marsupials, Afrotherians, Xenarthrans, and Sundatherians. Johns Hopkins University Press, Baltimore.
Prothero, D.R. 2017. The Princeton Field Guide to Prehistoric Mammals. Princeton University Press, Princeton.
Ungar, P.S. 2010. Mammal Teeth. Johns Hopkins University Press, Baltimore.
Upham N.S., Esselstyn, J.A., Jetz, W. 2019. Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation. PLoS Biol 17(12): e3000494. Reprint
Wilson, D.E. & Mittermeier, R.A. eds. 2018. Handbook of Mammals of the World. Vol. 2. Insectivores, Sloths and Colugos. Lynx Edicons, Barcelona.

Uncategorized

Introduction to Mammalogy Lab

Post author By Ted Stankowich
Post date August 19, 2021

A meerkat looking up into the sky to the right

Most mammalogy labs focus on morphology, diversity, and evolution, using skulls, skeletons, and study skins to demonstrate key concepts and teach differences between taxa. In this module you will learn some basics about museum collections and specimens, how to read dental formulae, the zoogeographic regions of earth, and how to identify specimens using a dichotomous key.

Jump down to:

Museum Tag Information
Zoogeographic Zones
Dental Formula
Dichotomous Keys

Museum Collections and Specimens

One of the primary ways scientists study the morphology and diversity of animals is using remains of individual animals that were collected from wild populations and preserved in different ways for long term study and storage in museum collections. There are several ways to preserve an animal, but the two most common are (1) storing the original full body (or parts thereof) in fluid in jars or buckets and (2) skinning the animal, preserving the skin, and cleaning the bones in a dry state.

Fluid Preservation

For fluid preparations, the specimen is first submerged in a fixing solution (formaldehyde and water) to halt the deterioration and decay process. Then it is put into a tightly sealed glass jar or other container filled with a fluid preservative, most commonly ethanol or isopropyl alcohol, for long-term storage. An identifying specimen tag (see below) is also inserted into the jar. Fluid preservations are most commonly used with aquatic invertebrates, fish, reptiles, and amphibians, and these specimens may last hundreds of years in this state, although they may shrink, swell, or discolor over time.

Jarred California pocket mouse (*Chaetodipus californicus*).

Study Skins, Skulls, & Skeletons

The most common way that mammals are preserved, however, is in a dry state. Before skinning, the preparer will write out one or more museum identification tags (see below) and take a variety of measurements of the whole carcass (head-body length, tail length, foot length, ear length, and body mass). Specimens are first skinned using scissors, a razor blade, or knife. The preparer may slice the skin down the ventral surface of the animal, pull the skin away from the muscular trunk, and then separate the skin from the legs, head, and tail. Usually, if the mammal is small, the bones are left in the paws; otherwise the paws are skinned entirely leaving just the claws, hooves, or nails in the skin. A preparer may also leave the feet and claws attached to the skeleton on one side, and attached to the skin on the other side. The tail bones may be extracted by pulling them out of the skin in one smooth movement, or by slicing the tail along its ventral surface and peeling the skin off. In the end, the preparer is left with a loose skin with no bones in it and a skeleton with muscles attached. Watch this video of the process of measuring and skinning a rodent.

The skull and bones may then be cleaned using a variety of processes. First, most of the muscles and organs should be cut away from the body and discarded so that the next step goes more quickly. The most common cleaning method is using flesh-eating dermestid beetles. A large colony of beetles in a dedicated tank can completely eat the flesh off of even large skeletons in a matter of days. Alternative methods of removing flesh are boiling, which requires care in order to not damage the bones, and maceration, which involves submerging bones in a container of water for weeks at a time to let bacteria gradually eat away and remove the meat from the bones.

When all of the meat is off of the bones, many preparators prefer to degrease them to remove natural oils and fats still remaining inside the bone (these make the bones appear brown and feel sticky). Degreasing is most commonly accomplished by soaking the bones for several weeks in ammonia or acetone, changing the solution regularly. Once the degreasing is complete, the bones are either left with their natural coloration or may be whitened for a more attractive/clean specimen. Whitening simply involves soaking the bones in diluted hydrogen peroxide for hours or a few days, depending on the specimen.

When the bones are cleaned and whitened, collections managers will assign a collection catalog number to the specimen and write that number with permanent ink on every single bone. The bones, if left loose, are typically housed in an appropriately sized cardboard box, and the skull may go in the same box or be stored separately with its own museum identification tag (see below).

With the loose skin, the preparer usually creates a museum study skin if the animal is medium or small in size or tans the hide if it is a large mammal. Creating a study skin is not the same thing as taxidermy. To make a study skin, the preparer will stuff the animal with rolled cotton (or some other soft absorbent material) and slide wires and/or wooden dowels into the legs, tail, and head so that the animal is posed with its legs outstretched in front and behind it, footpads down, and the tail straight behind (unless it is very long in which case it is curled back on itself). The skin is then sewn up with a needle and thread, pinned in position on a surface, and left to dry for days or weeks. A museum identification tag (see below) is then attached to the preserved specimen as it dries. These study skins are stored in large flat drawers in cabinets of museums, so their limb positions allow skins to be lined up in tight, compact formations for more efficient storage. Watch this video of the process of stuffing and pinning the rodent skin.

Specimen Identification Tags

All museum specimens are assigned an official specimen catalog number; the number is written directly on skulls and bones, but study skins (and sometimes skulls) have a paper tag attached to the specimen (as you saw in the videos above). This number identifies the specimen in the collection’s catalog. The specimen tag also has a variety of other important identifying information on it that will be useful to you. Look at the tag of a specimen. On one side of the tag is the species name written in pencil; writing in pencil allows us to change the name when taxonomists assign new nomenclature to a species or split it in two. The rest of the tag is written in ink, as the information is permanent. The catalog number may appear in different locations on the tag, depending on who fills it out. The catalog number is either in the upper right corner on the side with the species name or sideways next to the hole on the tag.

Front of a museum tag with the species name, *Phenacomys intermedius*, written in pencil and the collection catalog number, 11065, written in ink. The museum name is often printed on the tag as well.

On the reverse side of the tag, you will find information on the location and date the animal was collected or trapped, who collected it, who prepared the specimen, the sex of the animal, the catalog number, and possibly a series of measurements. These measurements, when complete indicate:
Total Length – Tail Length – Hind Foot Length – Ear Length ≡ Body Mass

Reverse side of a specimen tag indicating the catalog number (11065), name of the collector (Todd Lawton), location of collection (Kananaskis Valley in Alberta), date of collection (20 May 1980), and a series of measurements. The sex of this animal is not indicated.

These measurements are taken before the carcass is skinned, as the skin tends to stretch out as it is removed, which would distort any future measurements. As a result, the body length of the prepared study skin is usually a bit longer than the original carcass measurement. Using a specimen in front of you, use a ruler to confirm the measurements are close to what is written on the tag. Also, see how the catalog number on the skull matches the one on the skin; that way you know that skull belongs to that specific skin.

As you go through each lab, refer to the specimen tag to determine which species you are looking at, and compare catalog numbers on skulls and study skins to figure out which skull matches belongs to each study skin.

Zoogeography

The world can be divided up into zoogeographic regions (proposed by Wallace 1876 and recently revised by Holt et al. 2013). A species is endemic to an area of it is native to a given geographic region and not found anywhere else. As you learn about different mammal taxa, you will learn what zoogeographic region each group is endemic to. Often times, species from very different taxonomic groups look and behave very similarly but live different zoogeographic regions. Further, the distribution of species can sometimes be explained by historical zoogeography (the study of originations, extinctions, and migrations over time) where plate tectonics and movement of the continents demonstrates why some areas might be very close to each other geographically but have very different species composition because their plates were once quite far apart (e.g., Oriental and Australian zones). Below is the revised zoogeographic zone map of Holt et al. 2013. The zone names used in subsequent taxon diversity modules use the nomenclature below.

Zoogeographic zones proposed by Holt et al 2013
Redrawn from **Ben G. Holt et al. Science 2013;339:74-78**

Dental Formula

A dental formula is a system for recording the numbers of incisors, canines, premolars, and molars that an animal has. Because most mammals have the same numbers of teeth on the right and the left, a dental formula refers to only one side of the mouth at a time. In mammalogy, we use dental formulas to help distinguish different taxa, as different groups have varying numbers of each type of tooth. Sometimes simply counting the number of incisors or cheekteeth (premolars + molars) can help you identify a skull!

Dental formulas can be written in a variety of formats but they all convey the same information: the number of each type of tooth on one side of the mouth on the top and on the bottom. An opossum (Didelphis virginiana) is a relatively primitive mammal with close to the maximum number of teeth found in mammals. An opossum has, on one upper side 5 incisors, 1 canine, 3 premolars, and 4 molars, and on one lower side it has 4 incisors, 1 canine, 3 premolars, and 4 molars. The dental formula as: 5.1.3.4/4.1.3.4 or:

Use an opossum skull and/or the image below to identify each tooth type by the numbers in the formula.

The primitive dental formula exemplified by the opossum has been modified in other mammal lineages. Further, the number of certain types of teeth will vary between individuals of a species or between species within a family, and we may write that range of numbers into the formula using parentheses. The dental formula for a mountain lion (Puma concolor) is 3.1.(2-3).1/3.1.2.1. Examine a mountain lion skull and/or the images below to find the different types of teeth. Watch out! That last upper molar on the mountain lion is tiny!

In some groups, it may not be clear if certain teeth in the skull are premolars are molars, and we may combine the two tooth types in a dental formula (e.g., 1.0.(2-3)/1.0.(2-3)). Sometimes teeth can be so modified that it is difficult to determine dental formula. For mammals that have more homodont dentition like porpoises, sloths, and armadillos it’s more difficult to use dental formulas.

Check out this fantastic collection of 3D skulls with teeth painted according to type on SketchFab.

Mammal Identification

Being able to quickly identify a skull or a trapped animal in the field down to species is a critical skill for any mammalogist (and it will undoubtedly be important for you to learn to identify different mammal orders, families, or even species from museum study skins and skulls in the lab). Granted, after doing preliminary research and conducting pilot trapping sessions, most mammalogists know which species they are likely to capture at any given site and can learn to quickly identify a subject down to species by site from memory. But one of the best tools we have for distinguishing between species are dichotomous keys, which allows the user to identify something by answering a series of questions, beginning with broad differences and ending with a species name. The questions are grouped in to couplets, or pairs of characteristics (e.g., 1A: Body larger than 10cm; 1B: Body less than or equal to 10cm). Each line of a couplet directs the user to another couplet for a different question. Eventually, the couplets get more and more specific until even the most similar looking species can be distinguished.

Most local field guides have dichotomous keys to the species that are found in that region, allowing the user to fairly quickly narrow down what species they are looking at while still in the field.

Here is a basic key to the skulls of some major mammalian orders, followed by images of their skulls to practice with. Look at each skull, one-by-one, and work through the key to see if you can identify it and land on the proper couplet. Click on the images to zoom in to see more details.

	Couplet / Dichotomous trait	Go To Couplet / Order
	1A Teeth absent 1B Teeth present	2 3
	2A Zygomatic arch present 2B Zygomatic arch absent	Monotremata – echidna Pilosa – anteater
	3A Homodont teeth 3B Heterodont teeth	4 6
	4A Skull <6″ long, teeth blunt 4B Skull >6″ long, teeth pointed	5 Cetacea – dolphin
	5A Zygomatic arch complete 5B Zygomatic arch in complete	Cingulata – armadillo Pilosa – sloth
	6A Eye sockets face forward 6B Eye sockets on side of head	Primates – chimpanzee 7
	7A Canines large, no gap btwn teeth 7B No canines, large gap btwn teeth	Carnivora – dog 8
	8A Incisors 1/1 8B Incisors not 1/1	Rodentia – chinchilla 9
	9A Upper incisors present 9B Upper incisors absent	Perissodactyla – horse Artiodactyla – cow

External Field Identification

Using a field guide on live rodents you’ve just caught in the wild requires the use of more subtle morphological traits on the external body of the animal, instead of the skull. Pacific sage scrub habitat on the west coast of California is home to a diverse array of small mammals. Researchers have been surveying these populations in various capacities for over 100 years. One such study looked at small mammal populations in sage scrub habitats and urban/suburban habitats in Southern California, identifying 11 species found between the two habitat types. Below is a sample dichotomous field key created for this system (typically keys include foot, ear, and skull measurements as well). See if you can use the key to identify the species in the images below.

Couplet / Dichotomous Trait	Go To Couplet / Order
1a Tail length less than 1/2 body length (very short) 1b Tail length greater than 1/2 body length	Microtus californicus 2
2a Hindlegs/feet long, specialized for jumping, elongated tail with tufted tip 2b Hindlegs/feet shorter, not specialized for jumping	Chaetodipus californicus 3
3a Naked or nearly hairless tail 3b Short-haired tail	4 5
4a Small body size (<23 cm head-tail) 4b Large body size (>32 cm head-tail)	Mus musculus Rattus rattus
5a Large body size (>26 cm head-tail), white/buff ventrum, dorsum brown/gray 5b Small body size (<26 cm head-tail)	6 7
6a Tail very faintly bicolored 6b Tail conspicuously bicolored	Neotoma macrotis Neotoma bryanti
7a Body very short (< 15 cm head-tail) 7b Body longer (>15 cm head-tail)	Reithrodontomys megalotis 8
8a Tail length less than or equal to body length 8b Tail length greater than body length	9 10
9a Tail length about equal to body length 9b Tail length less than body length	Peromyscus boylii Peromyscus maniculatus
10a Total length usually less than 23 cm 10b Total length usually greater than 23 cm	Peromyscus fraterculus Peromyscus californicus

References

Holt, B.G., Lessard, J.P., Borregaard, M.K., Fritz, S.A., Araújo, M.B., Dimitrov, D., Fabre, P.H., Graham, C.H., Graves, G.R., Jønsson, K.A. and Nogués-Bravo, D., 2013. An update of Wallace’s zoogeographic regions of the world. Science, 339(6115), 74-78.

Wallace, A.R., 1876. The Geographical Distribution of Animals, Cambridge Univ. Press, Cambridge.

Uncategorized

Cetacean Evolution and Diversity

Post author By Ted Stankowich
Post date July 30, 2020

Paul Balfie https://flic.kr/p/2hnTwBY; Attribution 2.0 Generic (CC BY 2.0))

Scientists have known for hundreds of years that whales were mammals (e.g., they give live birth, have sparse hair, produce milk for young). For most of that time, however, where they fit in the mammal family tree was a mystery, and very few fossils had been found that showed their transition from land to water. But an explosion of discovery and research over the past 30-40 years has solidified our understanding of how, when, and where whales evolved from their terrestrial mammal ancestors. In this lab you will:

Watch a video describing the evolution of cetaceans and explore these transitions using 3D models. You will answer a set of questions about these transitions in the Whale Evolution Quiz.
Watch a video overview of extant cetaceans and learn to identify local whales and dolphins from their external features, especially those you can see from above water while on a whale watching tour. You may then be quizzed on your ability to identify these local species.

There are five questions throughout this lab. A .docx file with those questions can be downloaded here:

Lab Questions Download

Cetacean Evolution

Cetaceans can be categorized taxonomically into three major groups: Archaeocetes (ancient whales), Mysticeti (baleen whales), and Odontoceti (toothed whales). Arachaocetes have ancestral morphologies including external hind limbs, semiaquatic lifestyle (e.g., giving birth on land), and heterodont/diphyodont dentition. In contrast, extant whales have more telescoping (elongated) skulls and are completely aquatic (no external hind limbs). You’ll learn more about them in the next section.

In the latter half of the 20th century, increasingly powerful molecular tests confirmed that whales were most closely allied with artiodactyls, the even-toed ungulates like antelope, deer, pigs, and giraffe. By the 1990s, analyses were starting to place cetaceans (Order Cetacea) within Order Artiodactyla as a sister taxon to hippopotamuses. This meant Cetacea was no longer a valid order (you can’t have an order within an order!); Artiodactyla became Order Cetartiodactyla, whales and hippopotamuses were placed into Suborder Whippomorpha, and Order Cetacea was downgraded to Infraorder Cetacea (Mysticeti and Odontoceti are “parvorders”).

Phylogenetic tree showing several lineages of Archaeocetes branching off first, followed by extant Mysticeti and Odontocetic (Crown Cetacea). Hippopotamidae are shown as the sister taxon to all Cetaceans. — Evolutionary tree of Cetacea

1. Looking at the phylogenetic tree in the figure, Arachaocetes is a group that includes all of the stray branches off of the tree before crown cetaceans evolved. What type of taxonomic group is this (hint: it ends in “–phyletic”)? What type of taxonomic groups are Mysticeti and Odontoceti?

In 2001, two publications reported on the morphology of whale astragalus bones (ankle bones), which have pulley-shaped grooves that improve flexion and are adaptations for high speed running; extant Cetaceans lack these bones entirely as they lack hindlimbs. They compared the astragalus bones of several Eocene archaocetes to those of extant terrestrial cetartiodactyls and other mammals. Artiodactyls have a double pulley shaped (grooved) astragalus allowing flexion both above and below the bone; most other mammals have a single pulley above the bone, but the base is flat. Compare the Pakicetus (an archaocete) astragalus to those of other extant mammals:

Four astragalus bones side by side showing a single pulley morphology on Canis, and double pulley morphology on Pakicetus, Sus, and Odocoileus. — Astragalus bones from one carnivore (*Canis*), one ancestral cetacean that still had hindlimbs (*Pakicetus*), and two extant terrestrial ungulates (*Sus* and *Odocoileus*)

2. What does this discovery tell us about the evolution of cetaceans and what can cetacean fossils tell us that extant cetacean bones cannot?

Watch this video on whale evolution:

The earliest archaeocetes look fairly terrestrial, while the most recent look very similar to extant cetaceans. The earliest archaocete clade is Pakicetidae, which lived in Indo-Pakistan in the early to middle Eocene (47-53 Mya). They probably moved between terrestrial and freshwater environments, feeding on freshwater prey. They retain many features typical of early terrestrial artiodactyls (e.g., long limb bones, no telescoping of the cranium).

Pakicetus by monostudio on Sketchfab

Artist rendering of Pakicetus inachus — *Pakicetus inachus* (Nobu Tamura (http://spinops.blogspot.com) – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=19459344 )

Intermediate between Pakicetids and later archaocetes, were the Ambulocetidae, which lived in the middle Eocene (46-48 Mya). Ambulocetus was probably an ambush predator with an alligator-like lifestyle, waiting for prey in shallow fresh water.

Artists rendering of Ambulocetus swimming in water — *Ambulocetus* swimming (Nobu Tamura email:[email protected] http://spinops.blogspot.com/ – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=51409887 )

3. What changes in skull, limb, and body morphology do you see between Pakicetus and Ambulocetus?

Next up, Protocetidae first evolved in Indo-Pakistan but then diverged and spread rapidly through North Africa and North America from 37-48 Mya. They likely lived in marine environments in coastal areas foraging for fish and invertebrates. They still had a pelvis and well-developed hindlimbs, but the sacrum is weaker, suggesting limited ability to walk on land. The vertebrae are beginning to become more uniform, reflecting a more undulatory mode of swimming.

View this 3D model of the protocetid Maiacetus. Load the model and experiment with the controls. Leave this model open in a new browser tab for the next step.

The final archaeocete family we’ll explore is Basilosauridae. This group lived all over the world in the middle to late Eocene and were fully aquatic. They likely ate vertebrate and invertebrate prey and swam with both undulatory and oscillatory movements.

In a new browser tab, view the 3D model of the basilosaurid Dorudon. In these browser windows you can not only manipulate the model, but also take measurements. Move your mouse arrow over the tip of the tail of Maiacetus and press “S”. A dot should appear on the tail tip. Now move your arrow over the tip of the nose and press “E”. A second dot and a line connecting the dots appears and you should see a measurement in the upper left corner of the screen. Write this down. Now measure the length of the humerus bone in the same way. Do this for both Maiacetus and Dorudon.

4. Calculate the ratio of humerus:body length for each species and report them. What does the difference in this ratio suggest about the changing lifestyle of these groups? Describe THREE other pieces of morphological evidence of evolutionary change can you see in the models?

In addition to the shifts in morphology over the course of cetacean evolution, their lifestyles can also be studied using evidence from the fossil record. While most large bodied mammals require a source of freshwater and cannot desalinate large amounts of salt water, modern cetaceans have evolved this ability. Researchers have studied the drinking behaviors of extant and extinct cetaceans using stable oxygen isotopes. ¹⁶O and ¹⁸O are the two most common isotopes of oxygen and the ratios of the isotopes differ in fresh and salt water. Marine (oceanic) water has higher concentrations of ¹⁸O than does freshwater, and we can analyze the oxygen isotopes in the calcium phosphate of mammalian bones to determine what type of water they were drinking and, therefore, living in. Below is a figure showing the oxygen isotope ratios of modern marine and freshwater cetaceans along with those of extinct cetaceans.

Graph showing oxygen isotope ratios of modern and extinct cetacean species. — Stable oxygen isotopes of extant and extinct cetaceans. Modified from Thewissen & Bajpai 2001, *Bioscience* 51(12). Copyright Oxford Academic.

5. Interpret the figure above, and compare and contrast the types of bodies of water that early cetaceans (e.g., pakicetids, ambulocetids), later proto- and remingtonocetids, and most modern marine cetaceans live in.

Modern Whale Diversity and Identification

The modern extant cetaceans are divided into two groups: the Mysticeti (baleen whales) and Odontoceti (toothed whales). Baleen whales are named for their large plates of baleen (made of strands of keratin) and lack of true teeth, but baleen whales evolved from toothed mysticetes that eventually lost their true teeth and were either toothless or had/have baleen plates. Odontocetes have retained their teeth (although most now have simple homodont dentition) and have the ability to echolocate to detect prey, other whales, and underwater topographic features.

Watch the video below to learn about modern cetacean diversity:

Now that you have a handle on the types of extant cetaceans that live all over the world, it’s time to learn to identify some local whales. Many of you may have taken a whale watching trip at some point, and the waters off of Southern California have a number of baleen and toothed whales that you can spot with regularity. For the remainder of this lab, you will learn to identify the local species of whales using images from the surface of the water so that the next time you go on a whale watching trip, you will be able to identify what you see!

For each of the 10 species below examine the 3D model and associated photographs. You should, on your own, conduct Google Image searches of each species to find more images that show the same features and use these to study from. Learn to identify the distinguishing characteristics of each species. You may be quizzed using actual photographs (not the ones below) to demonstrate your ability to identify these species.

Odontoceti (dolphins and porpoises)

Short-beaked common dolphin (Delphinus delphis):

Most common year round with megapods in Summer. Look for a distinctive cream-colored patch on the side of the thorax with pale white underbelly; stout but distinctive beak, eye surrounded by pale color; 1.5-2m in length.

Long-beaked dolphin by 3d wild park on Sketchfab

Teddy Llovet; https://www.flickr.com/photos/teddyllovet/2843976434 ; Attribution 2.0 Generic (CC BY 2.0)

Stuart Wilson; https://www.flickr.com/photos/135081788@N03/42609790812; Attribution 2.0 Generic (CC BY 2.0)

Bottlenose dolphin (Tursiops truncates):

Common year round. Uniformly gray with pale white underbelly; distinctive crease between the melon and short stubby beak; 2-4m in length.

Bottlenose Dolphin by rohr3dsolutions on Sketchfab

Brandon Trentler https://www.flickr.com/photos/btrentler/29530040660; Attribution 2.0 Generic (CC BY 2.0)

Ed Clayton; https://www.flickr.com/photos/edclayton/6186231783/; Attribution 2.0 Generic (CC BY 2.0)

Risso’s dolphin (Grampus griseus):

Less common but mainly Summer and Fall. Lacks a beak; body pale gray and usually heavily scarred; 3m in length

Don Owens; https://www.flickr.com/photos/regexman/10293912405/; Attribution 2.0 Generic (CC BY 2.0)

Don Owens; https://www.flickr.com/photos/regexman/10293779264/; Attribution 2.0 Generic (CC BY 2.0)

Harbor porpoise (Phocoena phocoena):

Less common and only north of Pt. Conception. Small body with gray dorsum fading to a white pale underbelly, very small triangular dorsal fin; 1.4-1.6m in length

Nise (Phocoena Phocoena) – Harbour Porpoise by MotionCow on Sketchfab

Pacific White Sided Dolphin (Lagenorhynchus obliquidens):

Rare but mainly in Winter and Spring; Distinctive white patches on the side of the thorax and down the dorsal sides of the tail; bicolored dorsal fin that is more hooked than other dolphins; very short beak; 1.7-2.5m in length

Kvitskjeving ( Lagenorhynchus Acutus ) by MotionCow on Sketchfab

NOAA; https://www.flickr.com/photos/usoceangov/5265737381 ; Attribution 2.0 Generic (CC BY 2.0)

Joe McKenna; https://www.flickr.com/photos/jpmckenna/4329818373; Attribution 2.0 Generic (CC BY 2.0)

Mysticeti (baleen whales):

Humpback whale (Megaptera novaeangliae):

Very common especially in Fall and Winter; Humped back with oddly stumpy dorsal fin; variable color on underside of fluke (black – white blotches – all white); long white pectoral fins; 14-17m in length

Knølhval (Megaptera Novaeangliae) Humpback Whale by MotionCow on Sketchfab

Anna Borenstein; https://www.flickr.com/photos/142266229@N06/30871690208/; Attribution 2.0 Generic (CC BY 2.0)

Dan; https://www.flickr.com/photos/estudiante/25813757565; Attribution 2.0 Generic (CC BY 2.0)

Laurel’sPhotos; https://www.flickr.com/photos/46585251@N06/4832915243; Attribution 2.0 Generic (CC BY 2.0)

Gray whale (Eschrichtius robustus):

Very common but mainly in Winter and Spring. They make the longest migration of any mammal on earth when they travel from Alaska down the coast towards Mexico in December-February, and then back northward in March-May. No dorsal fin but small hump and many bumps down the posterior dorsum; Gray mottled skin and fluke; More pale top of the head; 12.8-16.8m in length

Stylized Gray Whale by J.R.R. Harlequin on Sketchfab

Sam Beebe; https://www.flickr.com/photos/sbeebe/6998652179; Attribution 2.0 Generic (CC BY 2.0)

Sam Beebe; https://www.flickr.com/photos/sbeebe/6998653627; Attribution 2.0 Generic (CC BY 2.0)

Sam Beebe; https://www.flickr.com/photos/sbeebe/6998646207; Attribution 2.0 Generic (CC BY 2.0)

Minke whale (Balaenoptera acutorostrata):

Common Winter through Summer; Short body; curved sickle-shaped dorsal fin; white band across pectoral fins; varying degrees of white bands on flank; doesn’t usually expose tail fluke; 7.8-8.8m in length

Vågehval (Balaenoptera acutorostrata) by MotionCow on Sketchfab

Vaughn Mullen; https://www.flickr.com/photos/outdoornovascotia/33529807465; Attribution 2.0 Generic (CC BY 2.0)

Jtweedie1976; https://www.flickr.com/photos/25921513@N00/2410441516; Attribution 2.0 Generic (CC BY 2.0)

RobOo; https://www.flickr.com/photos/105105658@N03/33565180385; Attribution 2.0 Generic (CC BY 2.0)

Blue whale (Balaenoptera musculus):

Less common but mainly seen in Summer; Huge body (largest animal to ever live); mottled gray or blue-gray; very small dorsal fin far back on body; white underside to tail fluke; 23-24m in length

Blue Whale – Textured by Bohdan Lvov on Sketchfab

David Slater; https://www.flickr.com/photos/davrozs/4761279590; Attribution 2.0 Generic (CC BY 2.0)

Jerry Kirkhart; https://www.flickr.com/photos/jkirkhart35/5909428163; Attribution 2.0 Generic (CC BY 2.0)

Kenny Ross; https://www.flickr.com/photos/61554530@N02/7278365718; Attribution 2.0 Generic (CC BY 2.0

Fin whale (Balaenoptera physalus):

Less common but mainly seen in Spring; Distinctive boundary between dark gray dorsum pale underbelly at pectoral fins; strongish dorsal fin but not hooked; doesn’t usually expose tail fluke; 18-19m in length

Fin Whale by Johann Woelper on Sketchfab

Charlie Jackson; https://www.flickr.com/photos/chaz_pics/48496867567; Attribution 2.0 Generic (CC BY 2.0)

Pam Gaynor; https://www.flickr.com/photos/pamgaynor/19398330489; Attribution 2.0 Generic (CC BY 2.0)

Now that you have seen all of the local species, go back and study your images and learn the names of each species. If you are interested in purchasing a field guide to learn more about identifying the marine mammals of California (including pinnipeds), I recommend Allen et al. 2011 (information below).

References

Allen, S.G., Mortenson, J., Webb, S., 2011. Field Guide to Marine Mammals of the Pacific Coast. University of California Press, Berkeley, 569pp.
Marx, F.G., Lambert, O., Uhen, M.D., 2016. Cetacean Paleobiology. Wiley Blackwell, West Sussex, 319pp.
Thewissen, J.G.M. & Bajpai, S. 2001. Whale Origins as a Poster Child for Macroevolution. Bioscience, 51(12), 1037-1049.

Tags Cetacea, Diversity, Dolphins, Evolution, Whales

Evolution

Mammal Origins & Evolution

Post author By Ted Stankowich
Post date July 28, 2020

Lateral view of Dimetrodon skeleton — By H. Zell - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12214271

Introduction

Mammals have been the dominant terrestrial megafauna on Earth since the extinction of the dinosaurs 65 million years ago. They have left an extensive fossil record, and their earliest ancestors that were distinct from other amniote lineages date back to the early Carboniferous period, 315 million years ago. But true mammals (members of Class Mammalia) wouldn’t appear until the mid-late Triassic, about 70-80 million years later! In this lab, you will explore the evolutionary history of the lineage that produced the mammals through media and 3D models of actual fossils. Then you will take virtual tours of two museums to learn more about mammal evolutionary history.

There are nine questions to answer in this module, and you can download a .docx file with just the questions here:

Lab Questions Download

The early mammal relatives and all of their decendants make up a giant branch of the vertebrate tree called Synapsida. Some people call the earliest groups “mammal-like reptiles”, but this is really misleading, as reptiles are members of an entirely different branch of amniotes, Diapsida. We’ll start with the earliest synapsids, the Pelycosaurs, but first watch this short overview video on the synapsids that preceded true mammals:

Now that you have a basic idea of how true mammals evolved from these early ancestors, let’s dive a little deeper into some of the most important groups and evolutionary transitions.

Pelycosaurs

The first great radiation of Synapsids included several different groups of large lumbering “reptile-like” lineages, including the famous Dimetrodon. Have a look at the model of a Dimetrodon skull chomping down on the ancient amphibian Euryops. Feel free to use your mouse to move around the skulls and explore.

Dimetrodon And Eryops by Idaho Virtualization Laboratory on Sketchfab

Recall that the distinguishing feature of Synapsids is the single temporal fenestra. In this model, you can see two large openings. The anterior/front hole is the eye socket. The rear hole is that single temporal fenestra. Keep an eye on that as we move forward to look at later lineages. Also note the different lengths of teeth. Yes, they all look sharp and pointed like a reptile’s teeth, but the fact that they differ slightly in size and shape means they could be used for different functions while chewing.

1. How might the longer canine-type (caniform) teeth be used differently than the smaller teeth in the rear of the jaw?

Now have a look at a 3D model of Dimetrodon with its skin on and watch this video about Dimetrodon.

Dimetrodon by Marcel Schanz on Sketchfab

Probably the most recognizable feature of Dimetrodon and many other Pelycosaurs of the time is the large “sail” formed by a row of long spines growing out of the vertebrae of the spinal column. While we still don’t know its exact function, there have been several hypotheses including thermoregulation or sexual signaling.

2. Based on what you learned in the video, explain how the current evidence suggests that its primary function was most likely as a sexual signal, and why the evidence is less supportive of the thermoregulation hypothesis.

By the end of the Permian, the pelycosaurs were gone and replaced by a more advanced group of synapsids, the therapsids.

Therapsids

Therapsids are another wastebasket group of synapsids that include big predators with long, sharp teeth, but also the very first large terrestrial herbivores! One of these groups, Dinocephalia, had bumpy heads with bony projections that they probably used as battering rams. Another group was the Dicynodonts (see Kannemeyeria below), huge herbivores with large canines, a “beak” that lacked teeth, and a bodies that weighed up to 1000kg (2200lbs)!

Artist rendering of the dicynodont Kannemeyeria latifrons. Large green stocky body with a robust head and stumplike canine teeth emerging from the mouth. Creature has a sprawling gait and short tail. — Dicynodont *Kannemeyeria latifrons* © Diego David Colmán

But there were still huge predatory hunters like the Gorgonopsians, with large canines, huge skulls with strong muscles for chewing, and bear-like bodies weighing about 300kg. Look at the skull and body of the gorgonopsian Inostrancevia.

Gorgonopsid (Inostrancevia) by MikeBG on Sketchfab

Inostrancevia alexandrii by kuzim on Sketchfab

3. Inostrancevia has the more advanced Therapsid skull and body adaptations. Looking at the skull, describe the shape and size of the temporal fenestra relative to Dimetrodon. Looking at the full body, what is happening to the position of the limbs?

Most of the therapsids died out in the Permian extinction 252 million years ago when three-quarters of terrestrial species went extinct. This led to the next major synapsid radiation, the cynodonts.

Cynodonts

The cynodonts are a wastebasket group of lineages that lived in the Triassic and were very similar to the therapsids that came before them. But they also possessed more mammal-like features than their predecessors. One of the most well-known cynodonts, Thrinaxodon liorhinus, was weasel-like in shape and one of the few carnivores of its time. Look at the skull model and the body image shown below.

Thrinaxodon liorhinus by slautenschlager on Sketchfab

Full body artist drawing of Thrinaxodon. Body has brown, short hair, with a somewhat sprawling limb posture, and a short tail. The head has sharp heterodont teeth with sharp canines. — *Thrinaxodon* © Dinoboy336

4. What new advancements do you see in the skull of Thrinaxodon relative to Inostrancevia?

In addition to changes in the skull, these groups had a muscular diaphragm to assist with breathing, whiskers on the snout, a complete secondary palate that allowed them to chew and breathe at the same time, and a jawbone with only few bones left in it (recall that pelycosaurs have 7 bones in the lower jaw and true mammals only have 1, the dentary bone). The last cynodont groups were even more mammal-like in their features, and it’s difficult to know where to draw the line between them and “true mammals”, but most consider the possession of a single dentary bone in the lower jaw with the jaw articulation between the dentary and squamosal bones to be the best indicator of what defines a true mammal.

Mammaliformes

The first true mammals are described as mammaliformes because they aren’t part of crown-group Class Mammalia, which branched off a little bit later and includes all of the extant true mammals. These mammaliformes were mostly mouse-like in size and appearance. Critically, they possess the defining synapomorphy of true mammals: a single dentary bone in the jaw articulating with the squamosal bone in the cranium to form the jaw joint. One of the most primitive true mammals was Morganucodon, a shrew-like Morganucodont. Mammaliformes only had one round of tooth replacement (diphyodonty), as opposed to multiple waves of tooth replacement of their ancestors (polyphyodonty). Look at the Morganucodon skull model and body drawing below.

Morganucodon oehleri by slautenschlager on Sketchfab

Artist drawing of Morganucodon watsoni. Shrewlike in appearance with pointed rostrum with small eyes and large whiskers. Undersides are white. Body has more upright posture with longer tail. — *Morganucodon watsoni* © By FunkMonk (Michael B. H.) CC BY-SA 3.0

The Docodonts, another major mammaliform group, were unique in that they had complex, square crowns on the molars (rather than three-cusped patterns of Morganucodon and other early insectivores), suggesting they had a more omnivorous diet. Discovery of full skeletons revealed that this group varied widely in ecology and lifestyle. There were arboreal, squirrel-like species (e.g., Agilodocodon), mole-like burrowers (e.g., Docofossor), and beaver-like swimming diggers (e.g., Castorcauda lutrasimilis).

5. What advancements in the skull do you see in Morganucodon compared to Thrinaxodon and the other skulls before it? Pay special attention to the teeth, zygomatic arch (“cheekbones”), braincase, and jawbone.

Subsequent mammaliformes, members of crown-group Class Mammalia including extant Monotremes and Therians, diversified greatly from these early forms, but most were still quite small as they were living among the dinosaurs. They were probably mostly burrowing or living deep in vegetation to stay safe, and remained in hiding for about 130 million years. The dinosaurs went extinct 65 million years ago, which means that for 2/3 of the age of mammals (~200 million years), mammals were living in hiding from the dinosaurs. It has only been in the last 1/3 that mammals have emerged as the dominant terrestrial animals and have grown in size and diversified into all ecological niches.

Virtual Field Trips

Finally, while you may not be able to see these amazing fossils in person, many museums now have virtual tours that allow you to walk through and see all of their exhibits for free! The American Museum of Natural History in New York City has one of the very best public exhibits of fossil mammals in the world, and today you will get a chance to see it virtually! Begin your walkthrough with the synapsids here to get to the best starting point for the tour.

As you move through the exhibits, look at each area (look for X’s on the floor and arrows guiding you around) and try to zoom in (controls in the bottom left corner) on the displays to read what they say. Below are seven skeletons to find that are labeled clearly enough to identify and bear adaptations that deviate significantly from the earliest small mammals (e.g., Morganucodon).

Simosthenurus – an extinct kangaroo
Edentates: Glyptodont – the traveling tank
Edentates: Giant ground sloth
Carnivores: Smilodon
Megaloceros – the giant Irish Elk
Stenomylus – a small camel
Mammut – mastodon

6. Find five of these skeletons, select ONE favorite and take a screenshot. Submit the screenshot to your instructor, and briefly describe TWO clear skeletal features of the skeleton you selected that differ from Morganucodon and what the function of each change might be (e.g., large antlers evolved for sexual combat).

Finally, we’re going to move forward in time to the last Ice Age (50K years ago) and take a tour of the La Brea Tar Pits in Los Angeles. Interestingly, “La Brea” means “the tar”, so you’re really visiting “The The Tar Tar Pits”! The tar pits are amazing treasure troves of fossils big and small, from microfossils of sea life and pollen to megafauna like mammoths, mastodons, dire wolves, and saber-toothed cats. Read the information in their virtual tour, look through the image slides, and watch the short videos.

7. Why did paleontologists reopen Pit 91, and what type of information have they learned?

8. How do the paleontologists use modern bones from outside the tar pit?

9. Why are there far more large carnivores in the tar pits than herbivores?

10. How and why have coyote diets changed from before the extinction of other large carnivores (e.g., sabre-toothed cats and dire wolves) to present day?