HORSES

Overview

Horses (equids) are herbivore ungulates (hoofed mammals) belonging to order Perissodactyla, which also includes tapirs and rhinos. The fossil record of horses ranges from Paleocene?, and early Eocene to Recent. Their Cenozoic biodiversity (number of families, genera and species) was highest during the first half of the Cenozoic when they arose on northern continents and later spread southward into Africa and South America. During the second half of the Cenozoic, their biodiversity has gradually and significantly been reduced. 

Most of the early Cenozoic equids (like the other perissodactlys) lived in woodlands and were rooters. During the later Cenozoic, with the spread of grasses (tough to chew and digest), many perissodactyls adapted to living on prairie grasslands.

With the changes in where and how they lived, as well as in what they ate, horses adapted accordingly. Their bodies became larger with longer legs, and the number of toes on each foot decreased. These changes enabled horses to become stronger and faster runners, thus better fleeing their predators. Their skulls also got bigger, thereby allowing for bigger brains. Larger skulls allowed also for larger teeth that were more complex internally, with vertical layers (folds) of enamel growing down, deep inside the teeth. These kind of teeth are called hypsodont teeth (see one of my previous posts), and they are very effective in increasing the strength of the teeth, thereby resisting wear and tear when the horses eat tough grasses with a high content of abrasive silica. Like humans, horses have only two sets of teeth: milk and adult. The latter can last up to 15 years in modern horses.

 About 10,000 years ago, domestication of horses and other livestock began.

Earliest Horses

The earliest known perissodactyls are two genera (one species each) of early Eocene age. Both are small dog-size—see one of my previous posts: Eohippus in Wyoming, northwest New Mexico, and Colorado; and Hyracotherium in London, England and in Paris, France. The morphologic and biogeographic histories of horses are summarized in the following chart.

Horse Teeth

Like in most mammals, the teeth of a horse are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as the cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The diagram above shows the right-side view of the skull/teeth of a modern horse.

Mammal-dental formulae typically designate the number of teeth (upper/lower) on just one side of both jaws. For example, the formula for many modern horses is 3/3, 0/0, 3/3, 3/3, and it refers to the presence of three incisors, no canines, three premolars, and three molars on one side of the upper and lower jaws = 18 teeth. In order to obtain the total number of 36 teeth (excluding the variations noted in the next two paragraphs), one has to include also the teeth on the other side of the skull. Thus, the horse has a total of 36 teeth (18 + 18), when both sides of the skull are counted. This dentition formula is the same for donkeys, zebras, and the Przewalski wild horse. It is important to mention that in all of these animals, their teeth continue to erupt (grow) during the life of the animal. This is because they eat coarse/tough vegetation which wears down the teeth. The cheek teeth of horses erupt in a tightly packed unit (called an "arcade") that acts like a single grinding tooth. Cheek teeth are a composite of three hard materials: enamel, dentin, and cementum.

Male horses typically have 40 teeth because between ages four and five years old, they develop upper and lower canine teeth. The canines are positioned nearer the incisors than the premolars (on both sides of the jaws). Only about 25% of female horses develop canines and are smaller in size than those found in males.  

There is an additional potential of dental variability: between 13 and 32% of both male and female horses can have small-sized vestigial (non-useful) premolar (referred to as a “wolf tooth”) located in front of the first premolar (typically only on the upper jaw). 

Reference Used:

Carson, D.M. and S. W. Ricketts. Equine dentistry. vcahospitals.com

OVERVIEW OF MAMMAL TEETH

This post is the second one concerning background information about the ancient history of various mammal groups that I shall be focused on in my upcoming series of posts (see my immediately previous posts about land bridges),

Before proceeding into the paleontologic details of various groups of land mammals, I discuss here the various types of mammal teeth. The shapes and arrangements of teeth in mammals are extremely important for the classification and identification of fossil vertebrates (as they are for living ones).

Some vertebrates (fish [including sharks], amphibians, and reptiles) generally have continuous replacement of teeth, so that as one is shed or lost, another erupts to takes its place. Land mammals, however, are fundamentally different is this regard; they have a mostly pre-determined number of teeth and very limited replacement of teeth.

Mammal teeth are differentiated into four types: incisors, canines, premolars, and molars. The premolars and molars are grouped together as cheek teeth. The teeth are, furthermore, very characteristic of the orders of the mammals to which they belong, and the morphology of the teeth reflect the diets of the mammals.

The number of teeth varies greatly in different groups of mammals. Dental formulae (i.e., the “ratios”) designate the number of teeth (upper/lower) on just one half of one side of both jaws. For example, the formula for dogs is 3/3, 1/1, 4/4, 2/3 = 40. This means that there are at least 10 teeth (namely, three incisors, one canine, four premolars, and two molars) on each quadrant (= one quarter) of the skull. But, as there are quadrants, the number of teeth is 10 x four ] = a total of 40 teeth on the entire adult skull.

The above diagram shows the side view of each of the jaws in a skull of a dog and the types of teeth present. The canine teeth are large, whereas the incisors are small. The large and pointy shape of the most forward molar tooth in the lower jaw and the most backward placed premolar in the upper jaw move pass one other, like blades in a pair of scissors, in such a way to allow for a shearing action (allows for eating flesh and bone). These two teeth (present are both sides of the skull) are referred to as carnassial teeth.  

The formula for a typical adult human is 2/2, 1/1, 2/2, 3/3, and it refers to the presence of two incisors, one, canine, two premolars, and 3 molars; for a total of 8 x 4 = 32 teeth. In some individuals there is an additional molar in each quadrant of the skull. This molar is the most posterior one and is referred to as a “wisdom tooth”. If there are four wisdom teeth, then the formula is 2/2, 1/1, 2/2, 4/4 = total of 36 teeth. 

In many mammals, including humans, there are two sets of teeth: the deciduous (“baby” or “milk” teeth) and the adult teeth. In humans, the “baby” teeth (2/2, 1/1, 0/0, 2/2), for a total of 20 teeth, are replaced by the adult teeth, which erupt starting around 6 years old or so. Note: baby humans do not have any premolar teeth. 

A COMPILATION BY R. SQUIRES OF CENOZOIC LAND BRIDGES MENTIONED IN THE GEOLOGIC LITERATURE AND REFERRED TO IN MY UPCOMING BLOG POSTS:  

This post provides vital background information that concerns a series of subsequent posts by me on various Cenozoic land-mammals (for example, hippos, sloths, camels, mastodons, etc.). One of the major themes that concerns all these animals is how and when they migrated from one continent to another. As you will see, some migrated from continent to continent via land bridges (listed below). Other groups, however, were/are confined to a single continent. 

De Geer Land Bridge

Connected various parts of the high Arctic in the early Cenozoic: Canadian mainland, Ellesmere Island, Greenland, and Svalbard to Norway. De Geer existed (intermittently?) at or near the end of the Cretaceous until the early Oligocene. See Brikiatis (2014).

Thulean Land Bridge

Extended from North America to Greenland, (intermittently?) from early Cenozoic until the early Oligocene. See Brikiatis (2014).

North Atlantic Land Bridge (NAtLB) or Greenland-Scotland Land Bridge

Extended from Greenland to Scotland during the middle Cenozoic.

Turgai Land Bridge

Extended between east and west Eurasia (intermittently in the Eocene?) during the Oligocene. See Brikiatis (2014).

Berginia 1 Land Bridge 

Extended from North America (Alaska) and Siberia. It occurred intermittently during Eocene and Oligocene time (between 55 and 25 million years ago (mya).

The following sequence of maps versus various intervals of geologic time (Eocene, Miocene, etc.)  provides summation of the major land bridges mentioned in my list:

Note: The general term “Beringia” has been used extensively in the published literature. The term is used mainly, however, for only for the most recent connection between North America and Sibera, but there have been at least two other similar land connections; hence  the overall term “Berginia” is misleading. That is why I labelled the first two "Berginias" as 1 and 2, and why I chose to label the youngest Berginia (the Ice Age one) as number 3.

Note: North America/Siberia land connection: It is possible that this connection occurred also during the latest Cretaceous (65.6 mya), thus explaining some dinosaur distributions. 

Beringia 2 occurred intermittently during the Miocene (between 25 and 5 million years ago, especially during late Miocene time. During that time interval, various groups of land mammals (many now extinct), as well as land plants, migrated primarily between North America and Eurasia. 

Greater Antilles-Aves Island (GAARlandia)

Latest Eocene/earliest Oligocene time only; shortly thereafter this island chain was dismantled and submerged. Today, only islands exist.

This very temporary land bridge allowed for some South American land mammals (ground sloths, capybaras) to migrate into Central America. Some current workers are hesitant to embrace this seemingly plausible migration route as not being all that viable. 

Great American Biotic Interchange (GABI) 

This acryonym refers to the greatest land animal exchange in Earth’s history = “Greatest American Biotic Interchange.” This exchange, which took place at the Isthmus of Panama portal, started at the Pliocene-Pleistocene boundary about 3 mya and is still going on. As shown in the following diagram, most of the animal migrations were southward out of North America into Central and South America (e.g., horses, mastodons, camels/llamas, tapirs, etc.); some animals migrated northward (capybaras, armadillos).

The "cats" arriving from North America were cougars, jaguars, and also saber-tooth cats. Other animal groups that migrated both ways were various insects, amphibians, snakes, alligators, turtles, tortoises?, and some flying birds.  

The large South American carnivorous land birds (phororhacoid ground birds----see one of my previous posts) reached also Texas and Florida during GABI.

Beringia 3

The latest high-latitude North Pacific connection between Alaska (North America) and Siberia (Asia) during the four Pleistocene Ice Ages (the most recent advance of ice was about 10,000 years ago). Glaciation caused the “Bering Strait” to be closed from time to time.

Note: The acronyonym “NALB” (North America Land Bridge), which has been used also for the Beringia 3-Pleistocene Ice Age connection, should be avoided because it can be confused with the North Atlantic Land Bridge (a.k.a. as the middle Cenozoic Greenland-Scotland land bridge during the Cenozoic).

OVERVIEW OF MAMMAL GROUPS THAT USED LAND BRIDGES FOR MIGRATION. All of the groups listed below (except humans) will be discussed in my upcoming posts.

Beringia 3 (Pleistocene time)

Mammoths

Humans

GABI (Pliocene-Pleistocene boundary)

Capybaras

Ground sloths

Glyptodonts

Cuvieronius gomphotheres

Beringia 2 (Miocene time)

Camels

Tapirs

Horses

Gomphotherium

Ambelodontidae (shovel-tusked gomphotheres)

Mastodons

Beringia 1 (Eocene time)

Unitatherium

Brontotheres

Horses

BALEEN (WHALE WHISKERS)

Baleen is an elastic horny material made up of keratin, the same fibrous protein material found in fingernails, hair, and feathers. Baleen is used in a filter-feeding system that hangs in rows from the roof of the mouth of a baleen whale. The whale pushes a scooped-up mouthful of water, partly shuts its mouth, and then uses it tongue to push the water through the “hairy” baleen. The baleen acts as a strainer/filter to catch small animals, such as krill, which are numerous tiny arthropods that float in sea water.

Partial view (100-cm-long) of piece of baleen from an eastern Pacific whale.

Baleen whales, which currently are made up of 16 species, belong to the mysticetes, which are also called the “whalebone whales.” Some examples of baleen whales are: the bowhead, gray whale, blue whale, sperm whale, humpback whale, beluga, and fin whale. Baleen whales all have two blowholes (nostrils) on the top of their heads.

Note: the other major group of whales are the toothed whales (e.g., killer whale), and they have only one blowhole.

Some gastropods, all of which have tiny limpet-like (inverted “V” shaped shells), can live in decaying baleen. There are four known families of these gastropods: pyropletids, cocculinids, osteopeltids, and neolepetopsids. A recent paper by Marshall and Walton (2021), which is given at the end of this blog post, described an osteopeltid limpet gastropod that was found to feed on baleen off the coast of New Zealand, in waters of 377 to 1042 m depth. How this gastropod and other ones like it metabolize the tough and essentially inert baleen is unknown, but it is probably related to the action of symbiotic bacteria within the gastropod gut. The shape of this gastropod is illustrated below.

Oval outline of the shell of the deep-sea osteopenia gastropod limpet Baleenopleta rotunda, top view, 12.5 mm diameter. Top view, height 4.5 mm of same B. rotunda specimen as shown below.

Side view, height 4.5 mm of same B. rotunda specimen as shown above.

Numerous minute specimens (small ovals) of Baleenopleta rotunda in living position in the narrow spaces between the “ribs” of a 110-cm-long baleen “frond.”   

Key Reference:

Marshall, B.A. and Walton, K. Jan., 2021. Baleenopelta rotunda, a newly discovered limpet from decaying baleen from New Zealand. Molluscan Research. Published online.  

RATTLESNAKES: From the Perspective of a Southern California Field Geologist.

During my nearly 45+ years of geologic mapping and associated fossil collecting, especially in the San Gabriel Mountains in Southern California just north of Los Angeles, I have been always been acutely aware that rattlesnakes are a constant hazard. 

My sightings of rattlesnakes during my field work have been “uncommon.” This post focuses on some of the rattlesnakes that I encountered and photographed. 

All of the following images, except the two are Crotalus atrox, the Southern Pacific rattlesnake. Most of these snakes that I have encountered were about two feet long (estimated), have a reddish gray "diamondback" pattern along their backbone, and have two grayish stripes on their cheeks.

IMAGE 1: On a road, 4 pm, cool and windy, May 30, 2011.

IMAGE 2: On a hiking trail, 10 am, warm, April 4, 2008.

IMAGE 3: Off-trail in a very remote area, warm, October 1, 2006. The visible part of the "Jacob Staff" (a measuring stick) is about 6 inches in length.

IMAGE 4: On a road, moderately warm, 9:30 am.

IMAGE 5: On a road 4 pm, August 2011. This snake is the longest rattlesnake (about 4 feet long) that I have ever seen (about 4 feet long) in the field. I identify it as Crotalus helleri, which has many snynonyms, including C. oreganus helleri, and C. viridis helleri. It is black with some white or yellow small spots (dots) aligned in widely spaced rows. The venom of this snake is highly toxic to humans and occasionally fatal.

IMAGES 6 and 7: Hiking trail with a steep dropoff just to the left, June 8, 2011, warm, 10 am. The snake is hidden in the black “hole” in the grass. The next image is a closeup of the black “hole.” I was never able to see the snake very well. It was black, and it might have been another Crotalus helleri.

The last two images bring to mind a very important lesson: NEVER WALK SIDE-BY-SIDE with another person when in the field. Always walk in a single-file. If you were walking side-by-side with someone situation like this one, it is likely that one of you would be walking in the grass. That person would have most certainly stepped on the snake. 

Pyrite Pseudomorphs

 This post concerns chemical weathering that can occur to the mineral pyrite [= iron sulfide FeS₂ = “fool’s gold].” The weathering transforms the pyrite molecules into another mineral, but the external shape of the pyrite crystal remains. This phenomenon, which is called a “pseudomorph after pyrite,” is not restricted to pyrite, as it can occur in other minerals, as well.

A pseudomorph is when one mineral is replaced by another mineral without any change in the external form. In the examples shown here, the original pyrite has largely not been replaced, but the surfaces of the original pyrite crystals have been altered by weathering. Thus, these altered specimens are only partial pseudomorphs. This kind of distinction is not commonly discussed in the literature.

Unaltered pyrite crystals are cubic, shiny, heavy (dense), and leave a black-color streak when scratched on a streak plate (see one of my previous posts). Also, the crystal surfaces of pyrites can be striated.

Unaltered crystals of pyrite (no striations).

The cluster is 5 cm across.

                        Unaltered crystals of pyrite with striations.

                            The cluster is 3 cm tall.

           Incipient pseudomorph of pyrite after goethite/limonite. 

                            The cluster is 2.5 cm tall.

Later stage of other partial pseudomorphs of pyrite after        goethite/limonite. The cluster is 3 cm tall.

The last image, which is of two co-joined crystal faces of pseudomorphs, is the most weathered. It has some areas of a soft-chalky coating of white, red, or brown material; its striations are still present but are not pristine. The composition of the coating material is a variable mixture of two minerals: goethite [FeO(OH)], a hydrated iron oxide-hydrous oxide and limonite [FeO(OH)•nH₂O], a hydrous iron oxide. Goethite typically is the dominant mineral. 

GYPSUM REVISITED: AN ADDENDUM TO THIS INTERESTING AND COMMON MINERAL

This post is an update (with new images and new text) of one that I did five years ago, on Sept. 30, 2017. 

There are numerous varieties of the mineral gypsum (selenite), a sulfate mineral, which has the chemical formula CaSO4.2H20. A keep clue is identifying gypsum is that it does not react with hydrochloric acid. Gypsum is normally the first salt precipitated and deposited in the evaporation of sea water or salty late water. As the salinity of the water increases, the gypsum is followed by the deposition of anhydrite and halite (sodium chloride). Gypsum can also form as vein-filling cracks in rocks.

Variety 1 is colorless selenite: it is transparent, smooth, and non-fibrous. 

(a)

lmage (a) shows a transparent crystal of selenite (43 mm long and 17 mm wide).

(b)

Image (b) shows a large transparent crystal of selenite (70 mm wide) that apparently has been artificially tapered by a machine.

                   

Variety 2 is colored selenite: image (c) shows a slightly colored crystal of selenite (30 mm long) of gypsum. This specimen has been scratched by fingernails. When I was teaching, I would pass this hand specimen around the classroom, and some students would   scratch this specimen. Gypsum is very soft, with a a Mohs Scale hardness of only 2 [see my previous post about the Mohs Scale].

                                              (c)

                                               (d)

Image (d) is of another slightly colored crystal (30 mm long) of selenite. This transparent crystal of gypsum has been scratched by fingernails. When I was teaching I would pass this hand specimen around the classroom, and some students would  convince themselves that they could scratch this mineral, which is very soft and has a Mohs Scale hardness of only 2 (see my previous post on the Mohs Scale).

Variety 3 is twinned crystals of selenite: See one of my previous posts about the subject of twinning in crystals.

                                               (e)

Image (e) is of two specimens of twinned crystals (the biggest one is 37 mm in length) of selenite. 

Variety 4 of selenite is satin spar: 

                                                (f)
Image (f) is of a rectangular hand specimen (4 inches at its widest and 1.5 inches thick) is milky white, silky, and  has fibrous with a characteristic parallel structure. shown here. You can clearly see the distinctive fibrous parallel, satiny structure of this soft mineral. Surprisingly, this piece of satin spar is quite heavy. Unfortunately, the name “satin spar” has also been applied to a fibrous variety of the mineral calcite (CaCO3), but that mineral which reacts with hydrochloric acid.

                                                 (g)

Image (g) is of a machine-cut (in a spiral (corkscrew) shape) of a piece of satin spar (6.5 inches tall). This sculpture demonstrates how the softness of selenite, which allows for it to be easily manipulated into complex shapes. 

Variety 5 of selenite is "desert rose": These are rosette-shaped crystals that can resemble a flower shape because of the curved gypsum crystals. These crystals are twinned (see comment mentioned above). Desert rose crystals can be gradational with so-called, "desert-flower" gypsum crystal.

(h)

Image h is a "desert rose," (2.5 inches across) consisting of a cluster of intergrown gypsum crystals.                                          

Variety 6 of selenite is alabaster. This variety of the mineral gypsum is massive material. It looks just like white limestone or marble. Alabaster is softer than those rocks and, therefore, is commonly used for carving statues, vases, and ornaments.

                                                (i)

Image i is a closeup of the surface of a slab of alabaster. The crystal boundaries are somewhat indistinct because of weathering of the surface of the slab.