Fossil Friday: The Doll Ponies of Southern California

by Outreach & Communications Coordinator Brittney Elizabeth Stoneburg

Last week the Western Science Center released a new paper about Micoene horses from Southern California in UC Berkeley Museum of Paleontology’s open access journal PaleoBios. This research has given us a clearer picture of what the area around San Bernardino National Forest looked like millions of years ago, and all it took was a few small horse teeth.

Excavations have been going on in the Cajon Valley Formation for years, but in 2018 Western Science Center researchers were granted a permit by the U.S. Forest Service. It dates back to the Miocene Epoch, which spanned 23.03 to 5.333 million years ago, and the Cajon Valley Formation fossils in the WSC collections are approximately 16.5 to 14 million years old. Horses living during this time period were much smaller than modern horses (a newspaper article on paleontologist John Merriam’s excavations nicknamed them “doll ponies” in 1913) and had three toes on each foot, unlike modern horses, which only have one toe on each foot.

The field site in 2018, with specimen positions marked. Lead author Brittney Elizabeth Stoneburg for scale.

We eventually identified three species of horses from the WSC sample - Archaeohippus mourningi, Scaphohippus sumani, and Parahippus brevidens (or as I tend to refer to their respective sizes in my head, tall, grande, and venti!). Each of them helped us gain a clearer understanding of prehistoric California, and what the environment there was like millions of years ago.

Teeth from the WSC Cajon Valley Formation sample. A. Parahippus brevidens. B. Archaeohippus mourningi. C. Scaphohippus sumani.

While it wasn’t particularly surprising to have found several Ar. mourningi specimens, as these tiny horses had been reported before in the formation, Scaphohippus and Parahippus ended up being a more complicated story.

Scaphohippus, for example, was originally separated into two species - S. sumani and S. intermontanus. As mentioned, you can identify many different species of horse based primarily on their teeth, and Scaphohippus is no different. S. intermontanus was said to have a less complex tooth surface than S. sumani, and was mostly known from the nearby Barstow Formation. But as we visited museum collections and tried to determine which species of Scaphohippus we had in our sample, we grew increasingly confused. These horse teeth looked so similar that it was incredibly difficult to tell the two species apart! Finally, instead of comparing individual teeth, we decided to look at whole tooth rows, and that sparked a question.

We had just assumed that the idea of there being two species of Scaphohippus was correct, but what if it wasn’t? What if the reason the teeth looked so similar is because they came from the same species?

How the realization felt to the authors.

Many of the fine details that help us identify horse teeth appear with a certain amount of wear, or in a particular tooth position. In Scaphohippus, the defining features are called “plications”, small enamel folds in the center of the tooth that increase in number as the horse would wear the tooth down with use. The number of plications in a particular tooth was part of what differentiated S. sumani from S. intermontanus, but when we looked at the tooth rows, we realized this difference wasn’t consistent enough to base an entire species on. And so our question was answered - our specimens belonged to S. sumani, because we no longer considered S. intermontanus a separate species! (S. sumani was named first, and so that name had what’s called “taxonomic priority”).

After that, it was time to turn our attention to a particularly strange little tooth that didn’t fit either Archaeohippus or Scaphohippus. Shaped like an ear lobe, this molar was markedly different from all the other teeth we had collected.

We searched high and low for an identification, but no previously published specimen from the Cajon Valley Formation matched. Then, at last, we stumbled on Parahippus brevidens, a Miocene horse species that had first been identified in the Mascall Formation in Oregon. Our tooth matched the holotype (the fossil a species’ description is based on) almost exactly.

Parahippus brevidens had been mentioned here and there in databases with Cajon Valley Formation specimens, but this paper is the first research confirming that these horses did indeed live in Southern California. This has extended the known range for these animals by about 400km and means they appeared up and down the west coast, at least from Oregon to Southern California! We learned so much new information about this horse species and its range, all from a single tooth.

So we figured out what three species of our horse we had in our sample - great! But what does that mean?

A faunal comparison between the Cajon Valley Formation and the Barstow Formation. Adapted from Figure 7 of Stoneburg et al (2021).

It means that this research is another piece in the puzzle of figuring out what California's ancient past looked like. Knowing now that we have three species of horses in the same place gives us a clearer picture of the Cajon Valley Formation's ecological profile. For example, we can now more accurately compare and contrast the formation’s fauna with the well-studied Barstow Formation, which is similar in age to the Cajon Valley Formation and is only around 65 miles away. Yet we see these consistent differences in the fauna found in each formation - for example, while Parahippus brevidens is now confirmed to have been found in the Cajon Valley Formation, it has yet to appear in the Barstow. Larger horses, like Hypohippus affinis and Megahippus mckennai, appear in the Barstow Formation, but haven’t appeared in the Cajon Valley Formation. What caused these faunal differences? Was there some sort of geological barrier impeding movement? Were the environments markedly different despite the proximity? We don't know quite yet, but these are the types of questions we can start asking.

There’s still so much we don’t know about the Cajon Valley Formation, and California during the Miocene as a whole, so every bit of research helps expand our understanding of a long gone environment. You can definitely expect more research about the “doll ponies of Southern California” to come!

Fossil Friday - Miocene horses

Hello! I'm Brittney Stoneburg, the Marketing and Events Specialist for the Western Science Center. While my job mostly entails communications and outreach at the museum, I've spent the last year dipping my toes into research!For my first research project, I've been working on fossils collected by the Western Science Center from our field site in San Bernardino National Forest. This means I'm up to my neck in horse teeth right now! I've been focusing on the small, three toed horses that roamed Southern California during the early to middle Miocene, approximately 18 million years ago.Multiple species of horse lived in this area, including Scaphohippus, whose teeth are picture above. While our collection from this site has only a few post-cranial bits, we can make pretty solid identifications just from the teeth. Horse teeth like these have numerous features that make it possible to identify them down to the species level. Knowing what horses lived in this area during the Miocene can tell us a lot not only about the horses themselves, but about the environment they lived in!I'll be presenting on my research at the North American Paleontological Conference in June, so if you'll be there, feel free to ask me about all of these horses!

Fossil Friday - Horse molar

One thing has become quickly obvious as we've been examining the Harveston fossil collection: there are a lot of horses.This is one of several isolated horse teeth from Harveston. This specimen is an upper right 1st molar, and is of particular interest because it is quite a bit smaller than most of our Harveston horses, as can be seen below:It turns out there were actually two species of horses at Harveston. The larger and much more common one is Equus occidentalis, while the smaller, rare species is generally called Equus conversidens (there are some nomenclatural issues with this name and with Equus species in general, but I'm not going to get into that here). E. conversidens makes up perhaps 5% of the Harveston horses. This seems to be a general trend for southern California sites, including Diamond Valley Lake, with both species generally present, and E. occidentalis being much more common.We've made a 3D scan of this E. conversidens tooth (WSC 24676) available for download on Sketchfab at https://skfb.ly/6yJNq.

Fossil Friday - horse dentary

Fossil Friday this week continues our examination of Pleistocene specimens from Temecula Valley, this time with a partial lower jaw from the horse Equus occidentalis.The image above was prepared for our poster at next month's regional GSA meeting in Flagstaff. It shows the dentary in medial, dorsal, and lateral views. The scale bar is 10 cm. In the dorsal view, you can see the occlusal surfaces of the last four teeth in the jaw, which in this case are (from front to back) the 4th deciduous premolar, and molars 1, 2, and 3. The 3rd molar had only just started to erupt, suggesting that this horse was about 3.5 years old when it died. In an oblique anteromedial view (actually a screenshot from a photogrammetric model), you can see the unerupted 4th premolar underneath the 4th deciduous premolar, and, in the background, the deep crown of the 2nd molar where the side of the jaw is broken:We've made a photogrammetric model of this specimen, and printed a 3d replica that we've already started using in programming:The 3D file of this specimen is available for download at WSC's Sketchfab site at https://skfb.ly/6yCx8.

Fossil Friday - horse scapula

This week we continue our documentation of Pleistocene fossils from the Harveston section of Murrieta, California, with a horse scapula.This specimen is a partial right scapula, shown above in medial view. It's missing a small part of the dorsal edge, on the right in the photo. The part of the scapula is cartilaginous in young animals, and actually only ossifies fairly late in ontogeny, so it's not unusual to be missing this area.The lateral view is shown above, with anterior at the bottom (this image is missing the scale bar because these are part of a set used to produce a photogrammetric model). The surface on the left is the edge of the glenoid fossa, the articulation with the head of the humerus (the "shoulder socket"). A bone ridge, called the scapular spine, is pointing straight at the camera and divides the scapula into concave anterior and posterior surfaces. The anterior portion is called the supraspinatus fossa, while the posterior part is the infraspinatus fossa; each serves as the attachment points for muscles of the same names, that are involved in rotating the forelimb.I've uploaded a 3D model of this specimen on Sketchfab, available at https://skfb.ly/6y9VM.

Fossil Friday - horse molar

As we continue to work on WSC's collection of Late Pleistocene fossils from Murrieta, it has become clear that, while the collection my be taxonomically diverse, it contains a lot of horse bones!The specimen shown above is a lower right first molar of the horse Equus occidentalis in lateral (labial) view. On the left is an unpainted 3D print of the same specimen. Below is the same specimen in occlusal view:Horses are highly hypsodont, meaning they have very tall, high-crowned teeth. As you can see in the lateral view at the top, we have both the roots and the occlusal surface preserved, showing that we have the whole tooth (it's not broken), yet the tooth is quite short. That's because almost this entire tooth has been worn away, indicating that it came from a very elderly horse.We've obviously scanned this tooth, since we've 3D-printed it. We've also made the scan available on Sketchfab:https://skfb.ly/6xyuR

https://sketchfab.com/models/dc9a5bba8ef64792bfd53465c66620d5/embed

WSC24698 Equus Right M1-dec by WesternScienceCenter on Sketchfab

Fossil Friday - horse lunate

We're continuing our efforts to document an describe the fauna from the Harveston neighborhood of Murrieta, a small but diverse collection that appears to be the only Rancholabrean-Age site in Murrieta. The bone shown here is the left lunate of a horse (Equus sp.), with the original bone on the left and a 3D print on the right. The lunate is one of 6 small bones that form the wrist. In horses they are blocky, tightly interlocking bones that can support the stress of holding the weight of a running horse. Unlike primates, which have highly mobile wrists, the horse wrist has essentially no ability to move side-to-side, as is limited to fore-and-aft motion, making it more stable during running.We're planning to scan and print as many of the Harveston bones as possible. The paint job on this print is too dark, so I need to lighten it a bit. O

Fossil Friday - horse mandible

We're continuing our review of our collection of Pleistocene fossils from the Harveston neighborhood of Murrieta, in southwestern Riverside County. This is a diverse fauna with a number of different genera, but it appears that in terms of shear numbers of bones this collection is dominated by horses.The specimen shown here is a partial lower jaw of Equus occidentalis, including about 2/3 of the right dentary and a fragment of the left dentary at the mandibular symphysis. At the top is the lateral view, with anterior to the right. Below is the medial view, with anterior to the left:And the dorsal view, also with anterior to the left, and showing the occlusal surfaces of the teeth:The entire right dentition except for the incisors is preserved, from the 2nd premolar to the 3rd molar. These are all adult teeth, and they are all in wear, so this was an adult horse. In fact, in the lateral view at the top, we can see the unerupted part of the 3rd premolar. There is only a very short crown present above the root. Horses have teeth with extremely tall crowns, so possibly as much as 70-80% of this tooth has worn away. This horse was not just an adult, but was elderly (or, at least its teeth show an amount of wear typically only seen in elderly horses).

Fossil Friday - horse metacarpal

One of the joys of paleontology is that every fossil has a story. Through our understanding of anatomy, geology, ecology, and a host of other field, we can often reveal part of that story, and even a relatively small, nondescript fossil takes on a larger meaning.The small, pointed bone shown above is the right fourth metacarpal from a horse (Equus). The metacarpals are the bones that make up the hand or forefoot. At the proximal end they articulate with the wrist bones (the carpals) and often with the adjacent metacarpals. At the distal end they normally articulate with the first finger bones (the phalanges). In humans, the fourth metacarpal articulates with the ring finger. Our fingers (or digits) are numbered from 1 to 5, with the thumb being Digit I and the pinkie finger being Digit V; thus the ring finger is Digit IV. Toes are numbered the same way, with the big toe as Digit I.Horses famously only have a single hoof on each foot, corresponding to a single finger (on the front feet) or toe (on the back feet). The distal end of the bone shown above (at the bottom of the photo) clearly does not have any kind of articulation for the bones that make up the single finger on the horse's right foot. So what's going on?In horses, the hand bone that supports the animal's weight is the third metacarpal, not the fourth one. The right third metacarpal (probably from the same individual) is shown below: This massive bone is well-suited to holding up a horse's weight. So what about the fourth metacarpal? If it doesn't support a hoof, what does it do?As far as we can tell, pretty much nothing.The fourth metacarpal is a remnant of the horse's evolutionary history. While modern Equus is limited to a single hoof on each foot, supported by Digit III, horses' ancestors and extinct relatives had multiple hooves on each foot. The lateral toes were gradually lost, starting with Digits I and V, and later with Digits II and IV. The key here is gradually. Each digit eventually became small enough that it no longer functioned for walking; there would be a tiny hoof, but it didn't reach the ground. The hoof would eventually be lost, leaving only the metacarpal with no attached finger or toe. After even more time and generations, even the metacarpal is lost. But the whole process takes several million years.In the horse lineage, the first and fifth metacarpals were lost tens of millions of years ago. But the reduction of the second and fourth metacarpals has happened much more recently. Within the last 10 million years horse ancestors still had functional second and fourth toes, even if they were smaller than the central third toe. A few million years ago the second and fourth toes were finally lost altogether, leaving their corresponding metacarpals and metatarsals as the last remaining remnants of these digits. And that's where horses are today. The second and fourth metacarpals (and metatarsals) have lost their toes, and even the articulation to attach to the toes, and they're greatly reduced in size. At the proximal end they still articulate with the wrist bones and the third metacarpal (see the posterior view below). The presence of the fourth metacarpal is evolution caught in the act, a snapshot of a dynamic process that seems static because it happens so slowly. If horses survive a few million more years, the fourth metacarpal will probably be entirely lost, so that there is only a small slice of time (geologically speaking) when such a fossil could be preserved. 

Fossil Friday - horse skull

Among the large animals recovered during the Diamond Valley Lake excavation, fully 1/5  of the specimens came from horses; only bison bones were more common among the large animals. There are two species of horse represented at DVL. The smaller species, Equus conversidens, is exceptionally rare in the deposit, and nearly all the recovered specimens belong to the larger Equus occidentalis. Reflecting this, there are a number of E. occidentalis skulls in the WSC collections.The specimen shown above is a partial skull of E. occidentalis seen in ventral view, with anterior to the right. The incisors are missing, but otherwise the right dentition is almost complete. The right teeth are labeled below:The only bones visible in this view are the maxillae, which hold the teeth, and a small part of the palatines, which form the curve at the posterior end of the palate (on the left in this image). That curve also forms the front edge of the interior nares, where the nostrils pass through the skull.This individual was an adult horse, with all of its permanent teeth erupted and showing wear. It was recovered from the West Dam area, which includes the most recent Pleistocene deposits in DVL. Carbon dates on samples from the West Dam area were all less than 20,000 years old, and most were less than 14,000 years (Springer et al. 2009).Reference:Springer, K., E. Scott, J. C. Sagebiel, and L. K. Murray, 2009. The Diamond Valley Lake Local Fauna: Late Pleistocene vertebrates from inland Southern California. In L. B. Albright III (ed.), Papers on Geology, Vertebrate Paleontology, and Biostratigraphy in Honor of Michael O. Woodburne. Museum of Northern Arizona Bulletin 65:217-235.

Fossil Friday - horse jaw

While the majority  of Ice Age fossils in Riverside County are from the Diamond Valley Lake excavation, there are a few other productive Pleistocene sites. As with DVL, these have mostly been found during construction projects. The specimen shown above was recovered from The Promenade shopping mall in Temecula.While badly crushed, the specimen is the tip of the lower jaw of a horse. It's shown in dorsal view, with anterior to the left. Mammal lower jaws are made up of two bones, the left and right dentaries. In most species the dentaries are fused together at the chin, in what's technically called the mandibular symphysis; that's what's preserved here.There are actually portions of 8 teeth present in this fragment, although they're poorly preserved. In the marked-up image below, the red areas are indicating the positions of the 6 lower incisors (3 on each side), while the blue arrows are pointing at the bases of the canines ("wolf teeth" in horse terminology Eric Scott pointed out to me that the "wolf teeth" are actually vestigial 1st premolars, not the canines, so that while these teeth are canines they are not wolf teeth):WSC volunteer Joe has been cleaning this specimen, and is working on other associated fragments. It looks like we probably have additional material from this jaw, which I'll post once it's further prepared.

Fossil Friday-horse skull fragments

Vertebrate paleontologists rarely get to examine whole skeletons (really, almost never). Even individual bones or regions such as skulls are usually fragments, often broken in odd ways, and identification can be a challenge. That's why paleontologists will keep around lots of well-illustrated references, reference collections of modern animal skeletons, and increasingly, 3D images of identified specimens. A lot of identification work is just comparing your unknown specimen to known ones and finding a match.For the last several weeks WSC volunteer Nathan Bonde has been preparing a block of sediment that came to the museum through a mitigation project (unfortunately with rather limited data). The sediment had several bone and possible tooth fragments sticking out, but it wasn't immediately clear what the material was or if it all belonged to the same animal.It quickly became apparent that the tooth was from a horse, apparently a upper left 3rd premolar or 1st molar (shown below next to a zebra skull from Brett's collection):The bone fragments were a little tougher. Continued cleaning showed that most of the bone joined together into a single large fragment, shown at the top of the page. The size and overall complexity suggested a skull fragment, which was bolstered by the presence of a bony tube that looked like the external auditory meatus, part of the ear structure. Since we knew that a horse was a strong possibility (based on the tooth), we compared the fragment to the zebra skull:If you're having trouble seeing how these match up, here's an annotated version:The area outlined in red on the zebra skull is the approximate preserved part of the fossil; note that the fossil is quite a bit larger than the modern specimen. The numbered features are:

1. Right occipital condyle (the articulation with the 1st neck vertebra)
2. Condyloid fossa
3. Jugular process
4. External auditory meatus
5. Glenoid fossa (the articulation point for the lower jaw)
6. Temporal fossa (opening for the muscles that close the lower jaw)
7. Zygomatic process of the squamosal (cheekbone)

Here's another view, oblique and from behind, showing more clearly how the zygomatic process projects out, forming the lateral side of the temporal fossa:Tooth size is apparently not a reliable indicator of body size in horses, and our modern specimen is both sub-adult and a relatively small animal. Nevertheless, in this instance we seem to be dealing with a fairly large horse; both the tooth and the skull fragment are much larger than our reference skull. If you look closely at the images above, you may also notice differences in the shape of the enamel ridges on the teeth, and somewhat different shapes and orientations in some of the cranial bones. While both these skulls are from the genus Equus, they do not represent the same species, which isn't really surprising considering that they're separated by at least 10,000 kilometers and at least 20,000 years.

Fossil Friday - horse deciduous premolar

Last summer I posted about a partial horse skull from Diamond Valley Lake that still had its deciduous premolars in place. Of course, if that horse had lived a little longer the deciduous teeth would have fallen out and been replaced with permanent teeth, with the remnant of the shed tooth left behind.The large tooth shown above is a shed deciduous premolar, probably the upper left dP4. Besides Max, for scale there is also a modern human deciduous premolar (P2 instead of P4, since humans only have 2 premolars) courtesy of my son Tim.Horses eat abrasive food such as grasses, and their teeth take a beating. By the time the 4th deciduous premolar is shed at around 3 years of age, it is worn to a nub. This is more apparent in an oblique view of the same tooth: In the Diamond Valley Lake collections we only have a few shed deciduous horse teeth. Many animals don't live enough to shed their teeth, and if they do live that long, by the time the teeth are shed they're worn so thin that they aren't as likely to survive to become fossils as a more complete tooth.This tooth will be on display in the "Stories from Bones" exhibit, opening at Western Science Center on October 31.

Fossil Friday - horse metatarsal

While the bulk of the Western Science Center's paleontology collection comes from Diamond Valley Lake, we have significant collections from other localities. One of the most interesting collections comes from Southern California Edison's El Casco Substation in northern Riverside County. This material, while probably still Pleistocene, is well over a million years old, about 4 to 6 times older than anything recovered from Diamond Valley Lake.Horses are among the most common remains from El Casco. Shown at the top is a horse left metatarsal (foot bone), seen in anterior view. The bone is lying on its side, with the proximal end (closest to the ankle) on the right and the distal end (closest to the toe) on the left. The bone is somewhat deformed, so even though this is anterior view, part of the left side is visible (the edge closest to the scale bar). Notice that the bone looks a bit swollen at one point, adjacent to where Max the Mastodon is holding the scale bar.Here's the same bone in posterior view:Again, the proximal end of the bone is on the right. In this view we can see that three bones are actually present, one large bone in the middle and a slender bone (commonly called "splint bones") on each side. In fact, all three of these bones are metatarsals.All modern horses in the genus Equus (and fossil Equus like this one) have a single functional toe on each foot. Like all mammals, horse ancestors had five digits on each hand and foot, but over time most of those digits have been lost. The one remaining functional digit is the third one, the middle finger on the hand and middle toe on the foot. These are attached to the 3rd metacarpal (hand) and 3rd metatarsal (foot).But what about those splint bones? Well, horses may only have one functional toe on each foot, but they still have remnants of the second and fourth metacarpals and metatarsals, which have not yet been completely lost. In the bone shown above, from the top you can see the 4th metatarsal, the 3rd metatarsal (by far the largest), and the 2nd metatarsal. The swollen area that is so visible in the top photo occurs near the distal tip of the 4th metatarsal, more clearly visible in lateral view:The distal tip of the 4th metatarsal is fused onto the side of the 3rd metatarsal. This is a pathological condition, is is likely a healed fracture of one or both of these bones. In medial view the 2nd metatarsal shows the normal condition, where it lies close against the 3rd metatarsal but with no fusion at the distal end:This injury was clearly not fatal, since it healed. There's no telling what caused the injury, but some type of blow to the outside of the foot is possible; maybe it was kicked or stepped on by another horse. But it does give us a snapshot, if an incomplete one, into the life of this horse.This specimen, along with many others in our collection, will be on display in our new exhibit Stories from Bones, which opens at the Western Science Center on October 31.

Fossil Friday - partial horse skull

Horses are among the most common large animals in the Diamond Valley Lake fauna, and there are several skulls in various states of preservation in the Western Science Center collection. One of these skulls stands out, however, because of the strange way in which it was preserved.The skull, which was collected at the East Dam of Diamond Valley Lake, is shown above in ventral view; essentially, you're looking at the roof of the mouth. While it has been mostly cleaned, it's still in the original field jacket, so this how the specimen was preserved in the ground. This view shows the complete set of posterior teeth (premolars and molars) from both sides of the upper jaw; the left ones are labeled below: You may have noticed that, while there are a lot of teeth in this jacket, there is no bone. Now, teeth tend to preserve better than bones, since tooth enamel is tougher than bone. But in this case we have the complete, well-preserved upper posterior dentition - 12 tooth positions - all in their correct positions relative to each other, with no bone to hold them in place! This suggests that the bone was removed after the skull was buried; otherwise the teeth would have shifted out of position. The only explanation I've been able to come up with is that, after burial, the skull was exposed to sediment or ground water chemistry that caused the bone to dissolve while leaving the teeth untouched. Differential dissolution rates are not unusual in fossils, but I don't recall previously seeing an example in which the bone was completely dissolved while the teeth were apparently untouched.This strange preservational history does provide us with certain advantages. With the bones removed, we can see features of the dentition that would normally require X-rays or CT-scans. Notice that M3 has a different appearance than the other teeth. That's because it had not yet erupted when this horse died, and was likely still imbedded in the upper jaw. This also enables us to estimate the horse's age. In modern horses M3 typically erupts when the horse is 3 to 4 years old, and this M3 was not even close to erupting, so this horse was almost certainly between 2 and 3 years old, likely closer to 2. That also means that its deciduous (milk) premolars should still be in the mouth, with the permanent premolars still developing. So let's look at a side view of the left premolars: Here's an annotated version:As expected, the permanent premolars were still developing when this horse died, and are present beneath the deciduous premolars (actually above them, since this is the upper jaw sitting upside down). Their presence also helps refine our age estimate, since P2 should erupt around age 2 to 3 years. Since dP2 is still in place but almost worn away, this horse was around 2 years old when it died.While in general I prefer to use X-rays or CT to look inside a bone, this skull gives us a great view of what was happening in this horse's dental development, and provides some interesting hints about the geochemistry of the sediments from this site.

Fossil Friday - horse pelvis

For this week's Fossil Friday we have the pelvic bones from a horse, collected from the Pleistocene deposits at the east end of Diamond Valley Lake.In most tetrapods (four-legged vertebrates that mostly live on land, and their descendants) the pelvic girdle is the primary structure for supporting the animal's weight and allowing it to move. As such this is usually a large complex structure made up of six separate bones (three on each side) plus several specialized, fused vertebrae (the sacrals). The pelvic bones are the ilium, the ischium, and the pubis, and in mammals these are usually fused to each other at a young age; the fused group of three bones is called the innominate.The field jacket shown above actually has portions of both the left and right innominates of a horse. The bone on the right side of the image is the left innominate, seen in dorsal view (from above), while the one on the left is the right innominate, seen in ventral view (from below). Below is a color-coded version of the same image:

The bones outlined in blue are the ilia; he right ilium is damaged but the left one is largely intact. The yellow outlines mark the ischia, and the red is the pubis. Only the right pubis is visible; the left one is either hidden underneath the ilium and ischium or (more likely) not preserved at all.Where the three bones come together they form a cavity called the acetabulum, outlined in green. This is the "socket" part of the hip's "ball-and-socket" joint where the leg attaches to the hip. There is also a large gap between the pubis and the ischium called the obturator foramen, which serves as a passageway for nerves and blood vessels that run through the pelvis.While there is a lot of variation is the details, all mammals that have back legs share this same basic hip structure, and even mammals that lack back legs (such as whales and sea cows) had ancestors with similar pelvic bones.

Fossil Friday - horse thoracic vertebrae

Our Fossil Friday specimen for this week is a pair of Pleistocene horse vertebrae, still in their field jacket.These two vertebrae are the sixth and seventh thoracics (see this link for an overview of vertebral regions in mammals). In the image the bottoms of the vertebrae are to the left and the tops are to the right. The sixth thoracic is the upper of the two, and you're looking at the posterior surface. In contrast, anterior surface of the seventh thoracic is visible. In the articulated skeleton these vertebrae lie roughly adjacent to the shoulder blades, as shown below in this mounted skeleton at the Texas Memorial Museum:

The long projections coming from the top of each vertebra are called "neural spines" or "spinous processes". They are rather tall in horses, especially in the anterior half of the thoracic region. These tall processes form the horse's withers, the hump present above the shoulders:

Tall neural spines are present in lots of different animals, and can serve a variety of functions including display and supporting fat bodies. They're pretty much always present in land mammals like horses that have large, heavy heads. Horses spend the vast majority of their time in a posture like the one in the image above; head to the ground, eating. It's hard to keep balanced with that heavy head stuck out like that, and ligaments that run along the neural spines transfer much of the load throughout the vertebral column. The neural spines also serve as attachment points for muscles that move and support the head, neck, and shoulders.These two vertebrae were collected in 1997 near the east dam of Diamond Valley Lake.

Fossil Friday - horse toe

Some of the most common animals in the Pleistocene deposits around Diamond Valley Lake are horses. While we don't have any complete individual horse skeleton in the WSC collections, across all our different specimens we probably have just about every bone in the skeleton represented. Shown here is an isolated horse hoof.Horses have highly specialized feet, with only a single toe on each foot. That toe is Digit III, equivalent to our middle fingers (on the front feet) and our middle toes (on the back feet). Modern horses actually also have non-functional remnants of the 2nd and 4th toes, a relict of their multi-toed ancestors.Fingers and toes are made up of a series of bones called phalanges (singular: phalanx) that articulate end-to-end with each other; the articulations are the joints in our fingers and toes. In horses, the last phalanx, or ungal, is enlarged into an arc-shaped wedge of bone to support the weight of the animal. In life, the ungal is covered with a keratinized sheath, the hoof itself, which doesn't usually preserve in fossils.The image at the top shows a horse ungal in dorsal view, with anterior at the top. The scalloped area at the back of the bone is the articulation with the next phalanx in the toe.Here' same ventral view of the same bone:

The lateral view, showing the wedge shape (the articulation with the next phalanx is at the upper right):

Note the how symmetrical this bone is in dorsal and ventral view. Horses are the only hoofed mammals in the valley that have a single toe, and as such are the only ones with a more-or-less symmetrical hoof-shaped ungal. Other hoofed mammals from these deposits, such as bison, have two toes with much more asymmetrical ungals.