Astragalus Dilemma ?

[jpevahouse]

My post comparing astragalus (ankle bone) generated some doubt/question as to the proper identification of a hyracodon astraglus I bought recently. So, I bought a subhyracodon astragalus from the same person found in the same area for comparison. Doubt arose from some forum users if the fossil I bought as a hyracodon astragalus since it agreeably doesn't have the characteristics of a rhino type animal. The hyracodon is thought to be the earliest ancestor of the rhino or general classification of such animals.

The fossils I picture are from Pennington County, South Dakota, White River formation, Oligocene era.

I searched the internet high and low, inside and out, relentlessly seeking a good clear picture of an hyracodon astragalus and failed miserably. I read many articles describing the hyracodon and in the end seemed to know less than when I started. The internet is a vast ocean of information but I find more and more conflicting, incomplete or simply misleading information. Getting clear definitive data can be difficult or impossible.

My suspicion is maybe it could be somewhat misleading to classify the hyracodon in the rhino family of animals since most everything I read said it was more horse like in appearance. The subhyracodon astragalus is amazingly similar to that of a modern horse. Yet, the subhyracodon as described was a true rhino in appearance so it should be no surprise it's astragalus looks like one of a rhino.

The other part of this story is I bought the two astragalus from a well established, well respected dealer who sells a lot of Oligocene era fossils. He also operates a web site dedicated to the White River formation. If anyone should know I figure it's him and have always trusted his judgement concerning the identification of the fossils I bought from him. Sure, no one's perfect but that would be a major boo boo for someone with his experience and reputation.

Edited September 6, 2014 by jpevahouse

[RichW9090]

JP, I'm unclear as to which bone is which in the picture you post, but I can say this: The one on the left is a perissodactyl, the one on the right is an artiodactyl. Many of the postcranial bones of both groups are figured in the volumes on the White Rive Oligocene published by the American Philosophical Society (unfortunately, I recently sold most of my Paleogene library, so I no longer have them to refer to). Simply because they are the most common artiodactyl in the White River, I'd check oreodont.

[jpevahouse]

JP, I'm unclear as to which bone is which in the picture you post, but I can say this: The one on the left is a perissodactyl, the one on the right is an artiodactyl. Many of the postcranial bones of both groups are figured in the volumes on the White Rive Oligocene published by the American Philosophical Society (unfortunately, I recently sold most of my Paleogene library, so I no longer have them to refer to). Simply because they are the most common artiodactyl in the White River, I'd check oreodont.

The bone on the right is the astragalus in question. Agreed, it looks like an oreodont astragalus. I have a couple of oreodont astragalus and the bone is over twice their size. It' similar to astragalus of other species of grazing/foraging mammals of the Oligocene era. It's also, similar to a modern pig, deer or camel astagalus in appearance.

The question arises, was the hyracodon a true rhino or rhino like animal with the same skeletal characteristics? Or was it a prototype?

Edited September 6, 2014 by jpevahouse

[Fruitbat]

Hyracodon is actually a member of the Hyracodontidae, sometimes called 'running rhinos', rather than the Rhinocerotidae (the 'true' rhinos). Both are currently included in the Superfamily Rhinocerotoidea. The Hyracodontidae were apparently built for speed and were, as far as I know, completely hornless. The family includes the so-called 'Indricotheres' or 'Baluchitheres' which were the largest terrestrial mammals ever.

As for the two astragali in your picture...the one on the right is definitely an artiodactyl and not a perissodactyl (rhinos, horses, tapirs, 'titanotheres',etc.). Perissodactyl astragali typically have a single 'pulley' like the one on the left in your picture while artiodactyls have a double 'pulley' arrangment. Your dealer did, in fact, make a bit of a mis-identification.

-Joe

[RichW9090]

Size of the artiodactyl astragalus doesn't much matter, since oredodonts range in size from tiny to massive. I'd still start with that.

[jpevahouse]

Size of the artiodactyl astragalus doesn't much matter, since oredodonts range in size from tiny to massive. I'd still start with that.

A question arose about the identity of the bone and I'm open to any possiblities. Hyracodon is one of the most common animals found in the White River formation. Someone, somewhere has an hyracodon astragalus which has been definitively identified. An example for comparison would put the question to rest.

Also, though there were numerous species of oreodont most were small animals. The promercychochoerus carrikeri the largest about the size of a cow lived over a very short period of about 1 million years.

Edited September 6, 2014 by jpevahouse

[Fruitbat]

jpevahouse...

There is no question that the astragalus on the right in your picture is not from Hyracodon. It is an artiodactyl astragalus, regardless of which artiodactyl it came from. Is the 2 1/4 inch measurement from top to bottom or from side to side?

-Joe

[siteseer]

Among modern mammals, the rhino has rather long fossil history. The earliest known form, Hyrachyus, actually dates back to the Early Eocene. It lived at the same time as the earliest horse, the earliest titanothere and early relatives of the tapir. I think the earliest chalicothere also lived then. These mammals would have resembled each other in life as their skeletons look much alike as well. It takes an expert to distinguish their teeth. The various features of their skeletons, including the single-pully astragalus, unite them as members of the Perissodactyla.

Also, remember that common names like "rhino" and "horse" have been given to modern animals. We don't have common names for animals that lived before humans but we can use those terms for their extinct relatives going back to their earliest ancestors. Hyracodon is a rhino in the same way Mesohippus is a horse.

Don't be too tough on the dealer. It could have been an oversight, but yeah, whoever it is should know the differences in astragulus shape. The other thing to keep in mind is that isolated bones can be a challenge to identify. I'm not sure you can always name an astragulus to genus especially if it fits within the size range of two horses, a couple of rhinos, and a tapir in the same fauna. I have a small astagulus from the Late Miocene of Nebraska. It looks like it belongs to a carnivoran so I am leaning toward one of the extinct dogs mostly because cats are more rare than dogs.

Jess

My post comparing astragalus (ankle bone) generated some doubt/question as to the proper identification of a hyracodon astraglus I bought recently. So, I bought a subhyracodon astragalus from the same person found in the same area for comparison. Doubt arose from some forum users if the fossil I bought as a hyracodon astragalus since it agreeably doesn't have the characteristics of a rhino type animal. The hyracodon is thought to be the earliest ancestor of the rhino or general classification of such animals.

The fossils I picture are from Pennington County, South Dakota, White River formation, Oligocene era.

I searched the internet high and low, inside and out, relentlessly seeking a good clear picture of an hyracodon astragalus and failed miserably. I read many articles describing the hyracodon and in the end seemed to know less than when I started. The internet is a vast ocean of information but I find more and more conflicting, incomplete or simply misleading information. Getting clear definitive data can be difficult or impossible.

My suspicion is maybe it could be somewhat misleading to classify the hyracodon in the rhino family of animals since most everything I read said it was more horse like in appearance. The subhyracodon astragalus is amazingly similar to that of a modern horse. Yet, the subhyracodon as described was a true rhino in appearance so it should be no surprise it's astragalus looks like one of a rhino.

The other part of this story is I bought the two astragalus from a well established, well respected dealer who sells a lot of Oligocene era fossils. He also operates a web site dedicated to the White River formation. If anyone should know I figure it's him and have always trusted his judgement concerning the identification of the fossils I bought from him. Sure, no one's perfect but that would be a major boo boo for someone with his experience and reputation.

[siteseer]

If you are getting serious about collecting mammal fossils, you should consider building a library of good references. You should get a general book about mammals. If you're really into Oligocene mammals or rhinos in particular, you should consider this book:

http://www.amazon.com/Evolution-North-American-Rhinoceroses/dp/0521832403/ref=la_B001IGO7HM_1_17?s=books&ie=UTF8&qid=1410064401&sr=1-17

There could be a figure of a Hyracodon astragulus in there.

I talked to forum member fossillarry the other day and he thought it was either Archaeotherium or an anthracothere (looked too big to be Leptomeryx, Poebrotherium, or the oreodonts of the time). It also depends on the layer. Most of the Badlands material from South Dakota or Nebraska comes from the Early Oligocene. The larger oreodonts come from Late Oligocene sites.

Jess

A question arose about the identity of the bone and I'm open to any possiblities. Hyracodon is one of the most common animals found in the White River formation. Someone, somewhere has an hyracodon astragalus which has been definitively identified. An example for comparison would put the question to rest.

Also, though there were numerous species of oreodont most were small animals. The promercychochoerus carrikeri the largest about the size of a cow lived over a very short period of about 1 million years.

[jpevahouse]

Among modern mammals, the rhino has rather long fossil history. The earliest known form, Hyrachyus, actually dates back to the Early Eocene. It lived at the same time as the earliest horse, the earliest titanothere and early relatives of the tapir. I think the earliest chalicothere also lived then. These mammals would have resembled each other in life as their skeletons look much alike as well. It takes an expert to distinguish their teeth. The various features of their skeletons, including the single-pully astragalus, unite them as members of the Perissodactyla.

Also, remember that common names like "rhino" and "horse" have been given to modern animals. We don't have common names for animals that lived before humans but we can use those terms for their extinct relatives going back to their earliest ancestors. Hyracodon is a rhino in the same way Mesohippus is a horse.

Don't be too tough on the dealer. It could have been an oversight, but yeah, whoever it is should know the differences in astragulus shape. The other thing to keep in mind is that isolated bones can be a challenge to identify. I'm not sure you can always name an astragulus to genus especially if it fits within the size range of two horses, a couple of rhinos, and a tapir in the same fauna. I have a small astagulus from the Late Miocene of Nebraska. It looks like it belongs to a carnivoran so I am leaning toward one of the extinct dogs mostly because cats are more rare than dogs.

Jess

Agreed, good advice. My original post comparing astagalus was to stimulate discussion about the similarity of bones among diverse animals and the difficult of identification. That post inspired this one because of questions about the ID of the one astagalus as hyracodon.

[jpevahouse]

If you are getting serious about collecting mammal fossils, you should consider building a library of good references. You should get a general book about mammals. If you're really into Oligocene mammals or rhinos in particular, you should consider this book:

http://www.amazon.com/Evolution-North-American-Rhinoceroses/dp/0521832403/ref=la_B001IGO7HM_1_17?s=books&ie=UTF8&qid=1410064401&sr=1-17

There could be a figure of a Hyracodon astragulus in there.

I talked to forum member fossillarry the other day and he thought it was either Archaeotherium or an anthracothere (looked too big to be Leptomeryx, Poebrotherium, or the oreodonts of the time). It also depends on the layer. Most of the Badlands material from South Dakota or Nebraska comes from the Early Oligocene. The larger oreodonts come from Late Oligocene sites.

Jess

The logic seems sound and points to the bone not being from a hyracodon. I read in various articles that even an animal as common during the Oligocene as the oredont has been defined and redefined many times over the last 100 years, now thought to be more related to the camel. Believing scientific assumptions and theories are enshrined in stone is a mistake considering all that scientist admit they still don't know about the past, like mass extinctions. One article stated the oreodont had not really been thoroughly studied and early studies used to classify the various species were often incorrect.

I read a couple of articles which stated that extinction of megafauna during the late Pleistocene was definintely a result of hunting by early people, a theory which in fact has very little factual basis and is hotly debated by scientist. Everyone, even scientist have their opinions but facts are much better if we can find them.

Edited September 7, 2014 by jpevahouse

[Fruitbat]

jpevahouse...

You're definitely right about science as being in a constant state of flux...but that is the essence of science itself! As newer (and perhaps better) information is gained, concepts are continuously revised to accomodate the new data. Your example of the phylogeny of the oreodonts is a classic one. In their seminal works on oreodonts, Schultz and Falckenbach erected a vast number of genera and species, many of which were later found to be based on variations caused by the vagaries of preservation or on normal individual variation. Researchers are still in the process of trying to wade through the muddle. One thing that is clear, however, is that oreodonts are artiodactyls...whether more closely related to the tylopods (camels, llamas, etc.) or the suids (pigs and their relatives) is still being debated.

Most of the time the 'opinions' of scientists are based on the information they have available to them...and in the case of extinct organisms that information is often woefully inadequate. Consider the incredibly small odds of an individual organism fossilizing in the first place. At best we are given snapshots into their lifestyles and all we can do is make educated guesses with the available evidence. If new discoveries throw those hypotheses into the waste bin then so be it! Having the 'facts' is often not all it is cracked up to be...because 'facts' aren't cast in concrete either! What is a 'fact' today may be laughable tomorrow. After all, Schultz and Falckenbach proposed their phylogeny of the oreodonts based on the 'facts' that they had available to them. They weren't just making up all those genera and species because they wanted to confuse everybody!

In the final analysis, your Hyracodon astragalus is not from a rhinoceros of any kind...it is from an artiodactyl (though which artiodactyl is a matter for more study).

-Joe