It’s all in the toes – Why Old World monkeys change their limb posture to run
Just by looking at its limbs, you can tell that a cheetah is born to run. Not only does this felid have non-retractable claws which act like cleats on a runner's shoe - a unique feature among big cats - but it also has the familiar tip-toe limb posture which allows the carnivore to reach further with each stride during the chase. Many other fast-moving terrestrial mammals share this limb posture, technically known as "digitigrade", but at least one group employs it in an unexpected way.
Variation in cercopithecine forelimb posture. The darker-colored bones represent full digitigrade posture, while the grey ones show an approach to the platigrade condition. From Patel 2009.
Although monkeys are often depicted as inhabitants of the tropical forest canopy, in many parts of Africa and Asia cercopithecine primates, or "Old World monkeys" such as baboons and macaques, walk on the ground. As they do so they adopt a digitigrade posture, with the palms of their hands and soles of their feet held off the ground. (If their palms or soles did contact the ground, their limb posture would be called "plantigrade.") According to studies of other digitigrade mammals, it would be expected that this would help the terrestrial monkeys speed up quickly. By walking on their toes, these monkeys would be extending their limb length for longer strides and more efficient locomotion at high speed.
Yet, as scientist Biren Patel describes in a recent American Journal of Physical Anthropology paper, at least three species of ground-dwelling cercopithecine monkeys don't run with a digitigrade limb posture. When Patel observed individual olive baboons (Papio anubis), Rhesus macaques (Macaca mulatta), and patas monkeys (Erythrocebus patas) walking along a runway at varying speeds, he observed that the monkeys held their palms and soles closer to the ground as they moved faster. Contrary to what is seen in other mammals, these monkeys adopted a less digitigrade limb posture when running, and the reason why may have everything to do with the anatomy of their hands and feet.
The hands and feet of primates can be useful for grabbing, grasping, manipulation of objects, and other activities, but they are not especially well-suited to running. There is an apparent trade-off between dexterity and adaptation to moving quickly - a horse can run very fast, but is ill-equipped to manipulate objects with its single, hoof-tipped toe. Thus, among the cercopithecines studied by Patel, it appears that they bring their palms and soles closer to the ground to lessen the physical stresses involved with running. The highly mobile joints of the primate arm, wrist, and fingers cannot cope with the forces associated with running as well as those of an animal - like a cheetah or horse - which lack such flexibility, and so the cercopithechine monkeys change their limb posture to compensate.
Stick figures showing forelimb movement of a male baboon at slow (top), intermediate (middle), and fast (bottom) speeds. The red lines represent the bones of the hand, excluding the fingers. Notice how these bones of the palm come closer to the ground at higher speeds. From Patel 2009.
The significance of this locomotor shifting in Old World monkeys is that their habitual tip-toe posture is probably not an adaptation for moving fast. This strongly contrasts with what has been hypothesized for other groups of fast-moving, digitigrade animals, and it would seem that the flexibility of the primate arm is one of the chief reasons for the difference. A digitigrade limb posture may help a baboon or macaque move farther, faster when walking at relatively slow speeds, but when it comes time to run for it, bolting on tip-toe just doesn't work as well if you are going to use those hands and fingers for things like grasping branches or picking the nits off your friend's back.
References:
Patel, B. (2009). Not so fast: Speed effects on forelimb kinematics in cercopithecine monkeys and implications for digitigrade postures in primates American Journal of Physical Anthropology, 140 (1), 92-112 DOI: 10.1002/ajpa.21039
Photo of the Day #958: Small-clawed otter
A small-clawed otter (Aonyx cinerea), photographed at the Bronx Zoo.
Teddy, Charlotte, and Chase – A writer’s best friends
Charlotte
Almost every day, my black cat Charlotte jumps onto my desk while I'm writing. She usually sits still for a moment, twitching the tip of her tail and staring at me intently, before bumping her head under my busily-typing arms and placing her paws on laptop. She will have my full attention, lest a string of cat-generated gibberish start appearing in Word.
Trying to remove Charlotte from the desk is not something to be taken lightly. She's small, but she's dense, and - because she hates having them clipped - her claws are very sharp. I was painfully reminded of this a few months ago when Charlotte started to go through her routine of strutting about the desk and shedding fur everywhere. When I picked her up to put her on the floor, she flung out her arms for balance and one of her claws went right into my upper lip - I still have a little scar from where she got stuck. It is better to give her a backscratch and let her lick my fingers when she jumps up.
Still, I adore Charlotte. When I am frustrated or just don't know where to go with a difficult essay or chapter, it is nice to get a visit from my little pamplemousse (because she is so round, you see). She also kept me company while I was going over the drafts of my first book during our unusually snowy winter. I know she was mostly looking for a warm place to rest, but it was still nice to have Charlotte curl up on my lap as I went through my edits.
Teddy
Teddy is a very different animal. Where Charlotte can be by turns by both very skittish and extremely affectionate, Teddy is a mellow cat, or at least he is until he thinks he sees a bug right above my office chair. One moment I'll be typing away, the next I will just catch the sound of Teddy bounding through the apartment before he launches himself through the air onto the back of the chair, his back claws scratching at the leather for purchase. Then he'll sit there for a few minutes, sniffing the air and staring at dust motes or whatever it was he thought he saw, before settling down on the head of the chair.
Chase
And then there's Chase. At about ten years old, he's the grumpy old man of the apartment, and unless he's out of food he rarely comes to see me while I'm writing. Instead he bides his time until I settle into bed at night. Almost as soon as the lights go out, Chase creeps up the bed to sit on my chest and stare at me for a while. During the day he mostly minds his own business, except when Charlotte tries to wash him and he inevitably attacks her for her troubles.
I had not intended to wind up with three cats of my own. Growing up, nearly all my pets were of the scaly and slimy variety - a small menagerie of frogs and anoles are buried in shallow graves behind the house I grew up in. After I married Tracey, though, the cats just found their way to us - Chase was given to us by friends who could no longer keep him; we adopted Charlotte because Chase didn't like Tracey (even though Charlotte ended up liking me better, anyway); and Teddy was the one foster kitten (out of 60+) that we could not let go. Since they often follow me into whatever room I'm working in, they are my near-constant companions while I'm writing.
Some writers think of their pets as their "editors", or at least friendly ears to listen to rough drafts. I can't. If I were to start reading my manuscript to Teddy, he would probably just crack an eyelid before going back to sleep (though I imagine that he would prick up his ears if the word "treat" was involved). He's just a big, fat, friendly grey cat, but that's why I am so glad he and the other cats are here.
I don't like my day job. As you might expect, that's an understatement. When I get home from work I am usually tired, have a headache, and just don't want to do anything, but - every day - the cats come out to greet me when I walk in the door. They purr, arch their backs, and jump up to have their heads petted, and their affection makes all the difference. They break my frustration with my job or whatever else might be bothering me, and that helps me settle my mind enough that I can write. For that, I think I can take a break every now and then to give my cats a little treat or bellyrub.
Taking a second look at the “Fire Beast”, Pyrotherium
A restoration of the head of Pyrotherium, from W.B. Scott's 1913 'A History of Land Mammals in the Western Hemisphere'
Until just a few years ago, I never paid that much attention to fossil mammals. Sure, I was impressed by the saber-toothed cat Smilodon and the American mastodon Mammut americanum - badass, extinct versions of living big cats and elephants - but beyond that they never really grabbed my interest. Although clearly different from living species, many fossil mammals seemed similar enough to extant forms that I didn't think twice about passing them by in museum fossil halls - get me to the dinosaurs, please.
How foolish I was! Since I never really looked, I never understood just how extraordinary fossil mammals were, and among the strangest was a hoofed mammal from the late Oligocene (~29-21 million years ago) of South America called Pyrotherium. This "fire beast" has been known for a long time - it was first described in 1889 - but it remains an especially enigmatic animal which has fallen through the cracks of academic literature. It is one of those poor fossil creatures which are rarely studied despite being well-known for their oddball status, but a recent redescription of the only known Pyrotherium skull by Guillaume Billet places this mysterious mammal in its proper evolutionary context.
A photo of the only known skull of Pyrotherium. From Billet 2010.
There are a number of features in the skull of Pyrotherium which stand out. Among the most prominent are its teeth. Jutting out from the upper jaw of Pyrotherium are two pairs of stout incisors, matched by a single pair in the mandible, and - along with a recessed nasal cavity which looks like it could have supported a trunk - the prominence of these tusks led some paleontologists to believe that this animal was an early type of South American elephant. That hypothesis was discarded long ago, but other scholars have not done much better - from marsupials to the knobbly-headed uintatheres, the problematic Pyrotherium has been shuffled around so much that some scientists have doubted whether it can be placed among any known group of mammals at all.
In his new description of the single Pyrotherium skull, however, Billet favors one hypothesis which has popped up over and over during the last century - that Pyrotherium was a notoungulate, or part of a diverse array of bizarre hoofed mammals which evolved in "splendid isolation" on the South American continent. Among the strongest evidence for this connection is provided by a small ridge of bone between the bulbous bony compartment of the ear bones and the side of the skull. It is quite similar to the ridge of bone which exists in the skulls of notoungulates, and along with other minute features (such as the placement of small holes, called foramina, around the ear region) it seems that the skull of Pyrotherium possesses distinctive landmarks only seen amongst this mammalian group.
A line drawing of the skull of Notostylops (left) and a photo of the only known skull of Pyrotherium (right). From Billet 2010.
Identifying Pyrotherium as a notoungulate is certainly progress, but Billet goes one better. Of the known notoungulates, Pyrotherium shares some unique dental characteristics with a rabbit-sized member of the group called Notostylops. Both, have large, forwardly-directed upper and lower incisors, and even though Notostylops has two pairs of lower incisors as opposed to the one in Pyrotherium, the difference in the development of the teeth suggest that the second incisors of Pyrotherium became so large that they crowded out the first set. In a sense, the front of the jaws in Notostylops looked like what one might expect for a forerunner of Pyrotherium. (Billet does not say this in the paper, but if his observations are correct I suspect that that anatomy of Notostylops may at least partially illustrate what the first pyrotheres were like.)
This close relationship between Notostylops and Pyrotherium (and, in fact, the whole swath of weird creatures called pyrotheres) was underscored by Billet's cladistic analysis of numerous notoungulate mammals. Through comparing each animal trait-for-trait with others in a computer program, it became clear that the shared traits Billet saw in both Notostylops and Pyrotherium really did indicate their close relationship and their membership among the notoungulates. Oddly enough, the previous inclusion of another pyrothere - Proticia - may have confused similar efforts to achieve resolution in the past. Its cheek teeth were similar to those of Pyrotherium, but otherwise so little is known of it that it often created uncertainty in this type of evolutionary analysis. Until it becomes better known, Proticia is best left out of the mix.
Yet, even though Billet has solved the mystery of Pyrotherium, much remains unknown about the mammals of South America and their relationships to each other. Numerous species are only known from single, fragmentary specimens, and many of the fossils which have been found have not been studied in years. For anyone looking to tackle interesting problems in mammalian paleontology, the questions surrounding the evolution of the notoungulates and other South American mammals present plenty of opportunities.
Billet, G. (2009). New Observations on the Skull of Pyrotherium (Pyrotheria, Mammalia) and New Phylogenetic Hypotheses on South American Ungulates Journal of Mammalian Evolution, 17 (1), 21-59 DOI: 10.1007/s10914-009-9123-0
Photo of the Day #957: Zeff the Amur Tiger
Zeff the Amur tiger (Panthera tigris altaica), photographed at the Bronx Zoo.
Prehistoric “bear-dog” Amphicyon was built like a “bear-lion”
A reconstruction of the "bear-dog" Amphicyon ingens, on display at the American Museum of Natural History.
In surveying the diversity of living organisms, the 20th century evolutionary theorist Theodosius Dobzhansky did not see "a formless mass of randomly combining genes and traits." Instead he perceived pockets of discontinuity organized around available ecological niches - clusters of occupied "adaptive peaks" separated by rifts and valleys representing vacant spots in the natural world. Big cats, for example, formed their own mountain chain distinct from the cluster of dog species, and dog species were likewise separated from bears. By looking at the ecological and evolutionary topography of species, life's pattern would emerge.
In such a view, it might be supposed that a valley between two clusters of peaks might be an evolutionary "no man's land". There is no living creature intermediate between a dog and cat, for example, so it seemed apparent that there was no niche for such a creature among the Carnivora (thus barring its evolution). Yet, when we take the fossil record into account, the landscape changes - extant mountain ranges erode away and what are present-day valleys are thrust up into jagged peaks. The topography of the past does not neatly match that of the present - there is no set array of niches which bring about the same forms over and over again - and the extinct carnivore Amphicyon is an imposing embodiment of the disjunction between the past and the present.
Popularly called a "bear-dog", the several species of Amphicyon were large mammalian carnivores which ranged over much of the Northern Hemisphere (as well as points in southern Africa) between 20 and 1.8 million years ago. Their popular name comes from their anatomy and their place in the Carnivora family tree. They sat on the caniform ("dog-form") side of a great split among the Carnivora, and Amphicyon species had robust, bear-like features while also being well-suited to chasing after prey as a dog might. Interestingly, however, paleontologist Christine Argot has proposed that the popular name for Amphicyon might be a misnomer. It might be more accurately called a "bear-cat."
The basis for Argot's new study is a nearly-complete specimen of the species Amphicyon major recovered from the approximately 15-13 million year old rock of Gers, France. Complemented with a few parts from other Amphicyon individuals from the same deposit, this skeleton is perhaps the best available guide to the species, and Argot compared its bones to the corresponding anatomical puzzle pieces in the grizzly bear, lion, and wolf. By examining Amphicyon in relation to these three distinct kinds of living carnivore, Argot could set about determining what kind of hunter it might have been.
Comparisons of the left scapulae of A) Amphicyon major B) a grizzly bear C) a lion D) a wolf. From Argot, 2010.
At the very least, Argot found that the "bear" part of the "bear-dog" moniker fits the prehistoric carnivore well. The shoulderblade of A. major, especially, was large and expanded much like a grizzly bear's, and even possessed an expansion of the bone called a postscapular fossa indicative of muscle attachments which are unique to bears among carnivorans. Its limbs were also very robust - much like a bear's - and the hands and feet of A. major were likewise suited to walking on the whole of these appendages rather than standing on tip-toe like cats and dogs.
The latter part of the "bear-dog" name did not hold up so well. In almost all the bone-for-bone comparisons, the wolf bones were far more slender than those of A. major. There is no doubt that Amphicyon and its close relatives shared a closer relationship with dogs than with cats, but the widest disparity between the species studied by Argot was between the extinct mammal and the living wolf. Clearly, A. major was a different kind of predator.
Interestingly, however, certain aspects of A. major anatomy resembled their counterparts in the lion, especially in the hips and spinal column. When looked at from above, for example, the hips of A. major don't have the same flaring expansions of bone seen in bears or dogs - instead, they have the more streamlined form of lion hips. Likewise, the first two vertebrae which articulate with the skull in A. major - the axis and atlas - most closely resemble the same bones in the neck of a lion. As Argot notes, this may mean that A. major was capable of moving its head through a similar range of motion as the big cat.
A restoration of Amphicyon by Charlene Letenneur. From Argot 2010.
When looked at as a whole, it appears that the limbs of Amphicyon were very similar to those of a grizzly bear while parts of its spine and hips were lion-like (with its long tail being unlike that of any of the taxa studied). It appears to be a hunter well-suited to grappling with prey, and may have used its strong arms to wrestle its victims to the ground.
Despite the anatomical details of A. major elucidated by Argot, however, just what it hunted and how it did so is somewhat difficult to determine. The site from which the specimen Argot studied was found used to be a body of fresh water adjacent to a hot open-woodland environment where relatives of living mammals mingled with representatives of now-extinct groups. There were elephants, rhinos, saber-toothed cats, deer, chalicotheres, antelope, and other mammals, but Amphicyon major was the largest of the carnivores, being the size of a large leopard or a small female lion. Given what is known about the hunting habits of living carnivorous mammals, it is likely that it hunted prey as big as - if not bigger than - itself, although studies of the microscopic wear left on A. major teeth hint that it would not have been above chewing some plant food some of the time.
How it would have tackled its prey is presently a mystery. There were many carnivores present in the same habitat, and in places where this is the case today (such as the African savanna) predators are pushed to use different techniques and pursue different prey. Based upon limb proportions, it appears that A. major might have been an efficient runner which ran after its prey at relatively low speeds until it could catch up and grapple its victim to the ground. Then again, it has been suggested that predatory mammals during this time were primarily ambush predators which stalked their prey before leaping out of the bushes. Unfortunately, such hypotheses are difficult to test, but given the bear-like nature of its limbs I could easily envision A. major chasing after a prehistoric deer until it got close enough to bring it to the ground and deliver a crushing bite.
Had Amphicyon survived to the present day, it would occupy a strange place among Dobzhanky's ranges of adaptive peaks. While more closely related to dogs and bears than to cats, in form (and perhaps ecological role) its own evolutionary hill would have poked up between cats and bears. This is why the fossil record is essential to understanding evolution. Living species are all too easily shuffled into discrete components with considerable gaps between them. When the evidence of the fossil record is brought to bear, however, unique formations entirely unknown today spring up, sometimes serving as intermediate forms between modern discontinuities. It provides the context for the natural world which we find ourselves in.
Christine Argot (2010). Morphofunctional analysis of the postcranium of Amphicyon major (Mammalia, Carnivora, Amphicyonidae) from the Miocene of Sansan(Gers, France) compared to three extant
carnivores: Ursus arctos, Panthera leo, and Canis lupus Geodivertistas, 32 (1), 65-106
Photo of the Day #956: Lowland gorilla
A young lowland gorilla (Gorilla gorilla), photographed at the Bronx Zoo.
Prehistoric mammal Prolibytherium had a “butterfly face”
The restored heads of a male (A) and female (B) Prolibytherium. Illustration by Israel M. Sanchez. From Sanchez et al, 2010.
Exactly what Prolibytherium magnieri was, no one is quite sure. Since the time it was described in 1961, the 17-16.5 million year old mammal from Egypt and Libya has been closely allied with prehistoric relatives of deer (Palaeomerycidae), ancient giraffes (such as Sivatherium), and a motley group of giraffe cousins (Climacoceratidae). Many experts now agree that it probably belonged to the latter group, but it was so bizarre that classifying it has been an extremely frustrating task.
What made Prolibytherium so strange were the huge structures growing out of its face. These bony appendages - which would have been covered by skin and hair in life - formed a butterfly-shaped plate which made Prolibytherium look like something Dougal Dixon might cook up for one of his speculative zoology books. Two bony support ribs stuck out towards the front of the face, and two more jutted out towards the back of the head, with a "web" of bone growing out from these structures to create the impressive ornament.
That Prolibytherium sported such hardware has been known for quite some time, but similar fossils found in the same deposits presented paleontologists with a petrified puzzle. These other fossils showed some similarities to the structure of the Prolibytherium appendages, but they were nowhere near as wide. Over the years scientists have hypothesized that this second kind of ornament might represent a member of the Palaeomerycidae, a juvenile or female Prolibytherium, or even the potential ancestor of Prolibytherium. Without more complete fossils, it was difficult to know which hypothesis was correct, but a short communication just published in the Journal of Vertebrate Paleontology reports that the mystery has been solved.
When the scientists behind the new study placed the complete Prolibytherium facial ornament next to the the smaller, cylindrical structures, the mystery ornaments looked very much like the supporting rods of the Prolibytherium cranial appendage. More than that, the structures were almost an exact match in terms of bone structure and their placement on the skull - the two types of ornaments belonged to the same type of animal. Being that there was no indication that a second species of Prolibytherium was present at the sites, the scientists propose that the second ornament type represents a female animal (since the degree of bone development on them indicates that the animals had been adults).
In effect, female Prolibytherium carried around the spiky framework which was the basis for the expanded, male ornaments. If the hypothesis that Prolibytherium belongs among the already strange Climacoceratidae, then it would even more of an oddball as the first known member of this group to exhibit such a distinctive degree of difference between the sexes.
Naturally, such conspicuous ornaments - both among males and females - beg the question of what they could have been used for. In living bovids, species in which males and females both have horns often use them for defense, but in these cases there is a decreased difference between the sexes. (The horns have to stay functional, so there is a limit to how far sexual selection can push them.) Clearly this pattern does not fit Prolibytherium, especially since the skin-covered wings of the males do not look like they would have been effective defensive weapons. Instead, it may be that the structures were primarily used for display or species recognition. Sexual selection - be it female choice or competition between males - appears to have created the two different types of ornaments in Prolibytherium, but the fact that females have conspicuous facial ornaments raises questions about how such structures evolved in the first place - the present function of a structure does not always explain its origin. The discovery made in the new report is fascinating, but Prolibytherium remains an enigmatic animal.
cranial wings among the males seem to beg the question of "What were they used for?" That is another mystery. Horns, antlers, and other such appendages are used for a variety of functions among mammals, not all of them mutually exclusive. In the case of Prolibytherium, it has been proposed that males could have used their cranial wings to wrestle with other males, but it is difficult to imagine what such conflicts would have been like. It may be that they evolved as display structures, perhaps as a result of female choice or competition between males for mates (the male with the biggest head wings, wins). Since Prolibytherium is long gone and there is no living animal quite like it, we may never know,
Sanchez, I., Quiralte, V., Morales, J., Azanza, B., & Pickford, M. (2010). Sexual dimorphism of the frontal appendages of the early Miocene African pecoran Prolibytherium Arambourg, 1961 (Mammalia, Ruminantia) Journal of Vertebrate Paleontology, 30 (4), 1306-1310 DOI: 10.1080/02724634.2010.483555
Fossil primate Saadanius provides context for the ancient ape/Old World monkey split
A gelada (Theropithecus gelada), one of many kinds of Old World monkeys. Photographed at the Bronx Zoo.
Imagine that there was no primate fossil record. No hominins, no Proconsul, Dryopithecus, no Eosimias, no Darwinius -- nothing. Now, given this dearth of fossil material, you could be excused for systematically organizing primates according to the stark divisions apparent between living species. Our species, while clearly a primate, would seem to stand by itself, and apes would be distinct from monkeys. Likewise, there would be a great division among monkeys - those of the Old World and those of the New - and the various lemurs, lorises, and galagos would fall within their own separate designation. Much like us, tarsiers would seem like another oddball group off by themselves. The terms "human", "ape", and "monkey" would have discrete meanings without any bleeding between categories.
While the basic premise is thankfully false - our knowledge of the primate fossil record expands with each year - primates were often organized in this kind of top-down manner. It was not so long ago that our species, and our species alone, was the sole living member of the Hominidae, and all apes were lumped within the Pongidae. This was the taxonomic signal of what paleoanthropologists believed to be a deep divergence between humans and apes, perhaps as long as 22 million years, but since that time genetic tests and new fossil discoveries have confirmed that we actually belong within the ape group, with our last common ancestor with chimpanzees existing around six million years ago. Expanding this general rule to primates as a whole, by the late 20th century it no longer became profitable to organize the group according to species in existence. The numerous expired branches of the primate family tree had to be taken into account, one in which words like "monkey" and "ape" can refer to more inclusive groups of primates of which we are a small part.
The trouble is that we are still stuck with the baggage of the old "top-down" system. For much of the public, a term like "monkey" identifies a specific grade of primate which cannot also be applied to apes or us - only a monkey can be a monkey. Without the right vocabulary, however, it can be exceedingly difficult to make sense of new fossil primate discoveries, especially since primate evolution is best represented by a wildly branching tree in which many lineages have been entirely eliminated. Such is the case with Saadanius hijazensis, a newly-discovered fossil primate which is causing paleontologists to revise what they thought about the timing of one part of primate evolution.
The skull of Saadanius, as presented in Zalmout et al, 2010. a, Cranium in anterior view. b, Cranium in lateral view, anterior to the left. c, Ventral view of cranium, anterior to the left. d, Anterior view of right temporal bone, ventral at top, lateral to the right. e, Ventral view of right temporal bone, anterior at top, lateral to the left. f, Medial view of right occipital condyle, anterior to the left. g, Ventral view of right occipital condyle, anterior to the left. cf, carotid foramen; egp, entoglenoid process; gf, glenoid fossa; pgp, postglenoid process; smf, stylomastoid foramen; te, tubular ectotympanic.
Known from a partial skull discovered near the Red Sea coast in Saudi Arabia, Saadanius lived in a freshwater mangrove swamp about 29-28 million years ago. As Iyad Zalmout and co-authors state in the introduction of the paper, it is the most substantive discovery of its kind for this part of earth's history - the later part of the Oligocene - but what has caused Saadanius to be catapulted into the headlines is its relationship to other primates. It appears that Saadanius sat close to the what would have been the last common ancestor of living Old World monkeys and apes, and thus provides scientists with some important clues about what the lineage leading up to this great split was like.
In technical terms, Saadanius was a catarrhine primate. Today, catarrhines are represented by Old World monkeys and apes (which together compose what is called the "crown group" of catarrhines), but there were also many lineages of catarrhines in the distant past which became extinct. Saadanius belonged to one of these groups, a catarrhine but not a member of the crown group, and therefore belonged to what is called a "stem group." In other words, Saadanius can be roughly described as a fossil monkey which was closely related to the last common ancestor of Old World monkeys and apes, but was not itself an Old World monkey or ape. (And, appropriately enough, Saadanius is actually derived from the Arabic term saadan used for both monkeys and apes.)
A hypothesis of how Saadanius relates to other catarrhine primates. In this evolutionary tree, Sadaanius is a stem catarrhine which sits just outside the split between the crown catarrhine groups (which, here, includes other extinct lineages like the Dendropithecidae). From Zalmout et al, 2010.
The reason the paleontologists who described Saadanius know this is because of how it compares to early apes, early Old World monkeys, and enigmatic groups which also fall around the crown catarrhine split. On the one hand, Saadanius lacks the conspicuously large canines, frontal sinuses, the broadened front of the palate, and other features which distinguish the earliest apes, yet, on the other hand, Saadanius has a peculiar condition of the inner ear called a tubular ectotympanic. This feature has only been seen among crown catarrhines before, so if it possesses it, then Saadanius was obviously close to the split. Overall, the skull of Saadanius resembles those of other early catarrhines, but this feature (along with some similarities in the way its face is constructed) place it much closer to crown catarrhines. Had its inner ear not been preserved, or had it only been known from teeth, it probably would have been placed further away from the split.
Significantly, Saadanius may also provide a test of hypotheses about when the earliest Old World monkeys and apes split from each other. On the basis of fossil evidence, the split has often been estimated between 25 and 23 million years ago, but divergence estimates based on genetic data have proposed an even earlier date - between 35 and 29 million years ago. The discovery of Saadanius, so close to the crown catarrhines, falls between these two estimates, putting the divergence between 29 million years ago and the earliest known crown catarrhines from 23-20 million years ago. As always, the discovery of other primates from within this timespan will be needed to sort things out, but Saadanius has provided paleontologists with a key with which to compare some of the traditionally enigmatic catarrhines from the later part of the Oligocene. It wasn't an Old World monkey, and it wasn't an ape, but it provides the context for the origins of both lineages.
Zalmout, I., Sanders, W., MacLatchy, L., Gunnell, G., Al-Mufarreh, Y., Ali, M., Nasser, A., Al-Masari, A., Al-Sobhi, S., Nadhra, A., Matari, A., Wilson, J., & Gingerich, P. (2010). New Oligocene primate from Saudi Arabia and the divergence of apes and Old World monkeys Nature, 466 (7304), 360-364 DOI: 10.1038/nature09094
Photo of the Day #955: Ring-tailed mongoose
A ring-tailed mongoose (Galidia elegans), photographed at the Bronx Zoo.