Ultradarwinian

2005-11-11T06:14:00.000-08:00

2005-10-27T04:44:00.000-07:00

2005-10-27T03:56:00.000-07:00

2005-05-21T06:40:00.000-07:00

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Searching for an Osteological Definition of the Class Aves

2004-10-16T18:55:00.000-07:00

The past decade has seen the triumphal expression of the Ostrom/Gauthier synthesis—the grand amalgam of a theropod origin of birds—with the discovery of "downy dinos" from the peerless limestones of Liaoning and elsewhere, in the People’s Republic of China. There remain persistent doubts that these "feathered dinosaurs" are in fact feathered or even attributable to Theropoda (e.g., Feduccia 1996, 1999, 2002, Ruben & Jones 2000, Czerkas et al. 2002, Lingham-Soliar 2003a, b). Though it is an unpopular opinion, the author will express nonetheless the possibility that these "filaments" in the Chinese theropods are not feathers or their antecedent homologs. Either way, it seems certain that at least some of these creatures—regardless of which side of the debate one favors—are secondarily flightless, post-urvogel birds (e.g., Caudipteryx, Protarchaeopteryx) and these discoveries whatever their meaning, force us to reconsider what makes a bird, a bird.

Feathers have always been used to traditionally define the class Aves (Feduccia 1996)—they are the apomorphy that has set birds apart from every other extant and extinct vertebrate known. Indeed, they are the quintessence of "birdness." It is to the good fortune of the scientific community that the immortal Archaeopteryx came preserved with its feathery pelage, captured in minute detail within the fine-grained limestones of the Solnhofen basin, some 150 million years ago, and as such, the avian status of Archaeopteryx has rarely been challenged. Let us examine, briefly, whether the traditional apomorphy based definition (sensu lato—this is not a digression on strict "phylogenetic nomenclature") remains sufficient in our attempt to answer the still contentious question of what exactly defines birds as a monophyletic grouping of organisms.

Within the two predominant theoretical contexts advanced for modeling the origin of birds, there have arisen candidates, which may display structures, which represent antecedent homologs to the avian feather, as seen, for instance, in the urvogel. On the one hand there are the now famous "downy dinos" and on the other, the intriguing little Triassic reptile from the former Soviet Union Longisquama insignis (Sharov 1970, Jones et al. 2001) which sports a series of extremely modified structures bearing a distinct resemblance (at least superficially) to feathers. Thus it seems, that both schools of thought in this most acrimonious of evolutionary debates, must abandon the classical usage of feathers as the defining criterion for the class Aves. At any rate, consider the dilemma if one recovers a fossil that is purported to be a bird, and yet lacks feather imprints (e.g., Chatterjee’s Protoavis).

The difficulty of defining Aves in an osteological sense is a persistent problem and yet is fundamental (James, pers. comm., Swofford pers. comm.), in my opinion, to keep abreast of new scientific discoveries which have shaken the existing conceptions of what autapomorphies define birds. Various potential suites of characters, usually correlated, have been advanced in this manner. For example, Chatterjee (1997) has favored the confluence of the orbits and infratemporal fenestrae via the breakdown of the temporal arcades as the sine qua non of "birdness" and a major synapomorphy of the class Aves. Or, in turn, Chatterjee has also advocated (1991) the loss of the ectopterygoid as an avian synapomorphy, but this seems doubtful due to the plesiomorphic presence of this element in the bony palate of Archaeopteryx (Elzanowski & Wellnhofer 1996).

Of course, part of the problem, lies in the taxonomic context in which we use the very term "Aves." Though advocates of the misled system of so-called "phylogenetic systematics" are correct in pointing out that the original sense in which Aves was used taxonomically was restricted to crown birds only, ever since 1861 the term has been implicitly understood to include all birds, extant and extinct. The migration of this designation to a node representing the avian crown so favored by adherents of the "phylogenetic nomenclature" system (e.g., Gauthier 1986) and the substitution of the new and obscure term "Avialae" where Aves once stood, has caused no end of trouble.

The practical effect of this alteration, introduced to increase utility, information content, and clarity, has been the antithesis of all these things. There is quite simply no defensible reason other than the simple and questionably egoistic desire to take credit for taxonomic hair-splitting to completely redefine the term Aves. For instance, under a crown-clade designation for the name Aves, one could not refer to Archaeopteryx—an indisputable albeit primitive, bird—as "avian," as it lies outside the crown! The absurdity of such a system is plainly evident, and thus the term Aves is within this piece used in the classical sense that most ornithologists and paleontologists continue to employ it in. For those most ardent of cladists, the seemingly most practical "phylogenetic definition" of Aves is as a node-based clade equivalent to the common ancestor of Archaeopteryx and Neornithes, plus all its descendants.
With that quagmire cleared up we can return to the pivotal issue: what are (if there are any) satisfactory osteological autapomorphies by which we might define the class Aves? I propose, though it is not by any means an original conception on my part, that the anatomy of the ear and otic capsule, and possibly the structure of the tympanic diverticula from the middle ear sac, might offer just that criterion. Witmer (2004) referred in a review of a recent study of the otic capsule of the urvogel to the nature of the ear in the earliest birds. He speculated on the unique structure of the avian ear and its association with the intense neurological demands of governing a flight apparatus and negotiating a complex three-dimensional world. This "flight ear" if indeed a quantifiable character complex, is eminently logical as a means to distinguish birds from other vertebrates, without reference to their integumentary structures. Other potential characters (e.g., furculae) may have broader taxonomic distribution and are thus of questionable utility.

Otic Capsule

The avian otic capsule is highly modified from the plesiomorphic archosaurian condition. Though Archosauria has in the past been arbitrarily redefined and restricted to the crown, usage herein is consistent with traditional definitions of this clade. The plesiomorphic construction of the otic capsule can thus be inferred from Euparkeria capensis, in which the otic capsule is composed of the prootic and opisthotic. There are two foramina piercing the stapedial recess laterally. The rostral-most foramen is the fenestra ovalis receiving the footplate of the stapes, ventral to which lies the vestibule. The caudal-most opening is the undivided metotic foramen separated from the fenestra ovalis via a robust bar of bone formed by the opisthotic (Ewer 1965, Gower & Weber 1998).

The term metotic foramen has been regarded as synonymous to "metotic fissure" but the metotic fissure is a strictly embryonic structure separating the otic capsule and basicranium of the chondrocranium. The foramen preserved into postnatal ontogeny serves rather as an exit for the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves as well as the internal jugular vein.

Both basal crurotarsal archosaurs and theropods display further refinement of this plesiomorphic condition in that the bridge of bone from the opisthotic has been vastly reduced to a slender crista interfenestralis. In sphenosuchid crocodylomorphs and Coelurosauria a new ossification arises from the rostrolateral and ventral portions of the exoccipital dorsal to the foramen for the hypoglossal (XII) nerve, projecting rostrally towards and ventral to the otic capsule. This subcapsular process encloses the rostral portion of the metotic foramen, subdividing it, and shifting the perilymphatic duct such that it opens as a secondary tympanic membrane, the so-called "fenestra pseudorotundum" (de Beer 1937). The vagus foramen is subsequently shunted caudally onto the occiput.

In Aves the otic foramina piercing the stapedial recess are arranged in a similar format, but homologies between the avian condition and that of their potential theropod (e.g., Gauthier 1986) or crocodylomorph (e.g., Walker 1972) ancestors, are highly confused. The avian otic capsule does not develop a subcapsular process sensu de Beer (1937). Rather a de novo ossification termed the metotic cartilage, forms opposite the center of the metotic fissure and ventral to the canalicular aspect of the otic capsule, fusing eventually to the exoccipital in later ontogeny (Saiff 1981, Chatterjee 1991, Gower & Weber 1998). This metotic cartilage floors the recessus scalae tympani and forms an attachment surface for the secondary tympanic membrane formed via the lateral diversion of the perilymphatic duct. Though de Beer (1937) and Walker (1985) regarded the subcapsular process of crocodylomorphs and theropods as homologous to the avian metotic cartilage, Rieppel (1985) presented compelling evidence to regard the structures as analogs. The subcapsular process is an outgrowth of the exoccipital whereas the metotic cartilage is a de novo ossification that only subsequently contacts the exoccipital. The impact of this potential lack of homology on assessments of avian origins is beyond the scope of this paper.

Further modification of the otic capsule in birds can be found in the drastic reduction of the opisthotic comparative to more basal archosaurs, forming only a rudimentary crista separating the vestibular and cochlear foramina. Most importantly, however, is the shift of the primary opening of the caudal tympanic recess (antrum pneumaticum centrale) such that the foramen for this entrance lies within the stapedial recess, caudodorsal to the fenestra pseudorotundum. Claims that a similar condition is observed in the maniraptoran theropod Sinovenator (Xu et al. 2002) are unconvincing due to ambiguous preservation of this region in the type material. The situation is most closely approached in Shuvuuia (Chiappe et al. 2002) and the crocodylomorph Dibrothosuchus (Wu & Chatterjee 1993). The former is likely a secondarily flightless bird (e.g., see Chiappe et al. 2002). Given these data, the opening of the caudal tympanic recess within the stapedial recess should be only cautiously considered an avian synapomorphy.

The Ear

The avian ear itself is essentially a tripartite structure, composed of the outer, middle, and inner ears. The outer ear is a simple tube, which conducts sound to the eardrum lying at its base. The middle ear is a cavity receiving the footplate of the stapes which transmits incident sound via the fenestra ovalis to the cochlea and inner ear, wherein are housed the sensory receptors relating to audition and balance.

From the middle ear in birds arise a series of five tympanic diverticula which invade and pneumatize either directly or indirectly the braincase, otic capsule, sides and roofing elements of the cranium, the quadrate, and the articular (Witmer 1990). These tympanic diverticula have been critical characters in discussions of bird origins (e.g., Whetstone & Martin 1979, Currie 1985, Witmer 1990) and have been most systematically reviewed by Witmer (1990). There is a variety of terminology by which the tympanic diverticula and their associated recesses are referred to, and that favored by Witmer (1990) is employed here in preference to that used by Baumel et al. (1979) in the Nomina Anatomica Avium or Whetstone (1983).

The dorsal tympanic diverticulum arises via an evagination of the middle ear dorsal to the columellar recess, passing medially to the stapedial artery, directly pneumatizing the prootic and squamosal. Contralateral communication between the right and left dorsal tympanic recesses via the skull roof is achieved in some taxa later in ontogeny through indirect pneumatization of these elements. Communication with the caudal tympanic recess within the paroccipital process and ventrally, the rostral tympanic recess within the prootic or lateral thereto, is usually observed in birds. Fibers of M. pseudotemporalis superficialis arise from the dorsal tympanic recess in some birds (Saiff 1974). The ramus occipitalis of the stapedial artery passes across the dorsal tympanic recess to exit the skull through the occipital foramen considered the homologue of the archosaurian posttemporal fenestra (Walker 1972, 1985).

The caudal tympanic diverticulum likewise arises as an evagination of the middle ear, located on the caudal wall thereof, within the columellar recess (housing the cochlea and fenestra pseudorotundum), caudodorsal to the fenestra ovalis and fenestra pseudorotundum. Witmer (1990) referred to the opening of the caudal tympanic recess within the stapedial recess (or columellar recess) bounded by the caudal and horizontal semicircular canals, utriculus, and caudal wall of the paroccipital process as the "proximal chamber." Extension of the sinus into the paroccipital process he termed the "distal chamber." Via these subsidiary diverticula the caudal tympanic diverticula proper pneumatizes indirectly the epiotic, squamosal, and basioccipital.

The rostral tympanic recess is integrally associated in birds with the parasphenoid, a highly complex bone with some seven centers of ossification (Jollie 1957). The portion of the sella of the parasphenoid, which expands dorsally is the alaparasphenoid, equivalent to the "tympanic wings" of some authors and is associated with a rostral evagination of the middle ear that passes rostromedial to the alaparasphenoid and basisphenoid, directly pneumatizing both. This is the rostral tympanic diverticulum, which creates the rostral tympanic recess sheathed by the alaparasphenoid laterally and arising rostral to the prootic. The laterosphenoid is directly pneumatized by the rostral tympanic diverticulum, which may develop contralateral communication ventral to the pituitary fossa. The region is highly vascular; the facial (VII) nerve exits the braincase within or caudal to the rostral tympanic recess and the palatine ramus of this nerve must cross the recess. The cerebral branches of the carotid arteries pass across the rostral tympanic recess within the parabasal canal.

The quadrate diverticulum of the middle ear sac, though always present in Neornithes, does not always pneumatize bone. In many diving birds (e.g., Gavia) the quadrate is apneumatic. The diverticulum invades the quadrate cartilage prior to the ossification of this element, entering in most cases ventromedially at the base of the orbital process. The last diverticulum of the middle ear invades the articular, passing between jaw muscles and penetrating the mandibular cartilage, though in many birds (e.g., Galliformes) it fails to do so and the articular in adult ontogeny is apneumatic.

Though Neornithes exhibit as a whole, consistently, pneumatic recesses and foramina associated with all five diverticula of the middle ear sac, it is difficult to ascertain with certainty, which of these recesses were present in the most basal birds. The situation is complicated by the uncertain status of Protoavis texensis. There is strong evidence to indicate that Protoavis is if not a species, at least a fauna incorporating avian material, and the braincase and otic capsule in both TTU P 9200 and TTU P 9201 seem indisputably to be those of a bird. Ad hoc alternative hypotheses, such as the material merely demonstrating the presence of coelurosaurian theropods within the Upper Triassic (e.g., Witmer 2002) can be dismissed for want of corroborative fossil evidence. If Protoavis were indeed avian, then its precocial otic capsule and braincase morphology would force serious reconsideration of the plesiomorphic distribution of tympanic pneumaticity in birds. Nonetheless, a full assessment of the Protoavis controversy cannot adequately be dealt with here, and thus for expediency, Archaeopteryx will be considered the most primitive known bird.

Archaeopteryx displays a dorsal depression on the prootic, bordered rostrally by the laterosphenoid, dorsally by the parietal, and caudally by the opisthotic and exoccipital. This depression was interpreted by Whetstone (1983) and Walker (1985) as corresponding to the dorsal tympanic recess of birds and indeed similar structures are observed on the prootics of juvenile neornithine birds (Walker 1985, Witmer 1990). Furthermore, the occipital foramen communicates with the dorsal tympanic recess in at least some modern birds, as well as the Cretaceous Hesperornithiformes, and a similar communication between the posttemporal fenestra and the dorsal tympanic recess appears to be present in the urvogel. These data strongly support the pneumatic interpretation of this depression. Nevertheless, Currie (1985) and Elzanowski (2002) expressed skepticism of the pneumatic nature of this recess based on the work of Saiff (1974), but there is little evidence to support their assertions in this regard.

The ventrolateral surface of the prootic in Archaeopteryx is pierced by several foramina within a shallow fossa, which expands rostrally across the surface of the bone ventral to the trigeminal foramen. These foramina were considered representative of a rostral tympanic recess by Whetstone (1983), Walker (1985) and Witmer (1990) and there is little caused to doubt this assessment. The prootic of Archaeopteryx was pneumatized thusly, as in neornithines, by both the dorsal and rostral tympanic diverticula.

The caudal tympanic recess is present in Archaeopteryx and of characteristically avian orientation. It opens caudodorsal to the fenestra ovalis and fenestra pseudorotundum within the stapedial recess, as noted prior. The structure directly rostroventral to the opening of the caudal tympanic recess may be a "threshold" to this recess (Walker 1985) or a distorted cotyla for the reception of a secondary quadrate articulation (the "facies articularis pro quadrato" of Whetstone 1983).

It is unknown if an articular sinus is present in Archaeopteryx, despite preservation of the articular in both HMN 1880 and JM 2257. Unfortunately in neither specimen is the dorsomedial aspect of the articular exposed, rendering it impossible to assess the presence of absence of this diverticulum (Witmer 1990). Alonso et al. (2004) reported the presence of a quadrate sinus in BMNH 37001 upon recent computerized tomography scanning.

It can thus be concluded that the plesiomorphic condition in birds with regard to tympanic pneumaticity arising from diverticula from the middle ear sac, is the presence of at least four of five pneumatic landmarks present in the neornithine skull. I will hypothesize, though until further preparation of the London and Eichstatt specimens it cannot be readily tested, that Archaeopteryx displays articular pneumaticity and that thus all five tympanic features were present in the primitive avian skull. If Archaeopteryx is considered the most primitive known bird, contralateral communication between tympanic diverticula is a derived character within Aves. If Protoavis is considered a valid taxon and a bird (e.g., Chatterjee 1991, 1997), then contralateral communication between the rostral and dorsal tympanic diverticula may be primitive to the class Aves, depending on the position of Protoavis therein.

Given the wide taxonomic distribution of many of these pneumatic recesses, it might justly be asked if any arrangement thereof can actually be advanced as synapomorphic of the class Aves. The opening of the caudal tympanic recess within the stapedial recess, as mentioned earlier, seems best interpreted as an avian synapomorphy in the absence of compelling evidence to the contrary. The rostral tympanic recess is present in crocodylomorphs amongst non-avian archosaurs (Walker 1990), and possibly theropods (e.g., Currie & Zhao 1993) though the homology the so-called "lateral tympanic recess" and the rostral tympanic recess is unclear due to the orientation of these structures relative to the prootic (Chatterjee 1991). The caudal tympanic recess itself, opening on the paroccipital process, is widespread amongst archosaurs (Chatterjee 1985, Raath 1977, 1985, Currie & Zhao 1993, Walker 1990, Chatterjee 1991). The crocodylomorph Dibrothosuchus (Chatterjee & Wu 1993) displays a configuration of the caudal tympanic recess in relation to the stapedial recess similar to that in birds. A dorsal tympanic recess is with certainty known only in crocodylomorphs and birds (Walker 1990, Chatterjee 1991) though a similar structure of contested homology is present in some theropods (Witmer 1990, Currie & Zhao 1993). Pneumatic quadrates and articulars are known in several archosaurs, including Postosuchus (Chatterjee 1985) and some theropods (Molnar 1985, Witmer 1990).

Like the middle ear, the avian inner ear shows considerable modification from the plesiomorphic archosaurian-state, related to the increased neurosensory demands of powered flight. The greater structural complexity of the avian inner ear is due to the enlargement of the system as a whole and increased differentiation of the canalicular system and cochlear process. Primitively, the bony labyrinth of the inner ear is contained within the prootic and opisthotic, while in birds (and also crocodiles), the epiotic also participates in the formation of this structure. The semicircular canals are consistently rounded in birds, while in reptiles they are angular. The vestibular cavity is highly reduced in birds, and the semicircular canals are situated away from this cavity, whereas in reptiles they surround the vestibule. The lagena has been elongated in birds forming a long, bony and tubular cochlea. Resultantly, the perilymphatic duct expands and is ultimately shifted laterally, exposed as a secondary tympanic membrane. The dorsal loops of the rostral and caudal vertical semicircular canals are completely separated from each other via the ventral migration of the common crus from which they both originate. The rostral vertical semicircular canal is elongated and usually longer than its partners in birds, possessing the largest ampulla and the majority of the canal lies within the epiotic as a function of its elongation.

The anatomy of the inner ear of Archaeopteryx was recently revealed in detail for the first time via computerized tomography studies (Alonso et al. 2004) and reveals a blend of primitive archosaurian characters and avian apomorphies. The position of the rostral vertical semicircular canal and the lateral semicircular canal are consistent with the plesiomorphic archosaurian orientation, but as in birds, the rostral canal is elongate and the caudal canal dips ventrally well below the lateral canal. The common crus is shifted in avian fashion. Alonso et al. (2004) plotted cochlea length and semicircular canal proportions in Archaeopteryx against those of other birds, non-avian archosaurs, and other reptiles. Their results found Archaeopteryx close to the range of modern birds in the proportions of the semicircular canals, and well within that of modern birds in the length of the cochlea.

If thus seems that the primitive state for the avian inner ear (again for expediency discounting Protoavis) already exhibits a shift in position of the common crus, the elongation of the rostral vertical semicircular canal, and ventral extension of the caudal vertical semicircular below the plane of the lateral canal. Additionally, the cochlea was already elongated in Archaeopteryx and in this character the urvogel was as equally refined in its acoustic acumen, as are its latter day relatives.

The Big Picture

If the distribution of characters offered herein and the presentation of homologies is accurate (and not all will agree to that), the following picture of the primitive avian ear emerges:
Pneumatic features in the braincase and cranial elements represent all five tympanic diverticula of the middle ear sac, though the presence of an articular sinus is hypothetical until new information arises. Three foramina, the fenestra ovalis, the fenestra pseudorotundum, and the primary opening or "proximal chamber" of the caudal tympanic recess pierce the stapedial recess. The metotic cartilage, a de novo ossification analogous to the non-avian archosaur subcapsular process, subdivides the metotic foramen, floors the recessus scalae tympani and provides a site of attachment for the secondary tympanic membrane.

The perilymphatic duct, with the expansion of the lagena and the formation of an elongate cochlea, has diverted laterally from its primitive course and opened as a secondary tympanic membrane within the fenestra pseudorotundum. The common crus of the inner ear has shifted ventrally, and thus the dorsal loops of the rostral and caudal vertical semicircular canals are entirely separate. The rostral canal is elongate and the caudal canal has extended ventrally below the plane of the lateral canal.

The combination of these characters seems to offer a practical method of diagnosing upon morphological as opposed to integumental grounds, the class Aves and with the possible exception of tympanic diverticula and their associated recesses, are suitable synapomorphies of birds. I particularly stress the presence of the metotic cartilage and the presence of three foramina within the stapedial recess, though the similarity in the stapedial recess of the crocodylomorph Dibrothosuchus qualifies this character.

If theropods are considered the ancestral stock of birds, then the dorsal tympanic recess and possibly the rostral tympanic recess, are avian synapomorphies. If the nearest taxa to birds are crocodylomorph archosaurs (Walker 1972) then none of the pneumatic features of the skull listed above are diagnostic of the class Aves. In either phylogeny, the structures of the inner ear listed and those of the otic capsule and stapedial recess, are diagnostic of birds. If the metotic cartilage is not homologous with the subcapsular process, then the foramen immediately behind the fenestra ovalis of the avian otic capsule is of questionable homology with that seen in both theropods and crocodylomorphs. In this case, the foramen in question may be synapomorphic of birds.

Conclusions

The ear region of birds, due to the substantial modification of the organs and structures associated with audition in response to powered flight, offers a practical means to define the class Aves via synapomorphies without reference to feathers. Caution must be exercised, however, in choice of characters related to the ear system in birds, as there is a considerable degree of homoplasy in at least some (e.g., tympanic pneumaticity) and homologies in many are as yet unclear (e.g., subdivision of metotic foramen).

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Witmer, L. M. 2004. Inside the oldest bird brain. Nature 430: 619-620.

Wu, X. & Chatterjee, S. 1993. Dibrothosuchus elaphrox, a crocodylomorph from the Lower Jurassic of China and the phylogeny of Sphenosuchia. Journal of Vertebrate Paleontology 13: 58-98.

The Impending Kenyan Domination (Old Content)

2004-10-15T22:39:00.000-07:00

With the 2004 Olympic Games quickly approaching, the latest issue of Science has a special section devoted to sport-related matters. Particularly interesting is a review [subscription required] of the physiology behind the African domination of sprinting and endurance running.

Since 1964, Kenya has racked up 38 Olympic medals, 13 of which were in men's races. Kenyan men hold the world records in a host of long-distance events, including the 3000-meter, the half-marathon and the marathon.

Three fourths of these amazing athletes can trace their ancestry back to the highland rim of Kenya's Rift Valley, right alongside Lake Victoria, the home of a group of tribes collectively known as the Kalenjin. From this pool of around 3 million people, the winners of more than 40% of all distance-running awards and nearly three times as many Olympic and World Championship distance medals as athletes from any other nation in the world is drawn.

What accounts for this remarkable feat? "Altitude is not the key to the riddle," the author avers, "because there's no difference between Kenyans and Scandinavians in their capacity to consume oxygen." What about the common training hypothesis, usually couched in the phraseology of "running-to-school"? It turns out that "Kenyan children aren't any more physically active than their Danish peers." It can't be that they try harder, because "Danes actually pushed themselves harder on a treadmill test, reaching higher maximum heart rates."

Instead, research indicates that "Kenyan runners squeeze about 10% more mileage from the same oxygen intake than Europeans can," as measured by lactate levels. The reason is that, "Kenyans have, on average, 400 grams less flesh in each lower leg. The farther a weight is from the center of gravity, the more energy it takes to move it. Fifty grams added to the ankle will increase oxygen consumption by 1%, Saltin's team calculates. For the Kenyans, that translates into an 8% energy savings to run a kilometer."

What East Africans like the Kalenjin accomplish in endurance-running, West Africans accomplish in sprinting. Perhaps the most memorable example is the well-deserved trouncing the Nazis received at 1936 Berlin games by largely West-African Jesse Owens.

Unlike their cousins to the east, "West African athletes are taller and a good 30 kilograms heavier." They also "averaged significantly more fast-twitch muscle fibers -- 67.5% -- than the French Canadians, who averaged 59%," or "endurance runners [who] have up to 90% or more slow-twitch fibers."
But all geographic human populations are genotypically identical, huh?

Full reference is:

Holden, C. 2004. Peering under the hood of Africa's runners. Science 305: 637-639.

New Dromaeosaurid Paper (Old Content)

2004-10-15T22:36:00.000-07:00

By J.A. Pourtless

This paper on dromaeosaur systematics, Senter, P., Barsbold, R., Britt, B. B., & Burnham, D. A. 2004. Systematics and evolution of Dromaeosauridae (Dinosauria, Theropoda). Bulletin of Gunma Museum of Natural History (8): 1-20, is the latest work from Dr. Phil Senter and his colleagues, and seems of significant interest. In an analysis of 101 osteological characters for 32 coelurosaurs with Allosaurus as outgroup, Senter et al. recovered a number of unconventional (vis-à-vis the most recent sheaf of coelurosaurian phylogenies, e.g., Clark et al. 2002) topologies. Deinonychosauria consists of an unnamed clade (Dromaeosauridae+ Microraptoria) + Sinovenator, which Senter et al. find to be a basal deinonychosaur and not a troodontid. Microraptoria is a new clade, defined as Microraptor not Velociraptor or Dromaeosaurus, ultimately including Sinornithosaurus, NGMC 91, and Microraptor zhaoianus, the latter which Senter et al. regard as conspecific with "Cryptovolans pauli" and "Microraptor gui". Bambiraptor is recovered as basal to this group. Dromaeosauridae itself (sensu Padian et al. 1999) cleaves into the monotypic Velociraptorinae and speciose Dromaeosaurinae.

The statistical support for their Deinonychosauria is not entirely impressive, and receives low Bremer support. Senter et al. have not made a convincing case from character-data for the dromaeosaurid affinities of Sinovenator to the exclusion of troodontids. However, the clade Microraptoria + Dromaeosauridae retrieves high bootstrap support of 75, and very high Bremer support of 6. Microraptoria itself receives moderate bootstrap and Bremer support values, while the clade therein, Sinornithosaurus + (NGMC 91 + Microraptor) receives massive bootstrap support (98) and high Bremer support (5). These values strongly indicate that the microraptorians represent dromaeosaurs and contradict my expressed opinion that they are not. Placing dromaeosaurs and microraptorians within Aves required 9-16 additional steps in Senter et al.’s analysis, which they conclude rules out their flightless status. I am hardly so convinced on that score.

Senter et al. recovers a monophyletic Bullatosauria within Arctometatarsalia (pro Holtz 1994), with high bootstrap support (83) although it lacks strong Bremer support (2). Protarchaeopteryx is considered congeneric with Incisivosaurus, for which a compelling case is made, and both this taxon and Caudipteryx fall out within Oviraptorosauria, the constitution of which in this analysis receives high bootstrap support but low Bremer support.

The data set for this analysis is largely recycled from Senter’s dissertation and will need to be reviewed for possible incorporation in part into our own. I find its conclusions interesting, especially in the polyphyly of Deinonychosauria as traditionally defined (e.g., Gauthier 1986), as well as several of its findings on the synonymous nomenclature of multiple new specimens recovered from China.

Acknowledgements:

I would like to thank Dr. Senter for generously making available a manuscript of the paper herein reviewed.

Clark, J. M., Norell, M. A. & Makovicky, P. J. 2002. Cladistic approaches to the relationships of birds to other theropod dinosaurs. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 31-61.

Gauthier, J. 1986. Saurischian monophyly and the origin of birds. Memoires of the California Academy of Sciences 8: 1-55.

Holtz, T. R. 1994. The phylogenetic position of the Tyrannosauridae: Implications for theropod systematics. Journal of Paleontology 68: 1100-1117.

Padian, K., Hutchinson, J. R., & Holtz, T. R. 1999. Phylogenetic definitions and nomenclature of the major taxonomic categories of the carnivorous Dinosauria. Journal of Vertebrate Paleontology 19: 69-80.

GFA adds: While it's perhaps true that "Senter et al. have not made a convincing case from character-data for the dromaeosaurid affinities of Sinovenator to the exclusion of troodontids," I do think they have effectivly demolished most of its putative troodontid synapomorphies, or at least those in the Theropod Working Group matrix. It remains to be seen what the topology would look like when Senter et al's character data is incorporated into a larger matrix, but Sinovenator's TWG coding errors dont affect its status as a troodontid in Holtz's very large data set, for example.

My thanks to Dr. Senter as well.

Welcome to the NEW Ultradarwinian

2004-10-15T22:31:00.000-07:00

Mblog, the old home of Ultradarwinian, has turned into a pay service. Because i'm very cheap, i've decided to make the wonderful blogger.com our new home. I'll be transfering old content ASAP.