Archive for December, 2009

Dood Have U Seen Avatar Yet?

31 December 2009

An excerpt from T. C. Chamberlin‘s outgoing presidential address to the American Association for the Advancement of Science published one century ago today. Enjoy 2010.

T. C. Chamberlin “A Geologic Forecast of the Future Opportunities of Our Race” Science 31 December 1909: 937-949.

Never trust a forged fossil

24 December 2009

Damsels in Eustress

23 December 2009

Together, by Jan Zajc - borrowed from Myrmecos

Two of my favorite blogs struck a peculiar resonance today.  Alex Wild has posted his favorite insect photos (plus a spider) of 2009.  All of them are very good and some, such as Jan Zajc’s photo of mating damsel flies shown above, are nearly as spectacular as some of Alex’s own work.  Meanwhile, BibliOdyssey has a selection of plates from August Johann Rösel von Rosenhof’s ‘Insecten-Belustigung’ (Insect Amusements) published serially from 1746 to 1761.

Insectorum aquatilium Classis II V.2

I can’t decide if I’m more impressesed with the amazing scenes captured by today’s best macro photographers or by the work of pre-photography nature artists like Rösel von Rosenhof who explored the same territory with paint, ink and paper.  I suppose it is the insects that impress me the most.

Oh yeah, and “eustress” is a real word.  Look it up.

Tool Academy

15 December 2009

The mainstream corporate science media is, to paraphrase Keith Olberman, going “cuckoo for coconut octopodes.”  A new paper in Current Biology shows how the cephalopod Amphioctopus deliberately carries debris (shells, coconuts, garbage) for later use as building material to construct makeshift hideouts:

Many of the news stories about this paper imply, or state outright, that this marks the first observed case of “tool use” in an invertebrate.  It’s not.  In fact, it is not even the first purported case of tool use by an octopus.

Jones (1968) described an encounter with a weaponized Tremoctopus:

Among the frequent visitors to the submerged light were a number of immature female octopods, Tremoctopus violaceus. I dip-netted one of these from the water and lifted it by hand out of the net. I experienced sudden and severe pain and involuntarily threw the octopod back into the water. To determine the mechanism responsible for this sensation 10 or 12 small (40 to 72 mm total length) octopods were captured and I purposely placed each one on the tender areas of my hands. The severe pain occurred each time, but careful observation indicated that I was not being bitten by the octopod…Subsequent examination of one of these female octopods, 72 mm long, which had been preserved, revealed fragments of Physalia [n.b. the not-quite-a-real-jellyfish better known as the Portuguese Man O’war] tentacles attached in an orderly fashion to each row of suckers of each of its four dorsal arms. (Jones 1968).

Crustaceans are also known to employ stinging cnidarians (jellyfish, anemones and the lot) for defense, both by placing them on their body as a passive defense and by directly wielding them as weapons held in their appendages.  Jones speculated that the octopus might use the Physalia tentacles to aid in prey capture as well as defense, but I’m not sure if that speculation has been confirmed by observation.

More reminiscent of the new Amphioctopus case is the report by Mather (1993) of careful selection of stones by Octopus vulgaris which were then carried back to an existing shelter and used to fortify the entrance.  The line between construction behaviors (which are well known in many invertebrates) and “tool use” is blurry, but the Octopus vulgaris case seems to show the same kind of advance planning that is so striking in the coconut octopus shenanigans.  Cephalopods are complex, intelligent even creative creatures and in retrospect these incidents are perhaps not all that surprising though no less impressive.

Another group of putative invertebrate tool users are more surprising from a cognitive standpoint.  Compared to cephalopods, insects are pretty dumb at least one the individual level.  However, ants are known to use tools in a variety of ways.  Ants will drop small bits of debris (sticks, grains of sand) into liquid food sources then carry the soaked objects back to the nest.  These makeshift vessels allow individual ants to increase their foraging efficiency by transporting larger volumes of food than they could carry in their crop alone.  Ants are also known to drop small stones in their lairs of rival insects such as burrowing wasps as an apparent tactic to impede competition (Pierce 1985).

Again, if you know something about the astonishing behavioral repertoire of ants (e.g. domesticating aphids, constructing enormous underground fungus farms, building living bridges and rafts to cross bodies of water) these cases aren’t actually that surprising – but they do serve to highlight that depending on how you define it (which is a whole other kettle of fish I won’t go into here) “tool use” does not require as much cognitive complexity as one might expect.

Some other cases of purported arthropod tool use include “sand throwing” by ant lions, corpse camouflage by assassin bugs, and the construction of special sound amplification systems from leaves by singing katydids (Pierce 1985).  Corolla spiders place stone rings around their lair.  When a potential prey item bumps into the stones, the spider can sense the vibration and leap out to capture the prey (Henschel 1995).  This is pretty clearly a modification of the prey capture strategies used by other ambush predator spiders that use systems of silken trip wires to detect prey.

You might quibble with any (or all) of these examples as representing the same sort of “tool use” as seen in Amphioctopus. But they do show a spectrum of specialized use of inanimate objects to further the biological goals of individuals and social aggregations.  In the past, we often had to rely on somewhat unsatisfactory (though often enjoyable to read) written anecdotal accounts of behavior observed in the field.  As the new paper shows, the ability to easily incorporate video in scientific communication–something that has only really taken off in the last few years–is fueling a much more sophisticated and well-documented record of animal behavior.  To put it in a coconut-shell: the evolution of human technology is transforming our understanding of the evolution of animal technology.

Cool.  Here’s a coconut octopus in a beer bottle, not really relevant, but too good to pass up.

Gods and Monsters

11 December 2009

Tawa as he appears in the Marvel Universe.

Today’s Science features the description of a new Triassic non-Hellasaurian diapsid (erm “dinosaur” if you prefer), Tawa hallae.  It is a remarkable animal represented by some really spectacular fossils, but there really isn’t much point in going into detail about it as the internet is already hemorrhaging with coverage and, as is the rule with celebrity fossils these days, dood’s already got his own multimedia enriched website (hosted by NSF…fancy).  Oh yeah, I haven’t exactly read the entire paper yet either.

But let me dust off the old cultural studies head-dress and discuss something that I have mentioned before: the introgression of ‘non-western’ cultural traditions into paleontological nomenclature.  The newly described dinosaur borrows its genus name from a Aboriginal American deity, Tawa, a Hopi sun god.  Tawa is seen as the creator of the universe in Hopi mythology and makes the occasional appearance as gift shop merchandise or Marvel superhero.

Tawa is not the first fossil genus with a divine namesake.  The protarded Azhdarchid pterosaur Quetzalcoatlus is perhaps the most familiar example, but recent years have given us Mahakala a dromaeosaur named for a Buddhist Dharmapala, and Beelzebufo an enormous extinct frog whose genus name might work out to something like “Philistine lord of the flies toad.”  Nor is this actually a new phenomenon, the 19th century gave us Sivatherium and Mauisaurus, an extinct giraffe relative named for the Hindu lord of destruction/protection and a plesiosaur named for a semi-divine Polynesian folk hero respectively. has a fairly lengthy list of mythically derived scientific names for both extinct and extant organisms.

Hopi Snake Dancers ca. 1887

All of which raises the interesting question, how those who practice the religious traditions from which these names are drawn feel about all of this?  I mean, I don’t imagine that many Ba’alists are that worried about Beelzebufo, and at any rate Ba’al has been disparaged a bit more directly over the last few millennia.  On the other hand, Hopi religious  ceremonies are still performed in the American Southwest.  I’m not sure if the authors of the new Science paper consulted Hopi elders when deciding to name a dead dinosaur after a Hopi deity, it is quite possible that they did.  Certainly I am aware of no established written (or even unspoken) ethical guidelines that cover such taxonomic practices.

Even for the blasphemous like me, there is no denying the mythic appeal of the American Southwest.  I think it is quite possible to read the name as a respectful homage to the connection between the living and the ancient cultural and natural history of the region.  Which is more of an honor, having a dinosaur named after one of your gods, or a comic book character?

Still, for a number of reasons, I can’t help but think that we won’t be seeing a ‘Muhammadosaurus‘ or ‘Yahwehia‘ any time soon.

For much more on Tawa (the dinosaur) check out this post on Chinleana including the emerging Q&A with one of the authors in the comments section.

Science 11 December 2009:
Vol. 326. no. 5959, pp. 1530 – 1533
DOI: 10.1126/science.1180350


A Complete Skeleton of a Late Triassic Saurischian and the Early Evolution of Dinosaurs

Sterling J. Nesbitt,1,2,*,{dagger} Nathan D. Smith,3,4 Randall B. Irmis,5,6 Alan H. Turner,7 Alex Downs,8 Mark A. Norell1

Characterizing the evolutionary history of early dinosaurs is central to understanding their rise and diversification in the Late Triassic. However, fossils from basal lineages are rare. A new theropod dinosaur from New Mexico is a representative of the early North American diversification. Known from several nearly complete skeletons, it reveals a mosaic of plesiomorphic and derived features that clarify early saurischian dinosaur evolution and provide evidence for the antiquity of novel avian character systems including skeletal pneumaticity. The taxon further reveals latitudinal differences among saurischian assemblages during the Late Triassic, demonstrates that the theropod fauna from the Late Triassic of North America was not endemic, and suggests that intercontinental dispersal was prevalent during this time.

1 Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA.
2 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
3 Committee on Evolutionary Biology, University of Chicago, Chicago, IL 60637, USA.
4 Department of Geology, Field Museum of Natural History, Chicago, IL 60605, USA.
5 Utah Museum of Natural History, 1390 East Presidents Circle, Salt Lake City, UT 84112–0050, USA.
6 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112–0102, USA.
7 Department of Anatomical Sciences, Stony Brook University, Health Science Center T-8 (040), Stony Brook, NY 11794, USA.
8 Ruth Hall Museum of Paleontology, Ghost Ranch Conference Center, Abiquiu, NM 87510–9601, USA. {dagger} Present address: Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA. Back

* To whom correspondence should be addressed. E-mail: <![CDATA[
var u = “nesbitt”, d = “”; document.getElementById(“em0”).innerHTML = ‘‘ + u + ‘@’ + d + ”
// ]]>

By the Late Triassic (~230 million years ago), Dinosauria had diversified into three major lineages: Sauropodomorpha, Theropoda, and Ornithischia (1, 2). In comparison to the later Mesozoic, fossils of Triassic early dinosaurs and their closest relatives are generally rare, fragmentary, and incomplete (3, 4). Indeed, the record from the Ischigualasto Formation, which provides some of the most detailed information on early dinosaur evolution (1, 2, 5), reveals that dinosaur specimens constitute less than 6% of the tetrapod assemblage (6). This depauperate fossil record has limited our understanding of early dinosaur interrelationships, diversification, and paleobiogeography, and the origin of modern avian morphologies during a critical interval of Mesozoic climate change and faunal turnover (79).

Here we report on a new carnivorous dinosaur represented by two nearly complete skeletons and several other partial specimens collected in a tightly associated small grouping at a single locality. Characterization of this taxon’s morphology and phylogenetic history enables us to solidify basal saurischian dinosaur relationships and bears directly on the evolution of early saurischian character systems, paleobiogeography, and diversification.

Systematic paleontology: Archosauria Cope 1869 sensu Gauthier and Padian 1985. Dinosauria Owen 1842 sensu Padian and May 1993. Theropoda Marsh 1881 sensu Gauthier 1986. Tawa hallae, nov. taxa.

Etymology. Tawa, Hopi name for the Puebloan sun god; hallae, after Ruth Hall, who collected many of the specimens that formed the genesis of the Ghost Ranch Ruth Hall Museum of Paleontology (GR) collections. Holotype. GR 241. A nearly complete associated but disarticulated skull and postcranial skeleton. Paratypes. A nearly complete skeleton of a larger individual (GR 242) and at least six other individuals found in the same area of the quarry [see supporting online material (SOM) (10)] including femora, pelvis, and tail (GR 155) and cervical vertebrae (GR 243). A complete right femur (GR 244) is from Hayden Quarry (HQ) site 3. Locality and horizon. Site 2, HQ, Ghost Ranch, Rio Arriba County, New Mexico, USA. The HQ has been dated to ~215 to 213 million years ago (11) and is in the lower portion of the Petrified Forest Member of the Upper Triassic Chinle Formation (12).

Diagnosis. A theropod diagnosed by the following combination of characters (autapomorphies are noted by an asterisk here and in Figs. 1 and 2): Prootics meet on the ventral midline of the endocranial cavity; anterior tympanic recess greatly enlarged on the anterior surface of the basioccipital and extending onto prootic and parabasisphenoid; deep recess on the posterodorsal base of paroccipital process*; sharp ridge extending dorsoventrally on the middle of the posterior face of the basal tuber*; incomplete ligamental sulcus on the posterior side of the femoral head and semicircular muscle scar/excavation on the posterior face of the femoral head*; small semicircular excavation on the posterior margin of the medial posterior condyle of the proximal end of the tibia*; “step” on ventral surface of the astragalus*; and metatarsal I similar in length to other metatarsals. See SOM for differential diagnosis (10).

Figure 1
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Fig. 1. The skull of T. hallae nov. taxa. (A) Reconstruction of the skull in lateral view. (B) The preserved skull elements of the holotype of T. hallae (GR 241) in left lateral view (j, qj, and the posterior portion of the mandible were reversed, and the matrix was digitally erased from the pm, mx, and n. The processes of the maxilla are present but obscured by matrix in lateral view, so they are represented here in outline). (C) Braincase of T. hallae in left lateral view (parabasisphenoid reversed). Abbreviations in the figure are as follows: antorbital fenestra (af), angular (an), articular (ar), anterior tympanic recess (atr), basioccipital (bo), dentary (d), descending process of the opisthotic (dop), external naris (en), frontal (f), jugal (j), lacrimal (la), laterosphenoid (ls), lower temporal fenestra (ltf), maxilla (mx), mandibular fenestra (mf), nasal (n), opisthotic (op), orbit (or), parabasisphenoid (pb), prefrontal (pf), premaxilla (pm), postorbital (po), prootic (pr), quadrate (q), quadratojugal (qj), supraocciptial (so), squamosal (sq), and surangular (su). Scale bar, 1 cm. Autapomorphies are noted by an asterisk.

Figure 2
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Fig. 2. Skeletal anatomy of T. hallae nov. taxa. (A) Anterior cervical vertebra (GR 243) in lateral view. (B) Right scapula (GR 242) in lateral view. (C) Right ilium (GR 155) in lateral view. (D) Middle caudal vertebrae (GR 155) in lateral view. (E) Left humerus (GR 242) in posterolateral view. (F) Complete right manus (GR 242) in posterior view. (G) Right proximal portion of the ischium (GR 155) in lateral view. (H) Right pubis (GR 155) in lateral view. The proximal portion of the apron is incomplete. (I) Left femur (GR 244) in anterior view. (J) Articulated right pes (GR 242) in anterior view. Abbreviations in the figure are as follows: acetabulum (ac), anterior depression (ad), astragalus (as), anterior trochanter (at), carpals (c), calcaneum (ca), deltopectoral crest (dp), distal tarsals (dt), femoral head (fh), fibula (fi), glenoid (gl), keel (k), metacarpal (mc), metatarsal (mt), parapophysis (pa), pubic boot (pb), pubic peduncle (pp), prezygapophyses (pre), radius (ra), supraacetabular crest (sac), tibia (ti), and ulna (ul). Matrix was digitally erased from around the manus. Scale bars, 1 cm and reconstruction scale = 0.25 m. Autapomorphies are noted by an asterisk.

Description. The holotype material is a juvenile or subadult individual, based on comparison to the largest femur among the referred material and the open braincase and neurocentral sutures. The premaxilla (Fig. 1) is similar to that of coelophysids in possessing unserrated premaxillary teeth and a narial process of the premaxilla that is elongate and forms a low angle with the alveolar margin. It differs from neotheropods by having a relatively tall maxillary process that extends dorsally beyond the posterior border of the naris as in Herrerasaurus. In dorsal view, the premaxilla-nasal suture is simple, lacking the W-shaped morphology present in Neotheropoda. Like basal neotheropods (such as Coelophysis bauri and Dilophosaurus wetherilli), a subnarial gap is expressed between the premaxilla and maxilla, but unlike these taxa, Tawa and Herrerasaurus lack an extensive antorbital fossa on the lateral surface of the maxilla. A concave narial fossa is located on the anterolateral surface of the nasal. A lateral ridge on the nasal forms the dorsal border to the antorbital fossa, similar to Eoraptor lunensis and C. bauri. Unlike most basal neotheropods, in Tawa the jugal participates in the antorbital fenestra and lacks distinct lateral ridges on the maxilla and jugal. The lacrimal is anterodorsally inclined, as in Herrerasaurus and more basal dinosaurs. The anterior process of the quadratojugal is elongate as in C. bauri but unlike that in Herrerasaurus. Tawa lacks several braincase character states present in most other dinosauriforms. Absent character states include a reduced and medially recessed descending process of the opisthotic [the crista interfenestralis (10)] and a metotic strut. A weak parabasisphenoid recess is present on the ventral surface of the braincase, similar to that in Herrerasaurus, Eoraptor, and Neotheropoda.

The vertebral column (Fig. 2) shares several apomorphic features with neotheropods. Cervical vertebrae preserve anterior pneumatic pleurocoels (as rimmed fossae) and anterior and posterior infrazygapophyseal fossae; these features are present in all basal neotheropods. The diapophyses and parapophyses of the anterior to midcervical vertebrae are close together and nearly contact. Tawa shares with Herrerasaurus pronounced ventral keels on the cervical vertebrae and elongate prezygapophyses in the distal caudal vertebrae, as in neotheropods. The dorsal vertebrae possess hyposphene-hypantra articulations and there appear to be only two sacral vertebrae.

The complete forelimb (found in articulation) and shoulder girdle share numerous apomorphic features with Herrerasaurus and neotheropods such as C. bauri and D. wetherilli. The elongate manus is particularly theropod-like, with metacarpals abutting each other along their shafts (without overlapping margins) and the presence of weak extensor pits (traits also present in the basal ornithischian Heterodontosaurus). The shaft width of metacarpal IV is reduced in Tawa, and the accompanying phalanges are greatly reduced. Moreover, digit V is completely absent, as in Herrerasaurus and other basal theropods. The manus of Tawa retains a plesiomorphically small medial-most distal carpal. The hand of Tawa also retains nine carpals, similar to the basal ornithischian Heterodontosaurus, whereas Herrerasaurus has seven and C. bauri has five.

The Tawa pelvis is generally plesiomorphic with respect to neotheropods. The preacetabular process of the ilium (GR 241) does not extend anterior to the pubic peduncle. Additionally, the anterior end is rounded, unlike the squared-off morphology of neotheropods. The supraacetabular crest projects laterally without any ventral deflection but is distally restricted in that it does not approach the articular facet of the pubic peduncle. The supraacetabular crest is continuous with the ventrolateral edge of the postacetabular process, as in coelophysoids. In contrast, the pubis displays a well-developed pubic boot similar to that present in neotheropods, Herrerasaurus, and Staurikosaurus.

The proximal articular sulcus of the femur, common to Dinosauriformes, is asymmetrically developed in Tawa and neotheropods. The fourth trochanter is symmetrical and bladelike in lateral outline, in contrast to the plesiomorphic saurischian condition of a thick asymmetrical ridge. The proximal condyles of the tibia align along the posterior edge as in Herrerasaurus and neotheropods (such as C. bauri). The tibia lacks a fibular crest, and the cnemial crest is not proximally expanded above the proximal articular surface. Two neotheropod character states—an expanded medial edge and a distinct proximodistally elongate posterior ridge—are absent on the distal end of the tibia in Tawa. The pes of Tawa is plesiomorphic in having metatarsals I to IV elongated. As in other basal saurischians, the fourth tarsal lacks a rounded posterior edge and the astragalus and calcaneum are not co-ossified. The astragalus retains a rimmed basin on the proximal surface posterior to the ascending process. However, the calcaneum of Tawa is reduced relative to the astragalus in a manner similar to that of neotheropods. It is mediolaterally compressed and completely lacks a medial process. Metatarsal I retains contact with the ankle in Tawa, as in Herrerasaurus and Eoraptor.

Cladistic analysis identifies T. hallae as the closest taxon to Neotheropoda (Fig. 3). The transitional morphology of Tawa present in both the skull and the postcranium results in the recovery of Herrerasaurus and Eoraptor as definitive basal theropods. Although initially described as early theropods (1, 2), the phylogenetic affinities of these taxa have been debated, with some authors arguing for a nondinosaurian position for Herrerasaurus (13), a nontheropod, but basal saurischian position for Herrerasaurus and Eoraptor (14), a basal saurischian position for Herrerasaurus and a theropod position for Eoraptor (15), or a basal theropod position for both taxa (5). In our analysis, Herrerasaurus forms a monophyletic Herrerasauridae with Staurikosaurus and Chindesaurus at the base of Theropoda, although clade support is weak (10). It takes six steps to recover Tawa and Chindesaurus as sister taxa. Eoraptor and Tawa form successively closer sister taxa to Neotheropoda.

Figure 3
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Fig. 3. Phylogenetic relationships of T. hallae among dinosaurs and the paleobiogeographic distribution of early dinosaur taxa [see SOM (10) for details of the analysis]. Relative temporal relationships for the early Mesozoic are indicated, with minimum ghost lineage extensions implied by phylogeny. The length of the gray bars indicates stratigraphic imprecision, and those with arrows continue through the Sinemurian. Abbreviations are as follows: Hett, Hettangian; Jr Ther, other Jurassic theropods; Rhaet, Rhaetian; and Sinem, Sinemurian.

Despite the absence of postcranial skeletal pneumaticity in the basal saurischians Saturnalia, Herrerasaurus, and Eoraptor, the presence of anterior cervical pleurocoels in Tawa and Chindesaurus supports the hypothesis that the origin of cervical air sacs predates the divergence of Neotheropoda and may be ancestral for Saurischia or possibly even Ornithodira [(16) and references therein]. The disarticulated braincase of the holotype of Tawa also documents the earliest example of an expansive pneumatic anterior tympanic recess, and the caudal expansion of this recess into the basioccipital. The weak excavation on the ventral surface of the basisphenoid in Tawa relative to neotheropods also suggests that development of the cavities associated with the middle ear sac (such as the anterior tympanic recess) preceded the elaboration of the median pharyngeal system into an expansive basisphenoid sinus in neotheropods. Despite the extensive nature of the anterior tympanic recess of Tawa, an anteromedial border to the recess is still provided by the basisphenoid and prootic. This reinforces the hypothesis that contralateral connections of the tympanic diverticula are not homologous in crocodiles and birds, and that the “interaural passage” used in sound localization by modern avians, and possibly some basal coelurosaurs (17), had not evolved by the time of the divergence of Tawa from Neotheropoda. Coelophysoid monophyly is unsupported in our analysis. Without Tawa and Eoraptor, phylogenetic analyses support a variety of characters as synapomorphies of a clade of “coelophysoid” taxa (Coelophysis, “Syntarsuskayentakatae, Liliensternus, Zupaysaurus, Cryolophosaurus, and Dilophosaurus), because they are absent in both tetanurans and neotheropod outgroups (10). With Tawa, these characters are more clearly inferred to have been primitive for Neotheropoda and later lost in the lineage leading to Tetanurae (10). Previous work (18) suggested that resolution of an inclusive Coelophysoidea may be artificial for two reasons: (i) the failure to sample Early Jurassic taxa that possess a mosaic of coelophysoid and more-derived neotheropod features, and (ii) the failure to recognize a broader distribution among basal dinosaurs of many coelophysoid synapomorphies. Tawa confirms these hypotheses because it possesses both coelophysoid traits (such as the low angle of the narial process to the premaxillary alveolar margin, the presence of a subnarial gap, and a strong ridge connecting the supraacetabular shelf and brevis shelf) and neotheropod plesiomorphies that collapse Coelophysoidea in our analysis (10). We suggest that the traditional basal theropod clade Coelophysoidea has acted as a phylogenetic vacuum cleaner, with deep theropod synapomorphies and ceratosaur/tetanuran reversals being “sucked up” and optimized as coelophysoid synapomorphies, because critical taxa were absent across the basal theropod tree. This result reiterates the centrality of new discoveries and increased taxon sampling to providing increased phylogenetic accuracy by breaking long branches [see references in (19)], is critically important for polarizing character evolution in more-derived theropod lineages, and bears directly on the magnitude of turnover in theropod faunas at the Triassic-Jurassic and Early-to-Middle Jurassic boundaries (7, 20).

The presence of multiple carnivorous theropod lineages (Chindesaurus, a coelophysoid-grade theropod, and Tawa) and an absence or rarity of sauropodomorphs suggest that the HQ saurischian assemblage was qualitatively more like that of the older Ischigualasto Formation (21), where only a single sauropodomorph specimen has been reported, than that of the overlying Los Colorados Formation, which is closer in age to the Hayden assemblage (21). In contrast, the Los Colorados saurischian assemblage contains diverse and abundant sauropodomorphs but only a single reported theropod. These patterns support the hypothesis that the evolution of Triassic dinosaur faunas was diachronous across Pangea (12).

The HQ taxa are spread throughout the stem of theropod phylogeny, and none are each other’s closest relative. This demonstrates that they do not represent a monophyletic Norian radiation endemic to the North American protocontinent. Instead, the HQ theropods are separated from each other by branches subtending taxa from other continental faunas, indicating that dispersal between these geographical regions probably occurred during the Carnian-Norian. Other contemporaneous theropod assemblages from Europe (22) and South America contain only members of Neotheropoda and do not match the diversity of theropods at the HQ.

Both parsimony (23) and likelihood-based (24) biogeographic methods for ancestral range reconstruction reject scenarios of an endemic North American theropod radiation (10). Analyses differ slightly in support for range reconstructions at individual nodes, but provide high relative support for inferring the South American protocontinent as the ancestral range through the spine of the basal dinosaur tree (10). In most analyses, the distributions of the three HQ theropods are explained by either dispersal to North America from South America or allopatric and/or vicariant speciation from an ancestral widespread range encompassing North and South America (10). This pattern is apparent in many other clades during the Late Triassic, including aetosaurs (25), crocodylomorphs (26), shuvosaurids (27), and “traversodont” cynodonts (28). The ubiquity of this phylogenetic pattern in clades encompassing markedly different ecomorphotypes argues against the presence of physiographic barriers isolating the Norian faunas of North America. Thus, the conspicuous absence of sauropodomorphs in the Norian of North America (3, 12) cannot be attributed to their inability to disperse to these areas but rather to their inability to become established in areas sampled by Late Triassic terrestrial sedimentary outcrops. Latitudinal differentiation of Norian faunas attributable to climatic differences and climatic tolerances remains an intriguing explanation for the global ubiquity of basal theropod taxa such as Tawa and the North American absence of sauropodomorphs. Indeed, recent paleoclimate models and proxy data for the Late Triassic reveal a marked dichotomy between low and high paleolatitudes (29). Alternative explanations, including smaller-scale ecological differences, community-level interactions, or facies-dependent sampling biases, cannot be ruled out, nor are these explanations mutually exclusive (12). Explaining these patterns remains an outstanding problem in early dinosaur evolution at the nexus of phylogenetic, geologic, and paleoclimatic studies of the Late Triassic.

Supporting Online Material

SOM Text

Figs. S1 to S8

Tables S1 to S5


References and Notes

Received for publication 10 August 2009. Accepted for publication 2 October 2009.