Weekly Spotlight: Daphoenus

29 05 2010

Good tidings and well-wishes!

At longtime reader Zach Miller’s request to cover an Amphicyonid “bear dog”, I’ve dedicated this post to one of the family’s most widely-preserved North American genera, Daphoenus sp.

Daphoenus sp. skeleton.

The genus was named by the famed American anatomist and paleontologist Joseph Leidy in 1853, with the first scientifically-described species being D. vetus. Five additional species have subsequently been established: D. hartshorianus, D. lambei, D. ruber, D. socialis, and D. transversus.

D. vetus skull.

Daphoenus has become the namesake genus for the Daphoneninae : one of the two known North American Amphicyonid subfamilies, with the other being the Amphicyoninae. According to Robert M. Hunt’s article in “The Evolution Of Tertiary Mammals In North America”:

“The Daphoeninae is considered here as a monophyletic North American endemic subfamily… [Its species are united by the following characters]: [Upper Molars or “M”]2-3 relative to M1 not enlarged in contrast to amphicyonines in which M2-3 are enlarged crushing teeth with amplified surface area; no reduction of premolars; p4 unreduced, often elongate, with squared posterior border; auditory bulla preserved only as an ossified ectotympanic crescent, loosely attached to the skull, without addition of any ossified entotympanic elements and without lateral prolongation into a bony external auditory meatus…; lack of expansion of the bulla posterior to the mastoid process; inferior petrosal venous sinus deeply excavated into edge of basioccipital; medial edge of petrosal in only slight contact with margin of basioccipital, not sutured to the basioccipital as seen in canids.”

Daphoenus Reconstruction.

In less technical terms, the Daphoeninae also bears the following non-exclusive generalized plesiomorphic characteristics:

-A generalized canine-like dentition.

-A relatively unspecialized and somewhat “feline” postcrania.

-Elongate cranium coupled with a short facial region of the skull.

-Presence of M3

-Lack of accessory cusps on anterior premolars.

-Elongation of lower limb elements (including the feet).

-A probable limitation in the ability to pronate/supinate the forelimb.

These features strongly insinuate that Daphoenus and its kin were cursorial beasts which were either overwhelmingly carnivorous or omnivorous with a bias towards predation.

In this figurine diorama, a Moropus is harassed by a fairly large Daphoenus.

As for Daphoenus itself, the skulls of it’s various species varied from a mere 14 cm in length (D. hartshorianus) to 24 cm in length (D. sp.) with the largest of these creatures rivaling a modern coyote in overall size. Some species are believed to have been sexually dimorphic, with the “males” sporting large canines and robust rostrums whilst the “females” maintain relatively small canines and gracile rostrums. The related species Brachyrhynchocyon sp. can be distinguished from this contemporaneous genus on the basis of the latter’s longer, narrower skulls and narrow premolars. Daphoenus is known from over sixty skulls (several of which contain associated lower jaws) along a number of postcranial skeletons, in addition to many isolated rostra, mandibles, and maxillae.

Daphoenus skull reconstruction.

The amphicyonids first emerged some 44 million years ago in Asia during the Mid-Eocene epoch before spreading into Asia and North America in the early Oligocene before eventually being out-competed by the precursors of modern ursids, canines, and felines by the Miocene’s conclusion. It should be noted that while these “bear-dogs” exhibited canine dentitions and a degree of homogeneous ursid-like cervical anatomy, they are not considered to have been members of either family: it would appear that all three groups have merely emerged from a common ancestor.

Daphoenus reconstruction.

May the fossil record continue to enchant us all!





Nectocaris Update

26 05 2010

Good tidings and well-wishes!

Last week, I leant some coverage to the bizarre little Burgess Shale creature Nectocaris pteryx. At the time, all of the publicly-available information about the species maintained that

A) Its known from but a single specimen.

B) Postcranially, Nectocaris was a sinuous, chordate-like creature.

However, shortly after my article was published, everyone’s favorite stuffed theropod pointed out that a pair of Burgess Shale scientists (The University Of Toronto’s Martin Smith & Jean-Bernard Caron) had argued that Nectocaris was, in fact, the earliest-known cephalopod and were working on a paper to this end. Evidently, the pair had stumbled upon NINETY-ONE additional specimens such as the one below:

That paper was released today, and the results have dramatically altered our perceptions of what this beast looked like. Ed Young of the brilliant blog “Not Exactly Rocket Science” has the scoop.

For those too lazy to click on the link to my earlier post, here’s a traditional reconstruction of Nectocaris as derived from the genus’ first and, for several decades, solitary specimen:

And here’s the updated reconstruction based on this wealth of newfound material:

I think it’s fairly obvious to say that this isn’t merely some miniscule alteration: it radically changes everything we thought we knew about this creature’s anatomy and phylogeny. To quote Mr. Young:

“Around four centimetres in length, Nectocaris had a soft, flattened, kite-shaped body with two fins running down its sides. Its small head was adorned with two long tentacles and two stalked eyes. Unlike the compound eyes that were common among Cambrian animals, probably had the camera-like structure that modern cephalopods use. From its neck protruded a flexible funnel, which opened into an internal cavity containing pairs of gills.

The funnel lay behind some of the earlier confusion about Nectocaris. In the original specimen, it was flattened so that it looked like a shield-like plate behind the eyes, reminscent of a crustacean’s body armour. The new specimens put paid to that interpretation. The structure is clearly a funnel, similar to those used by modern cephalopods. Nectocaris probably used it to swim the same way, giving it an extra boost of jet propulsion to complement the beating of its large fins.”

The homogeneous lack of shells throughout the newfound Nectocaris specimens have shattered the notion that cephalopods evolved from Monoplacophorans: an assertion which largely rested upon the fact that the earliest known representatives of this tentacled group had previously been the nautiloids. Evidently, the shells which graced several groups and species of subsequent (and current) cephalopods evolved independantly.

However, not every gap has been filled in assigning Nectocaris to this new role: none of the fossils appear to display the intimidating beak-like mouth and horny tongue (known scientifically as the ‘radula’) of squids, octopi, and their kin remains, as far as the authors can ascertain, absent from N. pteryx. The radula is of particular importance due to its presence in nearly every group of modern mollusks and has hence become a uniting feature.

Regardless of what Nectocaris’ true relations may be, this news provides yet another example of how fossilized species with which the scientific community feels it’s somewhat familiar can drastically suprise its members almost instantaneously by way of new discoveries (or even, in some cases, a second trip through the archives).

A POINT TO CONSIDER: How much time will elapse before PZ Meyers jumps all over this story?





Yet Another Awesome British Science Show…

26 05 2010

Good tidings and well-wishes!

A YouTube acquaintance of mine recently sent me the following clip from the British Channel 4 program “Inside Nature’s Giants” which, as far as I can ascertain, is an intriguing exploration into the ecology, evolution, and comparative anatomy of some of the planet’s largest creatures. The video I’ve posted appears to be a collection of clips from a trio of episodes involving the modern giraffe, what I believe to be a Nile crocodile, and the asian elephant. Sadly, it looks like there’s no news of a DVD coming along anytime soon. Fortunately, however, there’s no shortage of available online clips.

WARNING: This footage involves a great deal of dissection and is therefore NOT for the squeamish and/or faint of heart!

NOTE: Dawkins’ assertion that “[Crocodiles] haven’t changed much in a very long time” is a gross and inaccurate oversimplification of crocodylian evolution… for more information, I’d reccomend tracking down a copy of this book.





A Slime Mold’s View Of Time

24 05 2010

Good tidings and well-wishes!

I’ve recently been reading John Tyler Bonner’s very intriguing, albeit imperfectly written (his writing style can, on occasion, be needlessly vague), book entitled “Why Size Matters: From Bacteria To Blue Whales” which, as the name suggests, analyzes the role of an organism’s size in it’s evolution, ecological niche, population density, and anatomy/physiology.  Dr. Bonner is perhaps best known for his expertise in the field of Slime Mold research, a fact which resulted in the following essay that first appeared in the Buddhist magazine Tricycle following a request from the magazine’s editorial staff just before the dawn of the 21st century. Although some of the prose contains a degree of anthropomorphism (which, given the non-scientific context, is excusable), I think that, overall, it nicely articulates just how ‘relative’ time often is in the scientific sense:

“Time From The Point Of View Of A Slime Mold”

Time and life are intertwined in so many different ways, something all biologists are acutely aware of. Consider a few extremes: a single cell bacterium may have its entire life cycle in half an hour, but a generation for an elephant takes 12 years and a giant sequoia 60 years. One reason I work with slime molds, which are soil amoebae that start off as single cells, and then come together to form a multicellular organism, is that their generations are short, so that if I start an experiment on Monday, I will know the result by Wednesday or Thursday. This kind of biological time–life cycle time–is at the middle of the time scale of living phenomena.

At the faster end of that scale is physiological time: how many beats does a heart have in a minute, or how long does it take to swerve the car in order to avoid a squirrel on the road. As with life cycles, these rapid living activities are greatly affected by size, so a huge elephant will have about 25 heartbeats per minute, while a tiny shrew’s heart goes at the amazing rate of over 600 beats every minute. The elephant will step to one side with slow deliberation compared with a small sparrow on the willow ledge with its lightning movements. We can combine the concept of the time required for a life cycle and the time required for rapid physiological processes in an interesting way. A shrew will live only a year or two, but an elephant will average 40 to 50 years; yet they have one thing in common: the total number of heartbeats they have in their whole lifetime will be approximately the same. So life for the small beast goes faster because its engine is racing along compared to the larger beast, and the total budgets for their actions are the same.

Evolutionary time is another time scale in the realm of life. Now we are no longer dealing with one generation, but with a great series of generations going way back in time to the beginning of life on earth. We are no longer dealing with minutes or a few scores of years, but with millions, and even billions of years. We find in the fossil record an era when all life consisted of cells, which later were followed by simple multicellular organisms, leading ultimately to all the great variety of animals and plants that we see on the earth today. This has been an exceedingly slow and exceedingly grand evolution that has taken up a vast quantity of time–so much so that it is difficult to comprehend the magnitude of geologic time. And we know that even these time spans are modest compared with those of the astronomer, who thinks in terms of light-years*.

So what does a biologist think of this second millennium? It is too short a time for major changes in evolution, but time enough for many generations. Every 1000 years will allow some 50 human generations, but the shrew will have a new generation each year, which means [some] 1000 each millennium. So for slime molds and shrews, the second millenium has meant waiting impatiently for a huge number of generations, while for elephants and ourselves–the wait barely tries our patience.

* [(I fully realize that a light-year is a unit of distance, and not time. However, the enormous amount of time required to travel between them preserves Bonner’s point.)]

The sheer magnitude of time with which paleontologists work on a daily basis is every bit as humbling and awe-inspiring as the vastness of space so eloquently celebrated by a host of passionate astronomers and astro-physicists throughout the ages from Issac Newton to Carl Sagan. 

But alas, I’ve said too much already! To those among my readers who study the field of long-vanished life, either professionally or as amateurs such as myself, I’d like to ask how this knowledge of deep time affects you outside the realm of scientific pursuits and during the course of day-to-day life.

May the fossil record continue to enchant us all!





Burgess Shale Extravaganza: Canadia

21 05 2010

Good tidings and well-wishes!

I’ve saved the concluding entry in this week-long diagnosis of Burgess Shale weirdos for what I consider to be one of the most extravagant and yet under-appreciated denizens of this most paleontologically vital of fossiliferous congregations: the 3-centimeter annelid Canadia spinosa.

The bizarre, vaguely feather-shaped extensions of the creature’s anatomy as shown in the previous photograph are in fact an innumerable series of short, rigid bristles scientifically known as “setae”. More specifically, they can be classified as “notosetae”: these are setae which are rooted in an animal’s dorsal lobes. These rather “showy” contraptions cover essentially the entire dorsal surface of Canadia. In famed Burgess paleontologist Simon Conway Morris’ book entitled “The Crucible Of Creation: The Burgess Shale And The Rise Of Animals” (which was largely written from the perspective of a time-traveling scientific research team, hence the heavy editing I’ve provided), he writes:

“Each bundle [of notosetae overlaps] the one behind it, so as to give a tile-like covering to the upper surface of the body. There can be little doubt that this entire arrangement [was] primarily protective. The neuropodia [or “ventral branches”], in contrast, are much more lobe-like and strongly muscular. Each neuropodia bears a prominent podium of [setae], and it is on these structures that [the creature walked]…[theoretically] by a series of locomotory waves passing along the neuropodia. By precise coordination each neuropodium is first placed on the seabed and then pushed back so that the [notosetae] act as levers to push the animal forwards. Finally, at the end of the stroke the neuropodium lifts the [setae] clear of the sediment and swings them forward in preparation for the next shove against the sea floor.”

In addition to hypothetically providing a degree of protection, the notosetae may have enabled Canadia to swim were they rhythmically beaten while cruising above the sea floor.

Canadia‘s gut was straight and was capable of anteriorly extending from the main body to form a proboscis of sorts. Along with the frequent presence of sediment in the beast’s gut, this digestive setup has given rise to the hypothesis that Canadia was a detrivore. The function of the anterior pair of tentacles is unknown, though they likely assisted the animal’s sensory capabilities.

I’ll close this article with an intriguing idea championed by Conway Morris’ aforementioned volume: that Canadia may in fact be that most elusive of Cambrian fauna: a relative of WiwaxiaWere this true, it would greatly disambiguate the spiny little eccentric’s incomparably vague phylogeny. Consider the following passage:

“When the [setae] is placed under the microscope, it is seen to have a microstructure very similar to that observed in Wiwaxia. Evidently there is some evolutionary connection between Canadia and Wiwaxia.”

Furthermore, Conway Morris notes that the ventrally-situated gills of Canadia strongly resemble those of modern molluscs, which persuasively insinuates a kinship between the genus and the mollusca phylum as well. Hopefully, additional discoveries will yield a plethora of new information concerning this functional morphology, evolution, and taxonomic affiliations of most engaging organism.

May the fossil record continue to enchant us all!





Burgess Shale Extravaganza: Aysheaia

20 05 2010

Good tidings and well-wishes!

Although many self-described “hard-core” scientists refuse to acknowledge it, popular culture exerts an appreciable influence upon virtually every discipline of science imaginable. Dinosaurian afficionados are well aware of Gary Larson’s “The Far Side” leading to the anatomical term “thagomizer” (which, for those ill-versed in paleo-jargon, describes the unique  spike arrangements on the tails of stegosaurs), guitarist Mark Knopfler’s namesake theropod Masiakasaurus knopfleri, and, of course, the “Harry Potter”-inspired pachycephalosaur Dracorex hogwartsia.

Dinosaurs, however popular, are far from the sole examples of this trend, as evidenced by the Burgess Shale denizen Aysheaia sp. whose genus name derives from the Ayesha mountain peak which, in turn, was named for Aysha: a sorceress and the “title” character of Rider Haggard’s 1905 novel “She: A History Of Adventure”. As is the case with its literary namesake, the simultaneously “alien” and “familiar” appearance of the two-inch Aysheaia is, as with many Burgess Shale residents, quite spell-binding (how’s that for a labored segue?).

According to Stephen Jay Gould’s “Wonderful Life”:

Aysheaia has an annulated, cylindrical trunk, with ten pairs of annulated limbs attached at the sides near the lower surface and pointing down, presumably for use in locomotion. The anterior end is not separated as a distinct head. It bears a single pair of appendages, much like the others in form and annulation but attatched higher on the sides and pointing laterally. The terminal mouth (smack in the middle of the front surface) is surrounded by six or seven papillae. The head appendages bear three spinelike branches at their tip, and three additional spines along the anterior margin. The body limbs end in a blunt tip carrying a group of up to seven tiny, curved claws. Larger spines emerge from the limbs themselves. These spines are absent on the first pair, point forward on pairs 2-8, and backward on 9-10.”

It should be noted that the limb-like appendages of Aysheaia are not true legs but can best be termed “lobopods”. Despite the fact that each lobopod is divided into a series of transverse rings, these are not to be mistaken for the series of joints which compose the legs of arthropods. In life, these lobopods would have contained a thick, muscular core surrounded by a fluid-filled cavity. This setup would have served as a hydraulic pump of sorts designed to enable locomotion.

Interestingly, a large percentage of Aysheaia specimens are found in association with the remains of sponges–an essentially nonexistent occurence elsewhere within the Burgess faunal roster. This has given rise to the well-accepted hypothesis that Aysheaia may have fed and dwelt upon these most primitive of animals. The idea is granted additional credibility when one considers the uselessness that the animal’s many claws would have likely served on the muddy floor of the basin. However, these spikes could have easily been employed for the purposes of scaling sponge colonies. Furthermore, it’s logical to conclude that Aysheaia‘s preserved anatomy insinuates no recognizable form of defense: a dire predicament which could have easily been averted were the creature to seek refuge within its (theoretical) spongy home.

As for Aysheaia‘s phylogenetic relationships, most paleontologists acknowledge that the beast bears a strong overall resemblance to the modern velvet worms (phylum Onychophora), although whether or not the genera should be considered a member of their phylum remains the subject of debate.

May the fossil record continue to enchant us all!





Burgess Shale Extravaganza: Amiskwia

20 05 2010

Good tidings and well-wishes!

NOTE: Contrary to the assertions of WordPress, this article was in fact posted on the 19th.

The soft bodied and rather delicately-built Amiskwia sagittiformis is a superb example of the conflict which often arises when a fossil is subjected to exceptional preservation and a debatable amount of deformity. The soft composition and meager proportions of Amiskwia and its kin prevent their fossil record from becoming remotely extensive. Hence, we may never know the degree to which geologic forces have altered the appearance of this superficially worm-like animal. Nonetheless, enough information has been gathered to make a few tentative educated guesses concerning its lifestyle and possible taxonomic affiliations.

Though the statement may sound comical, at a full inch in length, Amiskwia was a comparatively large inhabitant of the Burgess Shale and the elder Maotianshan Shales . This considerable size appears to have come at the expense of abundance, with a mere eighteen specimens of this peculiar genus having been found in the former deposit as of this writing.

Amiskwia‘s entire body is dorso-ventrally compressed. Its head sports a pair of ventrally-situated tentacles, just behind and between which lies an ovular mouth. Though the creature’s “trunk” is unsegmented, it does contain a pair of lateral fins which are unsupported by any sort of stiffening device: the same is true for the caudal fin which comprises the beast’s tail. The trunk and caudal regions of Amiskwia‘s anatomy appear to have been quite muscular: a piece of evidence which supports the hypothesis that Amiskwia may have been a pelagic (free-swimming) organism, which would also help to explain its aforementioned rarity in the formerly muddy Burgess basin.

Intriguingly, the internal anatomy of Amiskwia is also relatively well preserved, as partially displayed by the following fossil specimen:

According to the Smithsonian Institution’s official page on the species:

“Note that in the fossil preparation the head shows a highly reflective area (cerebral ganglia? [aka: the bilobed ‘brain’ of many modern worms and arthropods])… The broad light area running along the trunk is the gut, while the narrow linear structures along the trunk may be traces of blood vessels and a nerve cord.”

 

Note the rather conspicuous pair of bulb-like organs in the head of this Amiskwia reconstruction: this is the artist's depiction of its cerebral ganglia.

The phylogenetic relationships of Amiskwia are a matter of some debate. However, the general consensus amongst Cambrian paleontologists is that while the genus exhibits characteristics akin to those of the chaetognatha and nemertea. it cannot be scientifically accomodated by either group.

May the fossil record continue to enchant us all!








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