Big love among the ostracods


How can anyone resist an article titled “Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode”? You can’t, I tell you. It’s like a giant brain magnet, you open the journal to the index, and there’s that title, and you must read it before you can even consider continuing on to anything else.

Some organisms have evolved immensely long sperm tails — Drosophila bifurca, for instance, has sperm cells that are about 60mm long, or 20 times longer than the length of the entire adult body. The excessively long sperm tail is obviously not a structure that has evolved for better swimming; instead, it is thought to act as a tangled barrier in the female reproductive tract to prevent other males from fertilizing the female, and there is also some very interesting evidence that sperm coevolves with the female reproductive tract, so some sexual selection at the level of the gametes is going on.

At the same time, sperm morphology is extremely diverse, and seems to evolve very rapidly. Perhaps these mega-sperm are a transient fad? Not all species of Drosophila exhibit the phenomenon, and those that do vary considerably from species to species. What we’d like to know is if there are any lineages that maintain these patterns of giant sperm over long periods of evolutionary time…so what do we need to do? We need to go spelunking for sperm in fossils!

That’s what this short letter in Science is about: the authors looked at ostracodes, a class of tiny crustacea that invests heavily in reproduction. About a third of their volume is their reproductive system, with males building giant (relative to their size) sperm pumps, and females having large seminal receptacles for sperm storage. The individual sperm are also large, often longer than the body length of the adult, and are also aflagellate — no flagellar tail at all, just a long, threadlike cell body. You can tell if a female ostracod is a virgin just by looking at those seminal receptacles, since they inflate hugely with all the giant sperm tucked inside.

So, if you look at the large orange blobs, the seminal receptacles, in this 3-D scan of a fossil female ostracod (bottom right of this image), you can tell that she was inseminated before she died, and that her mate had very large sperm. Her condition was also very similar to that of modern ostracodes (bottom left).

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Partial reconstruction of E. virens (extant) and H. micropapillosa (fossil). Anterior is to the left. Orange structures indicate central tubes of Zenker organs in males or seminal receptacles in females; brown, esophagus; turquoise, mandible; purple, upper lip; pink, lower lip; green, valves; and gray scales, whole-body reconstruction. All scale bars indicate 100 µm. (A) Lateral view of male E. virens with several organs included for comparison. (B) Male H. micropapillosa in lateral view with several organs in context of whole-body reconstruction. (C and D) Ventral views of several organs including tubes of Zenker organs of male H. micropapillosa. (E) Lateral view of female E. virens with several organs included for comparison. (F) Female H. micropapillosa in lateral view with several organs in context of whole-body reconstruction, including seminal receptacles.

So, the conclusion is that boinking with giant sperm is an enduring property of at least some lineages: they’ve been going at it for a hundred million years. The authors also suggest that this kind of technique could be useful for measuring sexual selection by assessing pre-mating parental investment in fossil invertebrates.

Matzke-Karasz R, Smith RJ, Symonova R, Miller CG, Tafforeau P (2009) Sexual Intercourse Involving Giant Sperm in Cretaceous Ostracode. Science 324(5934):1535.

Miller GT, Pitnick S (2002) Sperm-Female Coevolution in Drosophila. Science 298(5596):1230-1233.

Limusaurus inextricabilis


My previous repost was made to give the background on a recent discovery of Jurassic ceratosaur, Limusaurus inextricabilis, and what it tells us about digit evolution. Here’s Limusaurus—beautiful little beastie, isn’t it?

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Photograph (a) and line drawing (b) of IVPP V 15923. Arrows in a point to a nearly complete and fully articulated basal crocodyliform skeleton preserved next to IVPP V 15923 (scale bar, 5 cm). c, Histological section from the fibular shaft of Limusaurus inextricabilis (IVPP V 15924) under polarized light. Arrows denote growth lines used to age the specimen; HC refers to round haversian canals and EB to layers of endosteal bone. The specimen is inferred to represent a five-year-old individual and to be at a young adult ontogenetic stage, based on a combination of histological features including narrower outermost zones, dense haversian bone, extensive and multiple endosteal bone depositional events and absence of an external fundamental system. d, Close up of the gastroliths (scale bar, 2 cm). Abbreviations: cav, caudal vertebrae; cv, cervical vertebrae; dr, dorsal ribs; ga, gastroliths; lf, left femur; lfl, left forelimb; li, left ilium; lis, left ischium; lp, left pes; lpu, left pubis; lsc, left scapulocoracoid; lt, left tibiotarsus; md, mandible; rfl, right forelimb; ri, right ilium; rp, right pes; sk, skull.

What’s especially interesting about it is that it catches an evolutionary hypothesis in the act, and is another genuine transitional fossil. The hypothesis is about how fingers were modified over time to produce the patterns we see in dinosaurs and birds.

Birds have greatly reduced digits, but when we examine them embryologically, we can see precisely what has happened: they’ve lost the outermost digits, the thumb (I) and pinky (V), and retain the forefinger, middle finger, and ring finger (II-IV), which have been reduced and fused together. This is called Bilateral Digit Reduction, BDR, because they’ve lost digits from the medial and lateral sides, leaving the middle set intact.

Dinosaurs, when examined anatomically, seem to have a different pattern: they have a thumb (I), forefinger (II) and middle finger (III), and have lost the lateral two digits, the ring and pinky finger (IV-V). This arrangement has been advanced as evidence that birds did not evolve from dinosaurs, since they have different bones in their hands, and getting from one pattern to the other is complicated and difficult and very unlikely.

The alternative hypothesis is that there is no conflict, and that dinosaurs actually underwent BDR and their digits are II-III-IV…but that what has also happened is a frame shift in digit identities. So dinosaurs actually have three digits, which are the index, middle, and ring finger, but they’ve undergone a subtle shift in morphology so that their forefinger develops as a thumb, and so forth.

Now we could resolve all this easily if only the physicists would get to work and build that time machine so we could go back to the Mesozoic and study dinosaur embryology, but they’re too busy playing with strings and quanta and dark matter to do the important experiments, so we’ve got to settle for another plan: find intermediate forms in the fossil record. That’s where Limusaurus steps in.

Limusaurus has a thumb, a tiny vestigial nubbin, and has lost its pinky completely. This is a (I)-II-III-IV pattern, and is evidence of bilateral digit reduction in a basal ceratosaur. In addition, the forefinger has become very robust, and while still distinctly a digit II, has been caught in the early stages of a transformation into a saurian first digit. It’s evidence in support of the dinosaurian II-III-IV hypothesis and the frameshift in digit identity! It’s almost as good as having a time machine.

Want to learn more? Carl Zimmer has a summary of the digit changes, while one of the authors of the paper, David Hone, also discusses the digits (the story is a little more complicated than I’ve laid out), and also has more on the rest of the animal—it’s a herbivorous ceratosaur, which is interesting in itself.

Xu X, Clark JM, Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-k, Han F-l, Guo Y (2009) A Jurassic ceratosaur from China helps clarify avian digit homologies. Nature 459(18):940-944.

Darwinius masillae

This is an important new fossil, a 47 million year old primate nicknamed Ida. She’s a female juvenile who was probably caught in a toxic gas cloud from a volcanic lake, and her body settled into the soft sediments of the lake, where she was buried undisturbed.


What’s so cool about it?

Age. It’s 47 million years old. That’s interestingly old…it puts us deep into the primate family tree.

Preservation. This is an awesome fossil: it’s almost perfectly complete, with all the bones in place, preserved in its death posture. There is a halo of darkly stained material around it; this is a remnant of the flesh and fur that rotted in place, and allows us to see a rough outline of the body and make estimates of muscle size. Furthermore, the guts and stomach contents are preserved. Ida’s last meal was fruit and leaves, in case you wanted to know.

Life stage. Ida is a young juvenile, estimate to be right on the transition from requiring parental care to independent living. That means she has a mix of baby teeth and adult teeth — she’s a two-fer, giving us information about both.

Phylogeny. A cladistic analysis of the fossil revealed another interesting point. There are two broad groups of primates: the strepsirrhines, which includes the lemurs and lorises, and the haplorhines, which includes monkeys and apes…and us, of course. Ida’s anatomy places her in the haplorhines with us, but at the same time she’s primitive. This is an animal caught shortly after a major branch point in primate evolutionary history.

She’s beautiful and interesting and important, but I do have to take exception to the surprisingly frantic news coverage I’m seeing. She’s being called the “missing link in human evolution”, which is annoying. The whole “missing link” category is a bit of journalistic trumpery: almost every fossil could be called a link, and it feeds the simplistic notion that there could be a single definitive bridge between ancient and modern species. There isn’t: there is the slow shift of whole populations which can branch and diverge. It’s also inappropriate to tag this discovery to human evolution. She’s 47 million years old; she’s also a missing link in chimp evolution, or rhesus monkey evolution. She’s got wider significance than just her relationship to our narrow line.

People have been using remarkable hyperbole when discussing Darwinius. She’s going to affect paleontology “like an asteroid falling down to earth”; she’s the “Mona Lisa” of fossils; she answers all of Darwin’s questions about transitional fossils; she’s “something that the world has never seen before”; “a revolutionary scientific find that will change everything”. Well, OK. I was impressed enough that I immediately made Ida my desktop wallpaper, so I’m not trying to diminish the importance of the find. But let’s not forget that there are lots of transitional forms found all the time. She’s unique as a representative of a new species, but she isn’t at all unique as a representative of the complex history of life on earth.

When Laelaps says, “I have the feeling that this fossil, while spectacular, is being oversold,” I think he’s being spectacularly understated. Wilkins also knocks down the whole “missing link” label. The hype is bad news, not because Ida is unimportant, but because it detracts from the larger body of the fossil record — I doubt that the media will be able to muster as much excitement from whatever new fossil gets published in Nature or Science next week, no matter how significant it may be.

Go ahead and be excited by this find, I know I am. Just remember to be excited tomorrow and the day after and the day after that, because this is perfectly normal science, and it will go on.

Laelaps has some serious reservations about the analysis — the authors may not have done as solid a cladistic analysis as they should, and its position in the family tree may not be as clear as it has been made out to be.

Franzen JL, Gingerich PD, Habersetzer J, Hurum JH, von Koenigswald W, Smith BH (2009) Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology. PLoS ONE 4(5): e5723. doi:10.1371/journal.pone.0005723.

Puijila darwini


It’s yet another transitional fossil, everyone! Oooh and aaah over it, and laugh when the creationists scramble to pave it over with excuses.

What we have is a 23 million year old mammal from the Canadian arctic that would have looked rather like a seal in life…with a prominent exception. No flippers, instead having very large feet that were probably webbed. This is a walking seal.

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a, Palatal view of skull; b, lateral view of skull and mandible, left side; c, occlusal view of left mandible. Stippling represents matrix, hatching represents broken bone surface. The images are of three-dimensional scans. The brain case was scanned using computed tomography, whereas all other elements were surface scanned.

What it tells us is that marine pinnipeds almost certainly had an origin in the arctic, derived from terrestrial and semi-aquatic forms — these were more otter-like animals.

You’ll want to learn more about this beautiful creature. There is a website all about Puijila (in English, French, and Inuktitut) where you can find all kinds of images…and you can also find out how to pronounce “Puijila, something we’re all going to have to practice. Who knew paleontology was going to lead us all into learning a few words of Inuktitut?

Rybczynski N, Dawson MR, Tedford RH (2009) A semi-aquatic Arctic mammalian carnivore from the
Miocene epoch and origin of Pinnipedia. Nature 458:1021-1024.

Guiyu oneiros


A fish is a fish, right? They’re just a blur of aquatic beasties that most people distinguish by flavor, rather than morphology or descent. But fish are incredibly diverse, far more diverse than terrestrial vertebrates, and there are significant divisions within the group. Most people know of one big distinction, between the Chondrichthyes (fish with cartilaginous skeletons, like sharks and rays) and the Osteichthyes (fish with bony skeletons), but there’s another particularly interesting split within the Osteichthyes: the distinction between Sarcopterygians (the word means “fleshy fins”, and we call them lobe-finned fishes colloquially) and the Actinopterygians, the ray-finned fishes. The lobe-finned fishes most distinctive feature is the muscular and bony central core of their fins — extant forms are the coelacanth and lungfish. It is this lineage that led to us terrestrial tetrapods, but other than that successful invasion of the land, the sarcops were something of an aquatic failure, with only a few genera surviving. The ray-finned fishes, on the other hand, are a major success story, with more than 28,000 species today. To put that in context, there are only about 5,500 species of mammals.

The Sarcopterygii and the Actinopterygii must have begun diverging a long time ago, and a couple of questions of interest are a) when did the last common ancestor of both groups live, and b) what did it look like? We don’t have a good and specific answer yet, because Osteichthyes origins are lost far, far back in time, over 400 million years ago, but every new discovery edges us a little closer. What we now have is a well-preserved fossil of a fish that has been determined to be an early sarcopterygian, and it tells us that a) the last common ancestor had to have lived over 419 million years ago, the age of this fossil, and the divergence probably occurred deep in the Silurian, and b) this animal has a mosaic of primitive Osteichthyan features, which tells us that that last common ancestor may well have shared some of these elements. It is another transitional fossil that reveals much about the gradual separation of two great vertebrate groups.

And here it is:

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a, b, A near-complete fish in part and counterpart. c, Close-up view of the anterior portion of the trunk shield in dorsal view, showing MD1 and MD2 flanked by rhomboid scales. d, Close-up view of the dorsal fin spine. MD1, first median dorsal plate; MD2, second median dorsal plate. Scale bar, 1 cm.

That may be a bit disappointing at first — it looks like Silurian road-kill — but really, that’s a remarkable good and useful specimen. The animal was covered with thick bony scales, and the skull was built of thick bony plates, and so while it was squashed flat by pitiless geology, the pieces are all there, and it can be reassembled into a much more fishy state. This drawing may be more satisfying:

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a, Restoration of the entire fish in lateral view. b, Interpretive drawing of the holotype V15541. Areas shaded in grey are unknown, and are reconstructed from other early osteichthyans. ano, anterior nostril; br, branchiostegal ray; cla, clavicle; cle, cleithrum; drs, dorsal ridge scale; dsp, dorsal fin spine; et, extratemporal; eta, accessory extratemporal; f.add, adductor fossa;, glenoid fossa; gu, gular; ju, jugal; l.ext, lateral extrascapular; lj, lower jaw; m.ext, median extrascapular; mx, maxillary; n.sp., spiracular notch; op, opercular; pa, parietal shield; pcl, postcleithrum; pop, preopercular; ppa, postparietal shield; psc, presupracleithrum; pt, post-temporal; scl, supracleithrum; sop, subopercular; sp., pectoral spine; tr, lepidotrichia; vrs, ventral ridge scale.

Now it looks like a kind of armored, spiky salmon with a thick muscular body (and yes, I too wonder about flavor, and would like to taste a slab of that). It’s definitely not a salmon, though — the bony structure is a curious set of compromises where some features are distinctly sarcopterygian, some look like they belong on a primitive actinopterygian, and others are unique or show affinities to characters of ancient extinct fishes, like rhipidistians. This is very cool. What we see here are relics of an ancient common osteichthyan ancestor, which are being honed into the specific characteristics of the Sarcopterygii. The analysis of the totality of the animal’s features, though, place it more in the lobe-finned than the ray-finned clade. That places it on a branch of the line leading to us…a very, very old branch, making this your many-times-great grand uncle, or cousin only a few million times removed. Now my curiosity about a taste-test is making me feel mildly cannibalistic.

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The topology is the most parsimonious tree arising from a matrix of 23 taxa coded for 153 morphological characters (tree length = 292, consistency index = 0.572, retention index = 0.737, rescaled consistency index = 0.421). The numbers at nodes indicate bootstrap support (where the value is greater than 50%) and Bremer decay index (bottom and top numbers, respectively). Eif., Eifelian; Ems., Emsian; Fam., Famennian; Fras., Frasnian; Giv., Givetian; Gor., Gorstian; Loch., Lochkovian; Lud., Ludfordian; Prag., Pragian.

When you look at that diagram, what should jump out at you is all the diversity in the Devonian, the so-called Age of Fishes, and the paucity of representative fossils from the Silurian…which is exactly where all the interesting branch points in the fish family tree are located. Once again, paleontology is a predictive science, and this tells us where to look for the next batch of exciting and informative fossils.

Zhu M, Zhao W, Jia L, Lu J, Qiao T, Qu Q (2009) The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458:469-474.


I’m not going to say much about this since Ed Yong has an excellent write-up, but a new feathered dinosaur has been discovered, called Tianyulong. As you can see in this image of the fossil, it was bristling with a fuzz of thin fibers — proto-feathers.

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a, Main slab of the holotype (STMN 26-3). b, Broken slab. The scale bar in b refers also to a. c, Close-up of skull and mandible. d, Interpretive drawing of skull and mandible. e, Close-up of dentition. Abbreviations: a, angular; aof, antorbital fossa; ca, caudal vertebrae; cv, cervical vertebrae; d, dentary; dv, dorsal vertebrae; emf, external mandibular fenestra; en, external naris; f, femur; h, humerus; isc, ischium; j, jugal; l, lacrimal; m, maxilla; n, nasal; pd, predentary; pf, prefrontal; pm, premaxilla; po, postorbital; pub, pubis; q, quadrate; qj, quadratojugal; scaco, scapulocoracoid; sa, surangular; tf, tibia and fibula.

There are a couple of noteworthy features in this creature. One is apparent: feathers just didn’t bloom suddenly in evolution, but appeared in steps. This animal has ‘feathers’ that don’t branch like those of modern birds, but instead form more of a furry coat than a set of flat blades.

The other cool thing is that this is an ornithischian dinosaur; most of the other dinosaurs that have been discovered to have feathers were saurischian. What that means might be made more clear by this diagram:


It implies that just maybe the last common ancestor of the saurischia and ornithischia were also covered with proto-feathers, which means that feathers may be a primitive state in this lineage.

Zheng X-T, You H-L, Xu X, Dong Z-M (2009) An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458:333-336.

Octopods from the Cretaceous!


Several new and spectacular cephalopod fossils from 95 million years ago have been found in Lebanon. “Spectacular” is not hyperbole — these specimens have wonderfully well-preserved soft parts, mineralized in fine-grained calcium phosphate, and you can see…well, take a look.

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Keuppia levante sp. nov. from the Upper Cenomanian (Metoicoceras geslinianum Zone) of Hâdjoula (Lebanon). A,
holotype, MSNM i26320a. B, sketch of the holotype.

[Read more…]

How did dinosaurs sit down?


That question has an answer: they crouched like birds. A 198 million year old fossil trackway from Utah has preserved a print of a theropod dinosaur taking a break, resting with hands curled inward and knuckle down, and legs bent. Except for the forelimbs, of course, it’s very birdlike.

Restoration of Early Jurassic environment preserved at the SGDS, with the theropod Dilophosaurus wetherilli in bird-like resting pose, demonstrating the manufacture of SGDS.18.T1 resting trace.

Here’s the section of the trace fossil they used to reconstruct the animal’s posture.

A, Overhead, slightly oblique angle photograph of SGDS.18.T1 resting trace. Note normal Eubrontes track cranial to resting traces (top center) made by track maker during first step upon getting up. Scale bar equals 10 cm. B, Schematic of SGDS.18.T1 to scale with A: first resting traces (manus, pes, and ischial callosity) in red, second (shuffling, pes only) traces in gold, final resting traces (pes and ischial callosity) in green, and tail drag marks made as track maker moved off in blue. Note long metatarsal (“heel”) impressions on pes prints. C, Direct overhead photograph and D, computerized photogrammetry with 5 mm contour lines of Eubrontes trace SGDS.18.T1. Color banding reflects topography (blue-green = lowest, purple-white = highest); a portion of the berm on which the track maker crouched is discernible. Abbreviations: ic = ischial callosity, lm = left manus, lp = left pes, rm = right manus, rp = right pes, td = tail drag marks.

Milner ARC, Harris JD, Lockley MG, Kirkland JI, Matthews NA (2009) Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace. PLoS ONE 4(3): e4591. doi:10.1371/journal.pone.0004591