Exposing the intimate details of the sex lives of placoderms

i-e88a953e59c2ce6c5e2ac4568c7f0c36-rb.png

The media is getting another science story wrong. I keep seeing this discovery of an array of fossil placoderms as revealing the origins of sex, and that’s not right. Sex is much, much older, and arose in single-celled organisms. Come on, plants reproduce sexually. A fish is so far removed from the time of origin of sexual reproduction that it can’t tell us much about its origins.

Let’s get it right. These fossils tells us about the origin of fu…uh, errm, mating in vertebrates.

What we have are a set of placoderm fossils from the Devonian (380 million years ago) of Western Australia (The Aussies are going to be insufferable, now that they can claim to be living in the birthplace of shagging) that show two interesting features: some contain small bits of placoderm armor that show no signs of digestion, and so are not likely to be relics of ancient cannibal feasts, but are the remains of viviparous broods — they were preggers. The other suggestive observation is that the pelvic girdle has structures resembling the claspers of modern sharks, an intromittent organ or penis used for internal fertilization.

[Read more…]

Weird-eyed fish

i-e88a953e59c2ce6c5e2ac4568c7f0c36-rb.png

This is a photograph of Macropinna microstoma, also called barreleyes. It has a very peculiar optical arrangement. When you first look at this photo, you may think the two small ovals above and behind its mouth are the eyes, and that it looks rather sad…wrong. Those are its nostrils. The eyes are actually the two strange fluorescent green objects that look like they are imbedded in its transparent, dome-like head.

i-e4c1d2dbc8b451db6023adcf1a04981a-macropinna.jpeg
(Click for larger image)
Video frame-grab of Macropinna microstoma at a depth of 744 m, showing the intact, transparent shield that covers the top of the head. The green spheres are the eye lenses, each sitting atop a silvery tube. Visible on the right eye, just below the lens on the forward part of the tube, is the external expression of a retinal diverticulum. The pigmented patches above and behind the mouth are olfactory capsules. High-definition video frame grabs of Macropinna microstoma in situ are posted on the web at: http://www.mbari.org/midwater/macropinna.

It gets the name “barreleyes” because it’s are cylindrical, rather than spherical; this is an adaptation for better light collection in the dim depths where it lives, using very large lenses but not building a giant spherical eye to compensate. It’s ore like a telescope than a wide-angle camera. Here’s what a single eye in a side view looks like — the lens (L) is what is glowing so greenly in the photos.

i-448ffcbc59cdd27dd3b9fef803aea490-eye_diag.jpeg
Chapman’s (1942) mesial view of the left eye of Macropinna microstoma. Abbreviations: RS = rectus superior, L =lens, OS = obliquus superior, OI = obliquus inferior, RIN = rectus internus, RI = rectus inferior, RE = rectus externus, OP = optic nerve.

As if that weren’t weird enough, the animal has a completely transparent skull cap, and the eyes swivel about within the skull to look out through that translucent cranium. In the two pictures below, the animal is first looking straight up through its head (the eyes are in the same orientation as in the diagram above), and in the right frame it has rotated the binocular-shaped eyes forward to look ahead.

i-6801c74f55ec3677758ade36aac77a7c-macropinna_side.jpeg
Lateral views of the head of a living specimen of Macropinna microstoma, in a shipboard laboratory aquarium: (A) with the tubular eyes directed dorsally; (B) with the eyes directed rostrally. The apparent differences in lip pigmentation between (A) and (B) are because they were photographed at slightly different angles. (A) was shot from a more dorsal perspective and it shows the lenses of both eyes; the mouth is not sharply in focus. (B) shows only the right eye, with the lips in sharper focus.

Nature is always coming up with something stranger than we would imagine, and Macropinna is a perfect example. Apparently, the function of this arrangement is to give the animal a sensitive light detector for tracking its prey, bioluminescent jellyfish, and at the same time to shield the eyes from the stinging tentacles of the jelly while it’s eating it.

Robison BH, Reisenbichler KR (2008) Macropinna microstoma and the Paradox of Its Tubular Eyes. Copeia 2008(4):780-784.

Neandertal genome? Or a premature announcement?

In a potentially exciting development, researchers have announced the completion of a rough draft of the Neandertal genome in a talk at the AAAS, and in a press conference, and the latest issue of Science has a number of news articles on the subject. And that is a reason for having some reservations. There is no paper yet, and science by press release raises my hackles, and has done so ever since the cold fusion debacle. Not that I think this is a hoax or error by any means, but it’s not a good way to present a scientific observation.

Also, the work has some major limitations right now. They’ve got about 60% of the genome so far, and it’s all entirely from one specimen. From the age of these bones, degradation is inevitable, so there are almost certainly corrupted sequences in there — more coverage would give me much more confidence.

With those caveats, though, there are some tantalizing hints, and the subject is so exciting that it’s understandable why there’d be rush to announce. So far, they’ve identified approximately 1000-2000 amino acid differences in the coding part of the genome (human-chimp differences are about 50,000 amino acids), but there’s no report of any detectable regulatory differences.

I’m withholding judgement until I see a real paper; for now, you have to settle for a podcast with a science journalist, which just isn’t meaty enough yet.

Schinderhannes bartelsi

i-e88a953e59c2ce6c5e2ac4568c7f0c36-rb.png

Fans of the great Cambrian predator, Anomalocaris, will be pleased to hear that a cousin lived at least until the Devonian, over 100 million years later. That makes this a fairly successful clade of great-appendage arthropods — a group characterized by a pair of very large and often spiky manipulatory/feeding arms located in front of the mouth. Here’s the new fellow, Schinderhannes bartelsi:

i-2cbbddd52e8796e69083ea2eab19549d-schinderhannes.jpeg
(click for larger image)
Holotype of Schinderhannes bartelsi. (A) Ventral. (B) Interpretative drawing of ventral side. l, left; r, right; A1, great appendage; A2, flaplike appendage; sp, spine; fm, flap margin; te, tergite; ta, trunk appendage. (C) Partly exposed dorsal side, horizontally mirrored. (D) Interpretative drawing of dorsal side. (E) Interpretative drawing of great appendages, combining information from the dorsal and ventral sides. (F) Radiograph. (G) Reconstruction. Scale bar, 10 mm [for (A) to (G)]. (H) Mouth-part. Scale bar, 5 mm.

There are some significant differences between familiar old Anomalocaris and Schinderhannes. Anomalocaris was a monster that grew to about a meter long; this little guy is about a tenth of that, around 4 inches. He also has those interesting “wings” behind his head, which presumably aided in swimming.

Another significant feature of this animal is that it has characters that place it in the Euarthropoda, which makes great appendages paraphyletic and primitively present in euarthropods. Those great appendages have long been a curiousity, and we’ve wondered whether they are a unique innovation that was completely lost in modern arthropods, or whether they evolved into one of the other more familiar cephalic appendages; the authors suggest that this linkage with the euarthropod family tree implies that chelicera (what you may recognize as the big paired ‘fangs’ of spiders) are modified great appendages.

i-243541e68415b06eea4b32a38610d746-schinderhannes_clade.jpeg
Cladogram; tree length, 87. Consistency index, 0.5402; retention index, 0.6552. (1) Peytoia-like mouth sclerites, terminal mouth position, lateral lobes, loss of lobopod limbs, and stalked eyes. (2) Great appendages. (3) Sclerotized tergites, head shield, loss of lateral lobes, and biramous trunk appendages. (4) Stalked eyes in front and loss of radial mouth. (5) Post-antennal head appendages biramous and antenna in first head position. (6) Free cephalic carapace, carapace bivalved, and two pairs of antennae. (7) Maxilla I and II. (8) Exopods simple oval flap. (9) Two pre-oral appendages and a multisegmented trunk endopod. (10) Post-antennal head appendages biramous and tail appendages fringed with setae. (11) Long flagellae on great appendage and exopods fringed with filaments. (12) Trunk appendages uniramous and eyes not stalked. (13) No posterior tergites. (14) Tail spines and chelicere/chelifore on first head position. (15) Proboscis. (16) Six post-antennal head appendages.

Kühl G, Briggs DEG, Rust J (2009) A Great-Appendage Arthropod with a Radial Mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323(5915):771-773.

Titanoboa!

i-e88a953e59c2ce6c5e2ac4568c7f0c36-rb.png

Just wait — this one will be featured in some cheesy Sci-Fi channel creature feature in a few months. Paleontologists have dug up a fossil boa that lived 58-60 million years ago. They haven’t found a complete skeleton, but there’s enough to get an estimate of the size. Look at these vertebrae!

i-28fcb2529d62fd2ff334bcf3001b3874-titanoboa.jpeg
a, Type specimen (UF/IGM 1) in anterior view compared to scale with a precloacal vertebra from approximately 65% along the precloacal column of a 3.4 m Boa constrictor. Type specimen (UF/IGM 1) shown in posterior view (b), left lateral view (c) and dorsal view (d). Seven articulated precloacal vertebrae (UF/IGM 3) in dorsal view (e). Articulated precloacal vertebra and rib (UF/IGM 4) in anterior view (f). Precloacal vertebra (paratype specimen UF/IGM 2) in anterior view (g) and ventral view (h). Precloacal vertebra (UF/IGM 5) in anterior view (i) and posterior view (j). All specimens are to scale.

Just to put it in perspective, the small pale blob between a and b in the photo above is an equivalent vertebra from an extant boa, which was 3.4 meters long. The extinct beast is estimated to have been about 13 meters long, weighing over 1100 kg (for us Americans, that’s 42 feet and 2500 pounds). This is a very big snake, the largest ever found.

The authors used the size of this snake to estimate the temperature of this region of South America 60 million years ago. Snakes are poikilotherms, depending on external sources of heat to maintain a given level of metabolic activity, and so available temperature means are limiting factors on how large they can grow. By comparing this animal’s size to that of modern tropical snakes, and extrapolating from a measured curve of size to mean annual temperature, they were able to calculate that the average ambient temperature was 30-34°C (American cluestick: about 90°F); less than that, and this snake would have died.

From other data, they know that the atmospheric CO2 concentration at this time was about 2000 parts per million, and that the forests it lived in were thick, wet, and rainy. They also estimate that slightly later, about 56 million years ago, mean tropical temperatures would have soared to 38-40°C (102°F), and would have killed off many species.

So there you go…this is one place I think I’d avoid if I had a time machine. It was a thick-aired, muggy, sweltering oven, with giant snakes crawling about. They were likely to have eaten large crocodilians, so I suspect a time-traveling human would be nothing but a quick hors d’ouevre. They’re still interesting, though, especially as an example of evolution and climate science meeting in a mutually revealing fashion.

i-5320a599e877d3e70f1e48e0fb660ba2-titan_recon.jpeg

Head JJ, Block JI, Hastings AK, Bourque JR, Cadena EA, Herrera FA, Polly D, Jaramillo CA (2009) Giant boid snake from the Palaeocene neotropics
reveals hotter past equatorial temperatures. Nature 457(7230):715-718.

Maiacetus

My teaching schedule this semester is a major time-suck; I’m teaching genetics and all of its associated labs (you really don’t want to know how much prep time goes into setting up fly labs), I’m doing some major revision of the content this year, and I’ve got this asymmetric schedule that packs everything into the first half of each week. So I simply have to protest when those evil (Stein was right!) scientists announce a major discovery on a Tuesday, which just happens to be the very worst day of the week for me. They’ve gone and found another important whale transitional fossil, Maiacetus, and I’m just going to have to tell you to go read a bunch of other fine blogs that already have it covered.

i-bd63fdc1acedcd8fb4bd46a00b992235-maiacetus.jpeg
(Click for larger image)
Skeletons of the Eocene archaeocete whales Dorudon atrox and Maiacetus inuus in swimming pose.

(A, B)- Dorudon atrox (5.0 m; 36.5 Ma) based on UM 101222 and 101215 [11] in lateral and dorsal views, respectively. (C, D)- Maiacetus inuus (2.6 m; 47.5 Ma) based on male specimen GSP-UM 3551 in lateral and dorsal views, respectively.

It’s beautiful. It’s clearly adapted for aquatic life, but it has another revealing feature: this specimen was pregnant at death, and the fetus is oriented for a head-first birth, which is not good for birth at sea (the head would pop out, baby would take its first breath, and drown before the tail emerged), so this animal would have had to give birth on land.

But like I said, you’ll have to read Carl Zimmer, Ed Yong, Brian Switek, or Greg Laden this time around for all the details. Or read the paper yourself! It’s freely accessible.