Category: fossils

More on placental mammals

February 11, 2013 • 3:00 pm

by Greg Mayer

There have been a number of interesting comments by readers on my post on the recent paper on the radiation of placental mammals by Maureen O’Leary and colleagues. I want to respond briefly to a few of them here.

Biogeography. Does this paper imply that the origin and geographic distribution of the  major lineages of placental mammals are not well correlated with the breakup of the Mesozoic super-continents? Yes, it does. The authors explicitly say so, and therefore would invoke more dispersal events to account for mammalian distribution. Now their cladogeny and timing may be wrong, and lack of congruence with plate movement might be a reason for preferring an alternative phylogeny, but the authors do correctly recognize the biogeographic implications of their phylogeny.

G.G. Simpson, one of the founders of the Modern Synthesis, was also one of the most influential mammalogists of the 20th century, who often dealt with large scale issues of mammalian history. He delayed accepting plate tectonics until much later than most other zoogeographers, in part because many major continental movements took place well before the diversification of modern mammals, and thus plate tectonics seemed ‘unnecessary’ as part of an explanatory schema. He might well have been pleased by this aspect of O’Leary and her colleagues’ work.

Publication venue. I criticized the publication as a short paper in Science of a work that clearly deserves and requires monographic treatment. There is an obviously correct place to publish this work: in the Bulletin of the American Museum of Natural History. Many of the authors hold positions at the American Museum, and the Bulletin is explicitly designed for the publication of monographic works. Indeed, Simpson published one of his most important monographs on mammalian classification in the Bulletin. As the preceding link shows, the AMNH, quite admirably, makes all its publications available as free pdf’s, so there would be no question of access. In fact, access would be much greater, since all the material would be in a single freely available work, and not dependent on accessing a variety of websites of unknown permanence and varying cost.

Some readers have noted the tyranny of popularity and attention that journals like Science and Nature exert, and I can certainly sympathize with the authors’ desire to have their work widely read. But ultimately, scientific work must be judged by its data, methods, and conclusions, and publication in Science hampers the paper’s evaluation as a work of science. Science has published summary papers that present the main conclusions of monographic works; Jared Diamond’s 1973 paper in Science summarizing his 1972 monograph on New Guinean birds is an example. As Diamond wrote, “A recent book discusses in detail many of the examples summarized here”, but the monograph to explicate O’Leary’s work may never appear. Perhaps Science no longer does this, but a near simultaneous publication of short summary and Bulletin would have been far preferable.

Are the conclusions correct? This is the $64,000 question. I think the initial critiques come in two parts. First, don’t we already have fossil representatives in the Cretaceous of several of the modern orders of placental mammals? Well, a number of fossils have been so identified, but O’Leary et al. (and others) would dispute these identifications. Their paper does not include a careful analysis of these cases, and their fossil sample is not exhaustive, but does include most of the very well known Cretaceous mammals. Many fossil mammals are preserved only as teeth and thus hard to identify conclusively; O’Leary et al. commendably included only the most completely known forms so as to be able to observe as many as possible of the large number of characters they used. They do agree that there are basal placentals in the Cretaceous. (Or, to use the term they would probably prefer, “non-placental eutherians”. Eutherian and placental are treated as synonyms by some, but they formally distinguish the Placentalia as only members of the least inclusive clade that includes all living placental mammals; these taxon name questions are not important for their main points.) But these Cretaceous forms are, by their estimation, not in general ancestral to the Cenozoic forms– they believe only a single placental lineage survived into the Cenozoic.

Second, critics ask, isn’t using the literal fossil record a pretty crude way of determining ages of taxon splits, since such ages are always minimum ages? And shouldn’t the richer information available in molecular sequence data that is time-calibrated by securely known fossil dates be used? Well, the critics will answer “yes” to both questions, and will also point out that the fossil record is imperfect, so to say we don’t have any fossils dated to the Cretaceous is different from saying no such animals existed then. O’Leary et al. might reply that all molecular dating requires geological calibration, so that the fossil data is primary, not the molecular extrapolation; and that we have lots of Cretaceous mammal fossils, and none of them are obviously the varied precursors of the Cenozoic placental radiation.

(There are also questions about the exact sequence of splits in their phylogeny, and how molecular and morphological data agree or disagree. These discussion will be of most intense interest to specialists in the various groups, although there is considerable general interest in them as well.)

Who’s right? I don’t know. But that’s what the upcoming arguments will be about.

The Piltdown Hoax at 100

December 18, 2012 • 12:16 pm

by Greg Mayer

The Geological Society (London) is having a special meeting today to commemorate the 100th anniversary of the Piltdown  hoax. There will also be a tour of a new special exhibit at the Natural History Museum (which also has a nice Piltdown website). It was exactly 100 years ago today that Charles Dawson, a Sussex antiquarian, along with Arthur Smith Woodward and Grafton Elliott Smith, both accomplished biologists, announced the discovery of the remains of a heretofore unknown hominid in a gravel pit in Piltdown, East Sussex.

Piltdown skull
A reconstruction of the Piltdown skull. Bone dark, reconstructed elements in white. Note that the skull consisted of parts of the braincase, nasals, and a right mandible, with no connections between the parts. Note also that the mandible is missing the anterior teeth.

Dawson claimed that the right mandible (broken anteriorly) and the braincase pieces had been recovered from the gravel pit. It appeared to show that in the Pleistocene of Britain there occurred a large-brained but ape-jawed ancestor of man. Many of Britain’s most prominent paleontologists, anatomists, and zoologists concurred with this analysis.

Piltdown gang
The “Piltdown gang”. Standing: F.O. Barlow, Grafton Elliot Smith, Charles Dawson, Arthur Smith Woodward. Seated: A.S. Underwood, Arthur Keith, W.P. Pycraft, and Sir E. Ray Lankester.

From the start, however, there was controversy as to whether the man-like cranium and ape-like jaw were from the same species. The fact that the intervening parts of the skull were missing made it impossible to demonstrate the case morphologically. Regardless, the general conclusion was to accept the association of the jaw and cranium. When Australopithecus was discovered in Africa by Raymond Dart in the 20’s, the significance of his find was underestimated since the African form had a man-like jaw and an ape-like cranium– the opposite of Piltdown. As further discoveries showed that Piltdown man was aberrant, it came to figure less prominently in accounts of human evolution.

Beginning in the late 1940’s a series of investigations were begun that explained the source of the anomaly– the Piltdown remains were not only not associated, they were relatively recent, had been stained to look old, and filed down to change their appearance– they were a deliberate forgery!  In 1953, J.S. Weiner, K.P. Oakley, and W. Le Gros Clark published their results in a Natural History Museum monograph (reference below), and, appropriately, presented their results a few days later at a meeting of the Geological Society. A longer monograph appeared two years later.

Exposure of Piltdown hoax
Meeting of the Geological Society on 25 November, 1953, at which K.P. Oakley presented evidence for the fraudulent nature of the Piltdown specimens (Daily Mail; the Mail only identifies the photo as of the meeting at which the hoax was exposed; The Times for 26 November 1953 gives the further details of the meeting).

There had, in fact, long been doubts about the provenance of the Piltdown specimens. In 1914, W.K. Gregory (1914: 190-191) expressed it pretty directly:

It has been suspected by some that geologically they are not old at all; that they may even represent a deliberate hoax, a negro or Australian skull and a broken ape-jaw, artificially fossilized and “planted” in the gravel-bed, to fool the scientists. Against this suggestion tell the whole circumstances of the discovery as above related.

At the time, Gregory accepted the story of discovery. The following year, G.S. Miller strongly stated that the specimens were not associated, declaring unequivocally that the jaw was simian, the cranium human. More elliptically, he wrote (1915: 1):

Deliberate malice could hardly have been more successful than the hazards of deposition in so breaking the fossils as to give free scope to individual judgment in fitting the parts together.

Colin Groves was told that Miller actually thought it was a hoax, but that decorum prevented him from saying so without proof, and so he contested the find on scientific grounds; it would be interesting to see what is written in Miller’s papers about the matter (perhaps someone has already looked). Another American zoologist, W.D. Matthew, readily accepted Miller’s conclusions (Matthew in Eastman et al., 1916: 107):

In the present reviewer’s opinion [W. D. M.] Dr. Miller’s argument is convincing and irrefutable; the jaw belonged to a chimpanzee and the skull to a species of man comparable with that represented by the Heidelberg jaw. It is hardly to be expected, however, that this conclusion will be readily accepted by the European writers, who have with but few exceptions committed themselves more or less deeply to the opposite view.

And, changing his previous view, so did Gregory (1916: 313):

In an earlier paper (1914) I have reviewed the controversy over the Piltdown remains (Eoanthropus dawsoni), emphasizing the entirely human character of the brain-case, the essentially ape-like character of the lower jaw and teeth and the doubts as to their association already expressed by several authors.

While I take no delight in the predicament of those taken in by the hoax, I do delight in the fact that Miller, Gregory, and Matthew were among those to see the true nature of the material: I have had occasion to refer and use their work in my own researches and teaching, and Gregory is one of my academic grandfathers.

After the exposure of the hoax, the incident became chiefly of historical rather than scientific interest, and the question of who perpetrated the hoax has been of recurring interest. Suspicion first fell on Dawson, but has included Smith Woodward, Arthur Keith, Teilhard de Chardin, and even Arthur Conan Doyle!  Miles Russell of Bournemouth University has discovered that Dawson was involved in a number of frauds and fakeries, and an extensive summary of his case against Dawson is online. He is speaking today at the Geological Society, and has just published a new book on the subject (reference below).

In addition to the new exhibit and the conference, Chris Stringer of the Natural History Museum is leading a team that will conduct further tests on the bones that they hope will allow them to produce more definitive evidence of who was involved in perpetrating the hoax. Both the conference and the further testing have drawn considerable media interest in the UK (Guardian, Telegraph, BBC, Daily Mail). Further online information on the Piltdown story can be found at the late Richard Harter’s website, a Clark University site with transcripts of many original papers, and at the Talk Origins archive.

h/t Sigmund

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Dawson, C., A. Smith Woodward, and G.E. Smith. 1913. On the discovery of a palæolithic human skull and mandible in flint-bearing gravel overlying the Wealden (Hastings Beds) at Piltdown, Fletching (Sussex). Quarterly Journal of the Geological Society 69:117-147. (transcription)

Eastman, C.R., W.K. Gregory, and W.D. Matthew. 1916. Recent progress in vertebrate paleontology. Science New Series 43:103-110.

Gregory, W. K. 1914. The Dawn Man of Piltdown, England. American Museum Journal 14:189-200. (pdf)

Gregory, W.K. 1916. Studies on the evolution of the Primates. Bulletin of the American Museum of Natural History 35:239-355. (pdf)

Matthew, W.D. 1916. Note on the association of the Piltdown skull and jaw. Bulletin of the American Museum of Natural History 35:348-350. (in Gregory, 1916)

Miller, G.S. 1915. The Jaw of the Piltdown Man. Smithsonian Miscellaneous Collections 65(12):1-31. (Biodiversity Heritage Library)

Russell, M. 2003. Piltdown Man: The Secret Life of Charles Dawson. Tempus Books, Stroud, UK. (Amazon)

Russell, M. 2012. The Piltdown Man Hoax: Case Closed. History Press, Stroud, UK. (publisher)

Weiner, J.S., K.P. Oakley, and W.E. Le Gros Clark. 1953. The solution of the Piltdown problem. Bulletin of the British Museum (Natural History) Geology 2:139-146. (Biodiversity Heritage Library)

Giant arthropods, then and now

October 25, 2012 • 7:43 am

by Greg Mayer

The Naturmuseum Senckenberg in Frankfurt am Main, one of Europe’s great natural history museums, has announced the discovery in Laos of one of the world’s largest known daddy longlegs by Senckenberg researcher Peter Jager. The apparently new species is now being studied by Jager and his Senckenberg colleague, Ana Lucia Tourinho. Daddy longlegs are also called harvestmen, although I grew up with lots of daddy longlegs, and never knew them to be called harvestmen except in books.

(The current incarnation of the Senckenbergischen Naturforschenden Gesellschaft incorporates the museum in Frankfurt and several other German natural history museums and research institutes.)

Many people mistake daddy longlegs for spiders. While both spiders and daddy longlegs are eight-legged arachnids, daddy longlegs have a more compact body with the abdomen and cephalothorax not separated by a constriction as in spiders, and their legs are invariably thread-like. The new giant form from Laos, which has not yet been formally described as a new species, has a leg span of over 33 cm. The record is 34 cm for a South American species.

Although science fiction films abound in giant arthropods or arthropod-like aliens (my favorite: Attack of the Crab Monsters), it is in fact hard for an

Attack of the Crab Monsters (1957), original poster from Wikipedia.

arthropod to get very large. The size limitation of arthropods is due to constraints on their chitinous exoskeletons and their ability to breathe. The University of California, Berkeley, website Understanding Evolution has great animations explaining these size constraints. Both kinds of constraints arise from the disproportion of size and shape as animals get bigger. Basically, many of the functions of animals (including their weight bearing skeleton and their respiratory surfaces) increase as the square of their increase in size, whereas their need for the functions increases as the cube of their size. So, if you double the size of an animal, without changing its shape, you will quadruple its surface area, but its volume will increase by a factor of eight. Thus if you have a physiologically important surface area (say the lining of your respiratory system), you are going to be lacking by a factor of two, as your oxygen needs are set by your volume. The “square/cube problem” in biology has long been known, and J.B.S. Haldane, one of the founders of modern evolutionary theory, wrote an influential popular article on the subject, “On being the right size“, in 1926. (Haldane, by the way, a geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate, was the original most interesting man in the world, as we’ve noted before here at WEIT.)

The largest insects today are not very big. They grew larger in the distant past, and this is thought to be related to a higher concentration of oxygen in the atmosphere at that time (35% during the Carboniferous period, compared to 21 % today). The largest known arthropod ever, also an arachnid, but an aquatic one, a sea scorpion or eurypterid from the Devonian, was discovered by Simon Braddy and colleagues a few years ago (pdf).

Giant arthropods from the fossil record compared with the average height of a (British) human male; (a) the eurypterid Jaekelopterus rhenaniae, Early Devonian, Germany; (b) the trilobite Isotelus rex, Late Ordovician, Manitoba, Canada; (c) the dragonfly Meganeura monyi, Late Carboniferous, France; (d) the millipede Arthropleura armata, Late Carboniferous, Europe. Scale bar (a–d), 50 cm. (e) Chelicera of the giant eurypterid J. rhenaniae from the Early Devonian of Willwerath, Germany, PWL 2007/1-LS. Photograph, the disarticulated fixed (above) and (rotated) free ramus (below). Scale bar, 10 cm.

They were 2.5 m long, and longer if you stretched out their claws (and imagine the size of the turds produced by d!). This is, as shown in the figure above somewhat larger that the typical British male (who, I must say, appears admirably buff and well-muscled in this outline drawing; perhaps it’s all that wrist-wrestling, or is it elbow-bending, down at the pub).

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Braddy, S.J., M. Poschmann, and O.E. Tetlie. 2008. Giant claw reveals the largest ever arthropod. Biology Letters 4:106-109  (pdf)

Haldane, J.B.S. 1926. On being the right size. Harper’s Magazine (March) 424-427. (retyped pdf)

h/t Andrew Sullivan

230-million-year-old arthropods in amber

September 19, 2012 • 8:38 am

I won’t discuss this new observation in detail, since the detail is mostly of interest to specialists, but it’s still a cool observation. A new paper in Proc. Nat. Acad. Sci. USA by Alexander Schmidt et al. (reference at bottom) describes specimens of two groups of arthropods—flies and mites—from Italian amber (fossilized resin) that is 230 million years old.

That pushes the amber-preserved individuals of these groups back a full 100 million years, and makes these the earliest fossils of the mite superfamily Eriophyoidea, a highly specialized group of plant parasites. Most eriophyoid species feed on angiosperms (flowering plants), producing galls, while about 5% of the species feed on gymnosperms (conifers, cycads, etc.); the latter mites are considered to be ancestral because the most primitive group of these mites still live on gymnosperms, and gymnosperms precede angiosperms in the fossil record (the latter originated from gymnosperm-like ancestors only about 130 myr ago).

Here are some galls of Eriophyes tiliae on lime (see more galls here); weird, eh?

And here’s the beast who makes them; note how bizarre and specialized it is:

The amber came from gymnosperms in a family of extinct conifers (Cheirolepidaceae); here’s one of their fossils (all captions in photos and drawings are from the original paper):

Cheirolepidiaceous shoots associated with amber. Museo delle Regole, Cortina d’Ampezzo, Italy, MRCA 7170.

The authors examined 70,000 (!) ancient amber droplets for arthopod inclusions, and found only three. I’ll show them all.  The first is a midge, which is a fly (Diptera); it is disarticulated and small (1.5-2 mm).  The specimen, though not whole, is very detailed.

(G and H) Disarticulated nematoceran fly, showing details of antenna and apical tarsomere. Museum of Geology and Paleontology, University of Padova, Italy, MGP 31345. Scale bars: B, 2 cm; C–F, 1 mm; G and H, 200 μm.

And here are the two mite specimens, preserved whole. Note that the scale bar is 10 microns, or 0.01 mm. These things are small!

Eriophyoidmite in the Italian Triassic amber: Triasacarus fedelei gen. et sp. nov., Holotype, MGP 31343. (A and C) Habitus in ventral view [reconstruction and photomicrograph, respectively; photo is a stacked image using differential interference contrast (DIC) illumination]. (B) Dorsal structures of anterior region, as viewed ventrally. (D) Gnathosoma, arrow pointing to infracapitular ledge [bright field (BF) illumination; f.p (focal plane) 2,347]. (E) Detail of F; arrows pointing to empodial featherclaws (BF, f.p. 2,324). (F) First and second leg pairs, with tip of proboscis in focus (arrow) and empodial featherclaw of first left leg indicated with arrow (BF, f.p. 2,324). (G) First and second leg pairs, with some solenidia denoted, tibial one by phi, tarsal ones by omega (DIC, f.p. 2,160). Scale bars: 10 μm.

Eriophyoid mite in the Italian Triassic amber: Ampezzoa triassica gen. et sp. nov., Holotype, MGP 31344. (A and B) Habitus, dorsal view. (A) Digitally stacked photomicrographic composite. (B) Rendering of complete specimen, as preserved. (C) Anterior portion of body, including gnathosoma. White arrows indicate infracapitular guides; black arrow points to second left leg (f.p. 2,904). (D) Portion of prodorsal and coxisternal region; arrows point to shadowy images of right legs I and II below prodorsal shield (f.p. 2,692). (E) Posterior apex of body; arrows point to caudal setae h2 (f.p. 2,932). All photos in DIC illumination. Scale bars: 10 μm.

What’s the significance? Well, it supports the phylogenetic (“family tree”) evidence that eriophyoid mites did indeed evolve feeding on gymnosperms, for these samples date 100 million years before angiosperms even existed. So it’s a nice confirmation of what we suspected from other data.  It also supports the age of the mite group, since there was some doubt about when it originated.  Other findings are of interest mainly to arthropod systematists, but the quality of these ancient specimens is so nice that I thought I’d present them.

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Schmidt, A. R, S. Jancke, E. E. Lindquist, E. Ragazzi, G. Roghi, P. C. Nascimbene, K. Schmidt, T. Wappler, and D. A. Grimaldi. 2012. Arthropods in amber from the Triassic Period. Proc. Nat. Acad. Sci. USA 109: 14796-14801.

Sexual selection in ancient animals

September 12, 2012 • 6:01 am

Sexual selection, which is a subset of natural selection, is defined as “selection based on mate choice.”  It usually, but not always, takes the form of males competing for access to females, and results in the development of either armaments in males that help them compete in the battle for mates (antlers on deer, horns on stag beetles, etc.), or bright plumage, coloration, adornments, calls, or behaviors of males that catch the fancy of females (the bowers of bowerbirds, the plumage, colors, and strange behaviors of the New Guinea birds of paradise, the songs of male frogs, etc.).  We understand the competition scenario more than the “female preference” scenario, for it’s hard to figure out why females would prefer one plumage or adornment rather than another.

Nevertheless, the signs of sexual selection of living species are fairly clear (males have the weapons or adornments, females don’t), and we can do tests to see whether sexual selection seems to be a reasonable explanation.  In peacocks, for example, you find that males (the ones with the big tails!), who have more “eyespots” in their tails get more mates, sage grouse who display the most vigorously also win the mate race, as do African widowbirds with longer tails.

It’s harder to do these studies in extinct species, but a new paper by Knell et al. in Trends in Ecology & Evolution (reference below), gives some nice examples of possible sexual selection in fossils, as well as a list of things about the fossils that might indicate sexual selection.

First, a few fossils suggestive of sexual selection:

Skull of the Oligocene artiodactyl Protoceras celer with maxillary and supraorbital protuberances, a knob-like parietal protuberance, and enlarged upper canines.
The fan-shaped cranial crest of the Cretaceous hadrosaurid dinosaur Olorotitan arharensis.

Of course, features like the fan-shaped crest above could have had other functions (e.g., thermoregulation, species recognition, etc.), so we have to use other criteria, delineated below, to see if sexual selection is a likely explanation.

Here’s a weird one—perhaps the strangest trilobite of all, Walliserops trifurcatus from the Devonian of Morocco (Photo: Dr. B.D.E. Chatterton).  Check out those horns and the bizarre trident on its head.  Richard Fortey showed this one at a talk I saw in Madrid last year, and I believe he gave a functional explanation for it rather than imputing it to sexual selection. (The photo, by the way, is of a real fossil.)

Now, how can we get an idea whether such bizarre features arose by sexual selection as opposed to other mechanisms (direct natural selection not based on mate choice, genetic drift, as nonadaptive pleiotropic byproducts of other features that evolved, and so on)? The authors suggest five observations:

  • Sexual dimorphism.  This is the most obvious feature: if an exaggerated trait or ornament is found in only one sex but not the other (preferably males), that suggests sexual selection.  There are a couple of problems here. First, if you see two forms of fossils, one with a trait and one without, those could be males and females of one species, or they could be two different species, one of which has ornaments in both males and females. Also, there are forms of mutual sexual selection, in which males and females both prefer a trait, that could lead to both sexes having ornaments, and thus no difference to be seen in the fossil record.

But here’s one lovely example from the paper in which sexual selection is likely. It occurs in pterosaurs, flying reptiles of the Jurassic. As the TREE article notes:

Darwinopterus is a small pterosaur from the Middle Jurassic of China, known from numerous specimens. Some individuals are crestless, whereas others possess a bony crest located along the midline of the skull, which was probably associated with soft tissues that enlarged crest size substantially in life [77]. Crested specimens have a proportionally smaller pelvis and ventrally fused pelvic elements, whereas crestless specimens have an unfused, wider pelvis. Furthermore, one crestless specimen has a pterosaurian egg preserved in close association with its pelvis and so is clearly a female.

Here’s the uncrested female Darwinopterus showing her egg (arrow) immediately outside the cloaca (photo by Lü Junchang):

And here’s a reconstruction by Mark Witton that shows the sexual dimorphism of Darwinopterus:

  • Change in growth rate during development.  Because sexually selected traits are “expensive” in terms of metabolic/reproductive cost, and could make animals susceptible to predation, in living animals they tend to show up only at the end of development, when the animal is an adult and ready to reproduce. Indeed, that’s exactly where we’d expect to see them develop given the modern theory of evolution.  And this is indeed the case, as is seen in things like peacock feathers and deer antlers.  If one sees this in one sex of fossils (as we do with the bony crests of pterosaurs), it suggests sexual selection.
  • “Positive allometry”.  This means that as the body of an animals gets bigger during growth, the feature gets proportionally bigger than that. For example, if the linear dimension of body parts increases by a factor of 2 during development, sexual selection might be indicated if the size of the ornament increases by a factor of 3.5.  This is really related to the previous point; neither are conclusive, of course, because one can have such positive allometry for other reasons, for example simply because of the constraints of development or biomechanical reasons.  To deal with this, the authors suggest testing whether the degree of allometry is one expected under a hypothesis of sexual selection versus hypotheses like thermoregulation or use as a rudder (as in the crests of flying dinosaurs).
  • “Morphological disparity”.  This means that if there are related species in a group in which one or more species show evidence of sexual selection, that group will show a diversity of different traits that are elaborated.  For example, the birds of paradise of New Guinea are sexually selected, with the males having bright ornaments, coloration, feathers, and bizarre courtship behaviors, and yet the traits of males differ drastically among related species. This is probably because female preferences vary erratically among different species (I won’t go into why this might be the case), and so the males also come to differ. Click on this Google image search that shows some of the “morphological disparity” among sexually selected males in those birds.
  • Costliness.  Sexually selected traits tend to be “costly”: that is, they take a lot of metabolic energy that could be diverted to other traits, or even reduce survival by making the male bearer visible to predators. Male peacocks are not only highly visible to predators because of their coloration, but their long tails make it hard for them to fly.  Of course, if such traits are sexually selected, the advantage gained by attracting more females has to outweigh the “cost” of bearing the ornament; and experiments in some species show that this is indeed the case.  As the paper explains, if traits arose for another function, say to recognize members of the same species, one wouldn’t expect them to be so costly.

Remember that these features are indications of sexual selection in the fossil record, not absolute proof (which we don’t get in science anyway), and it’s hard to test various alternative hypotheses.  The evidence for sexual selection becomes stronger if the traits in a fossil species show a combination of the different properties shown above.  I find the example of Darwinopterus (above) pretty convincing.

I’ll leave you with one more example of a fossil species that might have experienced sexual selection: this is a Cambrian trilobite, Parablackweldeeria luensis (from the paper), showing a fossil (a) and reconstruction (b). The animal was unusual in having its eyes on the ends of long eyestalks:

This resembles the long eyestalks shown in diopsid flies, like this speciemen of Teleopsis dalmanni, in which males have eyes on long eyestalks as a result of sexual selection (the males “face off” against each other and butt each other with their heads; long eyestalks give an indication of head size and may help settle contests by forcing the smaller fly to give up sooner.

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Knell, R. I., D. Naish, J. J. Tomkins, and D. W. F. Hone.  2012. Sexual selection in prehistorical animals: detection and implications. Trends Ecol. Evol. Early online publication, Sept 7. DOI 10.1016/j.tree.2012.07.015

The last perambulation of an ancient arthropod

September 8, 2012 • 8:46 am

From BBC Science News via a series of tweets, including Barbara King, Steve Ashley, and finally an email from Matthew Cobb, an amazing fossil finds its way to us.

As reported by Nick Crumpton, a fossil “death march” of a horseshoe crab was found in the Solenhofen limestone—the same formation that yielded the famous transitional fossil Archaopteryx.  The fine-grained sediments from what was once a quiet lagoon produced exquisite preservation, and in this case we have what is interpreted as the final walk of a horseshoe crab flung into the lagoon by a storm (the storm part is speculative) 150 million years ago.  Here’s the animal:

Note the phenotypic similarity to modern horseshoe crabs, a similarity which makes this animal a famous “living fossil.” Of course they’re not externally identical to modern ones, and we know nothing about the changes in its anatomy, biochemistry, or simply DNA sequence, which presumably has changed via the molecular clock in the last 150 million years.

After it purportedly landed in the lagoon, the crab began to walk, and made 9.7 meters before it died. (Remember, horseshoe crabs are in the subphylum Chelicerata, not Crustacea, so they’re more closely related to spiders and scorpions than to “real” crabs, which are crustaceans.)

As the BBC reports:

The fossil records an entire walk, and the researchers believe that the abrupt beginning of the trace can be explained by the animal being “flung” into the lagoon during a storm, although they cannot be certain of this interpretation. . .

“The lagoon that the animal found itself in was anoxic, so at the bottom of these lagoons there was no oxygen and nothing was living,” Mr Lomax [Dean Lomax of the Doncaster Museum and Art Gallery] told the BBC.

“This horseshoe crab [Mesolimulus walchi] found itself on the lagoon floor and we can tell by looking at the trace that the animal righted itself, managed to get on to its feet and began to walk,” he explained.

However, the anoxic conditions of the lagoon floor quickly proved fatal to the arthropod and it soon began to struggle.

“We started to study the specimen closer and saw that the walking patterns and the animal’s behaviour started to change. The leg impressions became deeper and more erratic, the telson (the long spiny tail) started being lifted up and down, up and down, showing that the animal was really being affected by the conditions,” he said.

And here’s the fossil trackway, also preserved in those optimal conditions. The animal moved from right to left in the picture (tracing is below), and you can see the fossil at the end.

RIP, ancient arthropod.