Birds of Stone: Avian Fossils from the Age of Dinosaurs

January 27, 2016 • 8:00 am

by Greg Mayer

This coming Monday, February 1, at 7 PM in the Student Union Cinema, the University of Wisconsin-Parkisde will present Luis Chiappe, Director of the Dinosaur Institute of the Natural History Museum of Los Angeles County, will speak on “Birds of Stone: Avian Fossils from the Age of Dinosaurs”.

Many of the features commonly associated with birds (feather, wings, hollow bones, wishbones) were inherited from their dinosaurian ancestors, and these features arose at various times during the birds’ long Mesozoic history. New fossils have laid out this evolutionary saga in great detail, allowing us to trace the changes from the earliest birds, such as Archaeopteryx, to the dawn of modern birds. The talk, part of UW-Parkside’s Science Night series, is intended for the general public.

At noon on Monday, in Molinaro Hall D 139, Dr. Chiappe will present a more technical talk at the Biological Sciences Colloquium entitled “Birding in the Mesozoic: Recent Insights on the Early Evolution of Birds”. There’s also a small exhibit in the UWP Library, “Dinosaurs and Birds: The Art of Science”, that you can stop in and see.

Both talks are free and open to the public. For the evening talk, parking in the Student Union lot is free after 6:30 PM. For the noon talk, there are metered spots, but if any WEIT readers are planning to come, email and I’ll see what we can do. The talks are presented in conjunction with the exhibit “Dinosaurs Take Flight: The Art of Archaeopteryx”, by Silver Plume Exhibitions in conjunction with the Yale Peabody Museum, at the Kenosha Public Museum, on display now through March 27th.

This is a very well done exhibit, combining fine reproductions of almost all of the eleven known Archaeopteryx specimens (the real ones almost never travel!), with an exploration of how several distinguished paleo-artists create their works, including Julius Cstonyi, whose work we’ve highlighted here at WEIT before.

Anyone from Chicago to Milwaukee is within range, and you can make a day of it– the exhibit at the KPM, two talks, and a stop in UWP’s Library. Even if you can’t make it Monday, the exhibit at KPM is well worth a trip on some other day. Here’s a tidbit– a realistic sculpture– from Dinosaurs Take Flight; I hope to post a fuller report later.

Archaeopteryx at its nest. — *Archaeopteryx* at its nest.

David M. Raup, 1933-2015

July 16, 2015 • 4:39 pm

by Greg Mayer

David Raup, one of the leading figures in the return of paleontology to the “high table” of evolutionary biology in the late 20th century, died this past Thursday, July 9, at the age of 82. Raup attended the University of Chicago as an undergraduate, got his doctorate at Harvard, and was associated most prominently first with the University of Rochester, and then again the University of Chicago, from which he retired in 1995.

David M. Raup, 1933-2015 — David M. Raup (1933-2015) in 1981.

Beginning in the 1970s, paleontology was rejuvenated by a renewed interest in what the fossil record shows about both the broad scale patterns of changes in biodiversity through time, and the details of how particular lineages change through time. Raup was one of the most influential figures in this recrudescence, along with his colleagues Stephen Jay Gould, Tom Schopf, and Jack Sepkoski. The great British geneticist John Maynard Smith, said in 1984 of this flowering of paleontology, “The paleontologists have been too long missing from the high table. Welcome back.”

Raup’s most distinguished contributions came in two areas, both marked by a sophisticated, quantitative, approach. In the first, he made great strides in the area of theoretical morphology, developing mathematical descriptions of the possible shapes of mollusk shells, and then asking which parts of the morphological space defined by these equations are occupied, which are not occupied, and why. In a popular exposition based on Raup’s work, Richard Dawkins called this morphological space “The Museum of All Shells”. The mathematical description of shells that don’t exist (i.e. those that are in the parts of the morphological space not occupied) might seem odd or unnecessary, but understanding the possibilities of morphological transformation is key to understanding what it is that constrains, and what it is that enables or directs, evolutionary change. As A.S. Eddington put it, “We need scarcely add that the contemplation in natural science of a wider domain than the actual leads to a far better understanding of the actual.”

Raup's (1966) morphological space-- the — Raup’s (1966) morphological space– “The Museum of All Shells”.

In the second, and more extensive, area of his distinguished contributions, Raup looked at levels of diversity, origination, and extinction through time in order to describe the pattern of these events and to model processes that could account for them. He was particularly interested in the relative influences of random versus deterministic factors in explaining the broad patterns in the history of diversification and extinction, notably detecting a periodicity in the history of mass extinctions.

A 26 million year periodicity of mass extinction (still debated) from Raup and Sepkoski (1980). — A 26 million year periodicity of mass extinction (still debated) from Raup and Sepkoski (1984).

In addition to his intellectual contributions, Raup had a significant effect on the institutional development of the field. In 1975, he was one of the founding members of the editorial board of Paleobiology, a journal dedicated to advancing, and a marker of, paleontology’s growth and renewed influence in the broader discipline of evolutionary biology. He had two papers in the inaugural issue, one coauthored with Schopf, Gould, and Daniel Simberloff. His other major contribution to the institutional development of the discipline was the publication, with Steven M. Stanley, of the influential textbook, Principles of Paleontology (1971; second edition 1978). Unlike previous paleontological textbooks, Principles had nary a key for identifying fossils or a compilation of taxa and their geological distributions: it was about the principles: systematics, biostratigraphy, paleoecology, evolution, and biogeography. On my own bookshelf, it sits inches away from an earlier influential text– Moore, Lalicker, and Fischer’s Invertebrate Fossils— which has a lonely chapter on principles, followed by 22 chapters and 700 pages of dense taxonomic and morphological detail. Raup stood out from traditional paleontologists, even among his fellow young Turks, for doing little or no descriptive systematic and stratigraphic work– even Gould had a long (and little-known) parallel publishing career on the systematic and zoogeographic nitty gritty of West Indian land snails of the genus Cerion— and his textbook reflects this.

Principles of Paleontology (2nd edition, 1978).

The University of Chicago remained a hotbed of palebiology after Raup’s retirement, and his influence there is still strongly felt, with luminaries such as Dave Jablonski and Raup’s former student Mike Foote carrying on the tradition; Mike, with Arnold Miller of the University of Cincinnati, has brought out a third edition (2006) of Raup’s textbook.

h/t Bob Richards

Dawkins, R. 1996. Climbing Mount Improbable. W.W. Norton, New York.

Foote, M. and A.I. Miller. 2006. Principles of Paleontology. 3rd edition. W.H. Freeman, New York.

Maynard Smith, J. 1984. Palaeontology at the high table. Nature 309:401-402 pdf

Moore, R.C., C.G. Lalicker, and A.G. Fischer. 1952. Invertebrate Fossils. McGraw-Hill, New York.

Raup, D.M. 1966. Geometric analysis of shell coiling: general problems. Journal of Paleontology 40:1178-1190. pdf

Raup, D.M. and J.J. Sepkoski. 1984. Periodicity of extinctions in the geologic past. Proceedings of the National Academy of Sciences USA 81:801-805. pdf

Raup, D.M. and S.M. Stanley. 1971. Principles of Paleontology. W.H. Freeman, San Francisco. (2nd edition, 1978).

T. rex badinage

June 16, 2015 • 1:42 pm

Reader John sent this tw**t showing an unfortunate pleiotropic effect of theropod morphology:

And, fortuitously, reader Bruce just sent me this humorous headline, which he said he saw on Janice Ian’s Facebook page (remember “Society’s Child“?):

Dinosaur feathers found in amber

April 21, 2015 • 10:00 am

UPDATE: I’m a real dummy; I failed to check the dates of any of these items and a eagle-eyed reader noted that they’re all from 2011! I should have seen that from the dates on the Science paper, if not the links. Oh well, it’s still interesting stuff.

*******

A paper in Science by Ryan McKellar et al. (reference and link below; no access to full paper without $$!) reports the amazing discovery of feathers preserved in amber from the late Cretaceous (around 65-90 million years ago). The amber (fossilized plant resin) was discovered in deposits at Grassy Lake, Alberta, Canada. There’s also a nice piece in The Atlantic which summarizes the importance of the findings and a series of photos at io9.com with explanations of what each photo shows. The striking finding is that there are all stages of feather evolution seen in the amber, from simple filaments to very complex feathers.

Now we’re not sure what creatures these feathers belong to, so I’m jumping the gun a bit with the title. Our uncertainty is because there are no fossils of anything in the layer where these feathers were found. But the authors justify the conclusion that these are dinosaur feathers because they’re in layers near those containing near dino remains:

Although neither avian nor dinosaurian skeletal material has been found in direct association with amber at the Grassy Lake locality, fossils of both groups are present in adjacent stratigraphic units. Hadrosaur footprints are found in close association with the amber, and younger (late Campanian and Maastrichtian) strata of western Canada contain diverse nonavian dinosaur and avian remains. There is currently no way to refer the feathers in amber with certainty to either birds or the rare small theropods from the area. However, the discovery of end- members of the evolutionary-developmental spec- trum in this time interval, and the overlap with structures found only in nonavian dinosaur com- pression fossils, strongly suggests that the proto-feathers described here are from dinosaurs and not birds.

Since we already have all kinds of fossil dinosaurs showing imprints of feathers, and those feathers seem to range from simple filaments to more complex feathers similar to those of modern birds, what does this finding add to what we already know? The answer is that these are 3-dimensional feathers that aren’t compressed, and give us a much better look at what early feathers were like. And that view supports a previously-suggested scenario of where bird feathers came from.

That scenario, as sketched in the paper, is diagrammed below. It begins with single filaments (I) that branch out to form a tuft of filaments (II), and some of these either coalesce to form a central shaft, or “rachis” (IIIb), or develop secondary branches (IIIa), with the next step the development of “tertiary” branches (IIIa + b).

Then those tertiary bits can either develop hooks (“distal barbules”: “d.b.” in the middle figure) or unhooked “proximal barbules” (“p.b.”). At this point, stage IV below, we pretty much have a modern feather with a central shaft and side filaments that hook together to make the feather into a unit (essential for flying). In stage V, other specializations develop. Remember, this figure represents a hypothesis about how feathers evolved before the amber feathers were found.

What the authors found was that basically every stage of feather development could be seen in the eleven specimens of amber analyzed in the paper. This, then, supports the scenario given above. I’ll show some photos from io9, but let me first add four things.

First this scenario for feather development suggests that feathers evolved, as I and many others long suspected, for thermoregulation. The filaments are, according to the authors, present in densities that would help thermoregulation and “protection” (I’m not sure what they’d protect), militating against any use in gliding (filaments don’t help you glide) and perhaps against ornamentation as their sole function (though they could also have served to ornament the bird).

Second, some of the feathers are so well developed that, according to the authors, they would have enabled the dinosaurs bearing them to fly. I’m not an expert on this, so I’ll take their word for it.

Third, some of the feathers are pigmented, and in ways similar to those of modern birds. Further, some of the filaments are coiled at their base, a feature that modern birds like grebes still have, and use to trap water to ferry to their young. Apparently the coiling of a straight filament enables the groove in the middle to trap water through capillary action.

Finally, all of these stages of feather evolution were found in roughly the same time period (same deposit), implying that there were all sorts of dinos coexisting with different degrees of feather evolution. Certainly not all of them gave rise to modern birds; most surely went extinct along with the rest of the dinosaurs. In fact, it’s likely that none of these specimens are from a species that was ancestral to modern birds.

Now to the feathers; all indented captions are from io9:

An isolated barb from a vaned feather, trapped within a tangled mass of spider’s web in Late Cretaceous Canadian amber. Pigment distribution within this feather fragment suggests that the barb may have been gray or black. Image via Science/AAAS

This is one of the later stages of feather evolution.

Below is an earlier stage when there were just filaments. Notice that there are many all together, which supports the notion that they could have been involved in thermoregulation.

Numerous individual filaments in Late Cretaceous Canadian amber. These filaments are morphologically similar to the protofeathers that have been found as compression fossils associated with some dinosaur skeletons. Pigment distributions within these filaments range from translucent (unpigmented) to near-black (heavily pigmented). Image via Science/AAAS

Here are the water-retaining feathers with filaments coiled at their base:

Cross-section through a feather with basally-coiled barbules, accompanied by a microphysid plant bug. The helical coiling observed within these barbules is most obvious in isolated barbules within the image, and is directly comparable to coils found in modern bird feathers specialized for water uptake. The high number of coils in the amber-entombed feather is suggestive of diving behavior, but similar structures are also used by some modern birds to transport water to the nest. Image via Science/AAAS

Pigmented feathers!

Series of six feather barbs in Late Cretaceous Canadian amber. Localized pigmentation creates a beaded appearance within each barbule: This has implications for the structural interpretation of fossil feathers exhibiting this general morphology. Pigment distribution within the specimen suggests that the feather would have originally been medium- or dark-brown in color. Image via Science/AAAS

More colored feathers.

A feather barb within Late Cretaceous Canadian amber that shows some indication of original coloration. The oblong brown masses within the dark-field photomicrograph are concentrated regions of pigmentation within the barbules. In this specimen, the overall feather color appears to have been medium- or dark-brown. Image via Science/AAAS

And more complex feathers.

Overview of 16 clumped feather barbs in Canadian Late Cretaceous amber. Image via Science/AAAS

_________

McKellar, R. C., B. D. E. Chatterton, A. P. Wolfe, and P. J. Currie. 2011 A diverse assemblage of late Cretaceous dinosaur and bird feathers from Canadian amber. Science 333:1619-1622

A new paper claims that evolution has stopped in a bacterial species. Is it true?

February 4, 2015 • 8:59 am

Several readers called my attention to a new paper by J. William Schopf and colleagues in the Proceedings of the National Academy of Sciences (reference below; free download at link), a paper that has also gotten a great deal of attention in the press. Last week a journalist asked me to comment on it, but I was too busy then to read it. Now that I have, I’m not all that impressed. It’s a decent paper, and doesn’t fail the first test of a science paper—does it tell us something new?—but I don’t think it makes the case that’s gotten the press all excited.

What is that case? The authors claim that their finding—an example of extremely slow evolution (in fact, no perceptible evolution) in a sulfur-producing bacterium—constitutes a test of Darwin’s “null hypothesis of evolution.” That hypothesis, as stated by the authors, is this:

. . . if there is no change in the physical-biological environment of a well-adapted ecosystem, its biotic components should similarly remain unchanged.

In other words, if there’s no selection pressure on the organism, it will not evolve. The authors implicitly equate “change in the physical-biological environment of a well-adapted organism” with “selection pressure on that organism,” and hence with “evolution of that organism,” but that’s not correct. I’ll talk more about this below, but for the time being try to guess how organisms could still continue to evolve in a relatively unchanging environment. Let’s look first at the author’s data, which, they claim, shows no evolution taking place in more than two billion years.

What Schopf et al. did, which is good stuff, is to examine bacterial microfossils in two ancient Precambrian biota and then compare them with modern fossils living in a similar environment. They were concerned with sulfur-using bacteria living on the ocean floor. The oldest formation examined was the “Turee Creek Group Kazput Formation” in northern Australia, a formation that’s ancient—about 2.3 billion years old. That’s old, but it doesn’t take the prize for age, for the oldest known bacteria (cyanobacteria, formerly known as “blue-green algae”) come from about 3.5 billion years ago.

The second group of fossil sulfur-metabolizing bacteria are about 500 million years younger than those from Turee Creek: they’re from the 1.8-billion-year-old Duck Creek formation, also in Western Australia. So we have about 500 million years of potential evolutionary change intervening between the two fossil formations.

Finally, the authors compared fossil bacteria from these two formations with sulfur bacteria that are still with us: a community of sulfur-using bacteria discovered in 2007 in the seafloor off the west coast of South America. All of these bacteria are presumed or known to use marine sulfur compounds, first reducing them to hydrogen sulfide and then oxidizing them to produce elemental sulfur and sulfur dioxide.

So what are the similarities among these three types of bacteria that led the authors to suggest that they hadn’t evolved over 2.3 billion years? There are three. First, all three bacteria lived in communities not in shallow water, but in the sea floor in deeper waters. Paleontologists have ways of telling this, and you can read the paper if you want to know how.

Second, they all have similar morphology, forming long filaments of about the same size. Here is what they look like; the figures show both the ancient bacteria from both formations and the modern collection (“A”). They form long, cylindrical filaments of comparable size

Third, chemical analysis of the matrix around the fossil bacteria, and of the modern ones, showed that they all lived or live in anoxic (“oxygen free”) communities, produce the same sulfur isotopes, and yield a form of pyrite as a metabolic product. The authors conclude, probably correctly, that the metabolic pathways for sulfur use in all of these forms are similar

So have they remained evolutionarily static, as the authors argue? First, let’s review their claim about these bacteria:

Once subseafloor sulfur-cycling microbial communities had become established, however, there appears to have been little or no stimulus for them to adapt to changing conditions. In their morphology and community structure, such colorless sulfur bacteria—inhabitants of relatively cold physically quiescent anoxic sediments devoid of light-derived diel [daily] signals and a setting that has persisted since early in Earth history—have exhibited an exceedingly long-term lack of discernable change consistent with their asexual reproduction.

They also claim that not only did the morphology and biochemistry of these species remain unchanged, but they didn’t form new species, either, although the concept of bacterial “species,” as Allen Orr and I showed in our book Speciation, is a bit hazy.

But I think the author’s conclusion is premature, and for two reasons.

First, regarding the “null hypothesis of Darwinism,” I think that that notion is wrong—or at least incomplete. Even in an unchanging environment, organisms can still evolve in significant ways. If new mutations arise that adapt the species better to that unchanging environment, then we will have evolution. For example, the bacteria could evolve more efficient metabolism of sulfur. Alternatively, one could have mutations that simply allow the bacteria to divide faster, giving them a selective advantage over others. Neither of these would be a response to a changing environment, but could still cause evolution. Considerable retooling of the bacteria’s metabolism, DNA synthesis, and so on, could still occur as evolution experiments with various mutations.

There is in fact one natural experiment that showed evolutionary divergence in an unchanging environment. There are salmon in some West Coast rivers (pink salmon, as I recall), that form “year classes”: they have a two-year life cycle and come into the same rivers from the sea to breed. They are basically the same species of salmon, but long ago a few stragglers switched form breeding in “even” years to breeding in “odd” years. Since the breeding seasons of the two classes don’t overlap, they became instantly reproductively isolated from each other in a unique way: they couldn’t interbreed, but at the moment the year divergence began they were genetically identical; and they continued to live in essentially the same environment. Yet over thousands of years the even- and odd-year forms have diverged, to the extent that they are somewhat reproductively incompatible when bred together—the beginning of speciation. This shows that a eukaryotic species living in very similar environments can still diverge genetically, and begin the process of forming new species.

Second (and the authors note this in passing), there could be considerable internal biochemical evolution taking place that can’t be detected from simply looking at the fossil bacteria or seeing if they metabolized sulfur in similar ways. After all, bacteria are morphologically simple, and there’s simply not that much room for visible change, especially if you’re constrained to look at fossil bacteria in rocks. And it’s impossible to sequence the DNA of the fossil bacteria or grow them in the lab, so we can’t see how genetically and metabolically different they are.

This is not just a theoretical possibility, for biologists are well familiar with this phenomenon. We know of many cases of “sibling species”: distinct species of closely-related organisms that can’t be told apart through morphology alone, but have diverged considerably in their non-visible characters. For example, I worked on a group of 8 Drosophila species in which females couldn’t be told apart by just looking at them, even under the microscope. (Males differed very slightly in their genitalia). Yet despite their morphological conservatism, the species diverged profoundly in ecology and in their reproductive compatibility: they don’t like to mate with members of the other species, and in many cases the inter-species hybrids were either inviable or sterile. Genetic analysis showed considerable divergence in the DNA, including in important traits affecting reproduction. The problem is even more severe if you are constrained to look at fossils, in which only the hard parts become mineralized. Important differences in softer parts that could reflect genetic change (granted, not that much of a problem in bacteria) could be missed.

The lesson is that it’s dangerous to use fossils—even bacterial fossils—to conclude that evolution hasn’t occurred. And the “null hypothesis of Darwinism” is a bit dubious anyway. Yes, species probably change most rapidly when the environment is changing, but there’s no reason why environmental change is a sine qua non for evolution.

The paper by Schopf et al. does show an intriguing case of morphological and metabolic stasis over billions of years, and it probably does reflect a lack of environmental change. But what it doesn’t show is that evolution hasn’t occurred in these bacteria. To know that, we’d have to have them all alive to sequence their DNA and look at their physiology, reproduction, and so on—and that’s impossible for the fossil species.
_________

Schopf, J. W., A. B. Kudryavtsev, M. R. Walter, M. J. Van Kranendonk, K. H. Williford, R. Kozdon, J. W. Valley, V. A. Gallardo, C. Espinoza, and D. T. Flannery. 2015. Sulfur-cycling fossil bacteria from the 1.8-Ga Duck Creek Formation provide promising evidence of evolution’s null hypothesis. Proc Nat. Acad. Sci. USA, online Early Edition. 10.1073/pnas.1419241112.

Readers’ wildlife photos

September 21, 2014 • 5:14 am

I believe this is the first time we’ve had fossils as readers’ wildlife. But remember that fossils once were wildlife, too, and these are particularly good specimens collected and prepared by reader Bruce Thiel.

30-40 million years ago, parts of Oregon and Washington were underwater. Marine animals that fell into the sediment were sometimes fossiized and can be found in the uplifted areas that erode out in streambeds or roadcuts. Six years ago I became interested in collecting and preparing these animals and it has morphed into a retirement project. I do not sell them but hope they will someday go into a museum collection for public display. Here are some of the more interesting ones I’ve uncovered. In the first three photos they are all Pulalius vulgaris including the small one next to the big claw—both found within 50’ of each other.

[JAC: If you were to buy these, you’d pay a pretty penny due to the labor involved in finding them and making such nice preparations. For an idea of what they go for, go here or here. ]

An isopod:

After the isopod, two Maeandricampus triangulum meet two new Raninid crabs.

The final picture shows a breakdown of the preparation process, done under a microscope with air scribes, which are miniature jack hammers.

A new phylum of very weird sea creatures

September 7, 2014 • 6:57 am

Read some biology today; it’s good for you!

It’s not often that a new animal phylum has been described, but a new paper in PLoS ONE apparently does just that, basing the phylum on two enigmatic species, dredged up from the deep sea, that can’t be placed in any existing phylum. This may add one more to the 35 phyla that already exist (see the list here, and please look. It’s nice to review the major divisions of life.)

The paper is by Jean Just et al. (all authors are from the Natural History Museum of Denmark at the University of Copenhagen), and the reference and pdf, which is free, are below.

What we have is something that looks like a cnidarian (jellyfish, corals, and sea anemones) or a ctenophore, but with a stalk. (Some cnidaria do have a stalk). But it has features that keep it from being placed in the phyla Cnidaria or Ctenophora. Its placement on the tree of life is further complicated by two things: we don’t really know where some major groups fit on the tree of life already (see below), and we don’t have any DNA or molecular data from this group to see what it’s most closely related to, or whether it’s an outgroup (a more distant ancestor) to all metazoans (multicellular animals).

The problem is that these creatures, which I’ll show shortly, were dredged up off of Victoria, Australia in 1986 from 400-1000 meters down. They were then fixed in formalin and later transferred to 80% ethanol. I’m no molecular biologist, but I think that would pretty much destroy the DNA, preventing any molecular analysis. And the samples are now old, shrunken a bit and degraded, and so some features may be effaced.

What we have are two species placed in a new genus, Dendrogramma, which the authors consider members of a new phylum as well, though they didn’t formally name one in this paper—probably because the placement of these creatures is uncertain. Two species were named. Here’s the first, Dendrogramma enigmatica:

Like the other speciers, it has a flattened disc with a notch in it, a stalk (so it was attached to the substrate), and a mouth-like opening that leads into an “gastrovascular” canal in the stalk that also feeds into the radiating canals in the disc. The tissue types were not examined, so we can’t draw homologies between the types of layers and those of other metazoans. Here’s the other species, Dendrogramma discoides:

And both species together. You can see from the scale (1 mm) that they were very small (10 mm = 1 cm, and there are 2.54 cm per inch).

Because of the stalk and the inflexible disc, these things were probably unable to swim but attached to rocks or the sea floor. Given their mouthlike opening, the authors suggest that “they fed on microorganisms, perhaps trapped by mucus from the specialized lobes surrounding the mouth opening.”

Why aren’t they members of existing phyla like cnidarians and ctenophores? Because they lack features found in those phyla. As the authors say (my emphasis):

Dendrogramma shares a number of similarities in general body organisation with the two phyla, Ctenophora and Cnidaria, but cannot be placed inside any of these as they are recognised currently. We can state with considerable certainty that the organisms do not possess cnidocytes, tentacles, marginal pore openings for the radiating canals, ring canal, sense organs in the form of e.g., statocysts or the rhopalia of Scyphozoa and Cubozoa, or colloblasts, ctenes, or an apical organ as seen in Ctenophora. No cilia have been located. We have not found evidence that the specimens may represent torn-off parts of colonial Siphonophora (e.g., gastrozooids). Neither have we observed any traces of gonads, which may indicate immaturity or seasonal changes. No biological information on Dendrogramma is available.

Given the absence of DNA data or complex characters that might help us decide where these things fit in the tree of life, the authors can only speculate. One big problem is that we don’t really know where the major phyla of multicellular animals fit on the tree. For example, some biologists claim, based on both molecular and morphological data, that the “outgroup” (the most unrelated phylum) to all metazoa is the Porifera (sponges). Others (and the authors of this paper take this position) claim that the outgroup is really Ctenophora (which, based on morphology alone, I would have thought were more closely related to the cnidarians, as biologists once thought [they’re really distantly related groups, though]). So here’s the phylogeny presented in the paper, showing cetophores as the outgroup to other metazoans (including the Bilateria, the group of phyla that includes all bilaterally symmetrical animals, including us:

To hedge their bets, the authors have also included ctenophores within other groups, as its placement is uncertain. They’ve put Dendrogramma as either an outgroup to all other phyla, or perhaps more closely related to the ctenophores or cnidarians. We just don’t know yet.

Molecular evidence could potentially resolve the placement of all these groups, and, frankly, I’m surprised that we haven’t settled the issue. For Dendrogramma we clearly need fresh material to get DNA (the authors plead for someone to get more specimens), but we could get plenty of DNA from the other species. Either that hasn’t been done (which I strongly doubt), or the lineages diverged so long ago that DNA evidence is inadequate to settle the question of, say, whether sponges or ctenophores are the outgroup. Perhaps some reader can explain to us why this major issue remains unsettled.

I noticed that the discs of these species resemble some creatures described from the Ediacaran fauna (also called the “Vendian fauna”), a group that lived from about 580 million years ago to about 545 million years ago, when the “Cambrian explosion” occurred and Ediacaran animals (if they were animals!) disappeared. (For pictures of various weird Ediacaran creatures, see here.)

My friend Latha Menon, who is not only the trade science editor at Oxford University Press (and editor of the British edition of WEIT) but also a Ph.D. candidate at Oxford’s Department of Earth Sciences, would know more about this, as she works on discs that strongly resemble these, but lived hundreds of millions of years ago. I therefore asked her to relate the new finding to the old group, as they could be related. Her answer is below, along with references. As you can see, she’s a very good writer, and I’m grateful for her input on this issue.

by Latha Menon

The discovery of Dendrogramma from the deep sea off Australia has undoubtedly caused a frisson of excitement among researchers on early life. A living fossil? An Ediacaran that has been surviving quietly in bathyal regions for several hundred million years? Let’s not get carried away, but it is an intriguing find.

When Reginald Sprigg discovered, in the 1940s, a set of strange impressions, many of discoidal forms, preserved on surfaces of the sandstone and quartzite of the Ediacara Hills, South Australia, he called them “medusoids”. Further work by Martin Glaessner and Mary Wade in the late ’60s continued to describe the various discoidal forms as medusoids, while frondose forms such as Rangea were considered to be Pennatulaceans (sea pens), and Dickinsonia was thought to be an annelid. Since then, Ediacaran macrofossils have been found all over the world, including spectacular fossil assemblages from the White Sea coast in Russia, the Nama Group, Namibia, Lantian and Miaohe Formations in South China, and the remarkable “E surface” at Mistaken Point, Newfoundland (see e.g. Fedonkin et al., 2007). The biota gave its name to the Ediacaran Period (635-541 Ma) ratified in 2004, and the fossils themselves appear from about 579 Ma (perhaps earlier), soon after the Gaskiers glaciation, the last of several widespread glaciations, and stretch up to the Cambrian boundary. Close to the boundary, the earliest biomineralized forms, the “small shelly fossils” appear, along with intense burrowing activity (bioturbation), and the Ediacarans, as far as we know, disappear, perhaps in an extinction. So what were the Ediacarans?

Nearly 70 years after Sprigg’s discovery, with many more fossil impressions, the affinities of the Ediacaran biota remain uncertain. Remember, that’s all we have – impressions (and in some cases, carbonaceous compressions) in the rocks. No skeletons; no biomineralized parts; and certainly no DNA. Molecular clocks provide little help so far back in time; results are notoriously varied and unreliable. Fossils really matter. And in spite of the limitations, a great deal of work has been done to glean information from the often exquisitely detailed impressions and the sedimentology of the surrounding rock, which indicates the setting in which they lived and died. As more evidence accumulated concerning morphology and sedimentary context, the early interpretations of medusoids, pennatulaceans, and annelids was increasingly questioned. Some may reach 30 cm and more in size, but were they necessarily early animals? The late Dolf Seilacher proposed that these enigmatic forms represented a “failed experiment”.

Discoidal forms are particularly hard to interpret. Some simple forms may be pseudofossils formed by physical processes; others have been persuasively explained as microbial colonies (Grazhdankin and Gerdes, 2007). Still, some possible affinities with familiar taxa have been suggested, with evidence put forward for bilaterian traces from about 555 Ma, and the claim that Kimberella may have been an early mollusc (Fedonkin & Waggoner, 1997). Our own group has found evidence in the early Ediacaran Avalon assemblage of Newfoundland for horizontal and vertical motion associated with a discoidal form (Liu et al., 2010; Menon et al., 2013), suggesting that some of these discs may indeed have been simple polyp-like forms. Two weeks ago, we published a paper describing Haootia quadriformis n. gen. n. sp. (Liu et al., 2014: – an extraordinary fossil impression that appears to indicate muscle bands, and bears a striking similarity to modern stalked jellyfish (Staurozoa). The idea that some of the Ediacaran discoidal forms may have been stem-group medusoids has made a big come-back.

And then we hear of Dendrogramma. The authors have referred it to Metazoa incertae sedis [“of unknown placement”]. The organism resembles cnidarians and ctenophores, but lacks the characters to establish a certain affinity with either group, though molecular analysis of further individuals might yet show that it belongs to one of these lineages (the existing specimens were damaged in preparation and not suitable for DNA analysis). Whether or not Dendrogramma turns out to represent a new phylum, it does seem to be a relatively primitive form, lacking cnidocytes, colloblasts, and other more sophisticated characters. From the discription, Dendrogramma appears to be a simple diploblastic animal with a disc showing a distinct pattern of gastrovascular branches and, in the case of one of the two species, D. discoides, a stalk with a possibly trilobed mouth-field. Various Ediacaran discoidal forms, particularly those from the diverse assemblages of South Australia and the White Sea, Russia, have trilobed structures within the disc, most obviously Tribrachidium. The authors point out the similarity of D. discoides with Albumares brunsae, and Anfesta stankovskii, as well as the less obviously trilobed Rugoconites from South Australia. There does appear to be a morphological similarity, particularly with the former two forms, both in the trilobed structure and in the pattern of radial ridges compared with the gastrovascular branching on the disc of Dendrogramma.

So can we conclude that Dendrogramma is a living Ediacaran? That’s almost certainly going too far. But it does seem quite possible that some of the trilobed Ediacaran discs may represent stem-group forms of such a lineage, lacking in such modern armoury as cnidocytes (for what would they sting?) and possessing a simple small mouth with no surrounding tentacles. As for all the other kinds of Ediacaran forms, even the many other discoidal forms, well, the work goes on.

Latha’s References:

Fedonkin, M.A., et al. (eds), 2007, The rise of animals: Evolution and diversification of the Kingdom Animalia: Baltimore, Maryland, Johns Hopkins University Press

Fedonkin, M.A., and Waggoner, B.M., 1997, The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism: Nature, v. 388, p. 868–871

Glaessner, M.F., 1959, Precambrian Coelenterata from Australia, Africa and England: Nature, v. 183, p. 1472–1473

Glaessner, M.F., and Wade, M., 1966, The late Precambrian fossils from Ediacara, South Australia: Palaeontology, Vol 9 (4), pp. 599-628

Liu, A.G., McIlroy, D., and Brasier, M.D., 2010, First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland: Geology, v. 38, p. 123–126

Menon, L.R., McIlroy, D., and Brasier, M.D., 2013, Evidence for Cnidaria-like behaviour in ca. 560 Ma EdiacaranAspidella, Geology, v. 41, p. 895–898

Sprigg RC. 1947. Early Cambrian (?) jellyfishes from the Flinders ranges, South Australia: Trans. R. Soc. S. Aust. 71(Pt. 2):212–24

______________

REFERENCE TO THE NEW PAPER: Just, J., R. M. Kristensen, and J. Olesen. 2014. Dendrogramma, New Genus, with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia, Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara. PLOS One DOI: 10.1371/journal.pone.0102976

	Norman Gilinsky on Readers’ wildlife photos
	Norman Gilinsky on Wednesday: Hili dialogue
	Peter N on Jesus ‘n’ Mo…
	Bryan on Jesus ‘n’ Mo…
	Brooke ONEILL on Wednesday: Hili dialogue