Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
This find, which was reported in May in the Proceedings of the National Academy of Sciences (click screenshot below for free paper with Unpaywall app; pdf is here, and reference is at bottom) is important not because it gives us a bit of stunning biological knowledge, but because it poses a conundrum: how did an ammonite (a shelled cephalopod and member of a now extinct group) get trapped in amber, which is fossilized tree resin? Ammonites were all marine organisms, while resin comes from trees, which of course grow on land. (The amber also has some marine snails.)
Further, the lump of resin, which is about 100 million years old, contains a number of terrestrial creatures as well: a spider, mites, beetles, a millipede, and so on. How could such a mixture have formed? I’ll show the creatures first and then give the authors’ speculations at the end, so this post be short. First, the paper:
The amber came from a famous amber-mining site in Myanmar, and isn’t that big: the piece examined is 33 mm long, 9.55 mm wide, and 29 mm high, weighing only about 6 grams—that’s 1.3, 0.4, and 1.1 inches respectively, weighing 0.2 ounces. But that small piece contained a wealth of life. Here’s what the whole thing looked like, and you can see the ammonite at lower right (scale bar is 5 mm):
Using a variety of microscopic techniques, they found these things in it (most pictured below):
One ammonite, which was abraded a bit and filled with sand (you can see it in the first picture). This indicates that the ammonite was dead when it was trapped by the resin and later fossilized. The authors identified the ammonite as a juvenile of the genus Puzosia, and, since its occurrence has been dated from other sediments, also dates the amber at about 100 million years old. This is a rare case of amber being dated from the creatures within it.
Here’s figure showing the ammonite (note the sand and broken shell); I’ve added the caption from the paper:
Figure 2 from paper: Ammonite Puzosia (Bhimaites) Matsumoto. (A) Lateral view under light microscopy. (B) Flattened sutures reconstructed by microtomography. (C) Microtomographic reconstruction, apparent view. (D) Microtomographic reconstruction, surface rendering; (E) Microtomographic reconstruction, virtual section. (Scale bars, 2 mm.)
Four isopods. Isopods are crustaceans that can be marine, freshwater, or terrestrial organisms (on land they’re called “pillbugs”). Two of these appear to be terrestrial and one marine or intertidal. Here they are, with caption:
(Figure 3 from paper): Isopods of uncertain taxonomic affinity, but generally consistent with littoral or supralittoral taxa. (A) Isopod 1. (B) Isopod 2. (C) Isopod 3. (D) Circular structure attached to the dorsal surface of isopod 2. (Scale bars, 1 mm in A and C. Scale bar, 0.5 mm in B and D.)
And a partridge in a pear tree (only kidding. . .)
These are all of course terrestrial organisms.
Here are some of those fauna with a caption (from Figure 5 of the paper):
Amber inclusions. (B) Acari: Phthiracaridae. (C) Acari: Euphthiracoidea. (D) Araneae: Oonopidae. (E) Diplopoda. (F) Diptera: Phoridae. (G) Hymenoptera: Chrysidoidea. (H) Coleoptera. (I) Blattodea. (Scale bars, 1 mm in E and H. Scale bars, 0.5 mm in B–D, F, and G. Scale bar, 2 mm in I.)
Okay, so here’s the big question: How did both sea creatures and terrestrial creatures get fossilized together in one bit of amber?
The authors give three possibilities (this is my paraphrase of what they say):
1.) There were resin-producing trees (most amber-source trees are confers) growing near a beach. Resin dripped from a tree, trapping the terrestrial stuff as it flowed down the trunk, and then landed on the sand, which had marine gastropod shells and the ammonite shell. The resin was subsequently fossilized. This is supported by the fact that the ammonite is filled with coarse shell sand and had already been abraded, as it would be if it were washed up on a beach (many ammonites inhabited shallow near-shore waters). The amber also contains sand, as if it had dripped on a beach.
2.) A tsunami flooded trees growing near the ocean, bringing with it marine shells that went into globs of resin that already had trapped terrestrial invertebrates. This seems unlikely because, as the authors note, “resin barely solidifies when submerged in water”. Resin that turns into amber has already hardened, and this wouldn’t be the case if this second hypothesis were true.
3.) A tropical storm blew both seashells and sand inland into a forest, and then scenario #1 would apply. The authors consider this unlikely because, were this true, we would presumably have found more bits of amber containing marine organisms. Yet we have almost none.
Of course we don’t know which possibility is true, but what is clear is that the ammonite was dead when it went into the resin.
Whatever happened, this is a unique piece of amber, and would be worth thousands of dollars on the open market. Fortunately for science, though, it’s been deposited in the Linpoge Amber Museum in Shanghai, China.
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Yu, T., R. Kelly, L. Mu, A. Ross, J. Kennedy, P. Broly, F. Xia, H. Zhang, B. Wang, and D. Dilcher. 2019. An ammonite trapped in Burmese amber. Proceedings of the National Academy of Sciences 116:11345-11350.
The evolution of whales from a small, deer-like artiodactyl took about ten million years: from about 50 million to about 40 million years ago. That’s remarkably fast evolution, especially when you consider the amount of morphological and physiological change that occurred, and the fact that the divergence between chimps and modern humans from their common ancestor, which (I think) took far less morphological and physiological change, took more than 6 million years.
Fortunately, we have a good fossil record of whales from Egypt, Pakistan and intervening areas, and so we can document this rapid change. Although the closest living relative of whales are hippos, the ancestor of modern cetaceans might have looked something like the first photo below, a reconstruction of the fossil Indohyus, a terrestrial, raccoon-sized artiodactyl (note hooves) similar to the modern chevrotain (“mouse deer”), which is known to stay underwater for long periods to escape predators.
Indohyus (reconstruction)
Here’s a living chevrotain, the Lesser mouse-deer (Tragulus kanchil). It’s hard to imagine that something like this could evolve into the mighty whale!
At any rate, a new paper in Current Biology by Oliver Lambert et al. documents a transitional form between ancient and modern whales, a species they name Peregocetus pacificus, dating from about 43 million years ago. It was found not in the Middle East but in Peru, and so also provides information about how whales colonized the Atlantic Ocean after their presumed origin off Southeast Asia. You can read the paper for free by clicking on the screenshot below, or by getting the pdf here. If somehow your access is blocked, judicious inquiry will get you a pdf.
This “whale” (if it can be called that) was dug up on the southern coast of Peru, with a large proportion of its skeleton remaining (see below; solid lines are recovered bones). From this skeleton we can conjecture that it was amphibious—it could swim and also walk on the land. Here’s the tally of found bones, and a reconstruction of how they were used to both swim and walk. As you can see from the scale, it was about 4 meters long:
(From paper): Schematic drawings of the articulated skeleton of MUSM 3580 showing the main preserved bones, in a hypothetical swimming and terrestrial posture. For paired bones, the best-preserved side was illustrated (sometimes reversed), or both sides were combined (e.g., mandible). Stippled lines indicate reconstructed parts and missing sections of the vertebral column; cranium, cervical vertebrae, and ribs based on Maiacetus inuus.
Some questions:
Why do we think it could swim?Peregocetus had a well-formed pelvis, attached to the rest of the skeleton, and well formed legs that would have stuck well outside the body. It had a long tail, and the configuration of the tail vertebrae, compared to those of other swimming mammals like otters and beavers, suggest that the tail was somewhat paddle-like, powerful, and thus could be used for swimming. (This is seen in the reconstruction at the bottom.). It’s not known if the tail ended with a boneless “fluke” (as in modern whales), as that would not have been preserved.
The rear feet were long, as were the rear digits, and those digits were flattened with flanges on the side, indicative of webbed rear feet. Based on the foot anatomy, the authors suggest that “Propulsive movements were either alternate or simultaneous hind-limb paddling or body and tail undulations, as observed in modern river and sea otters, alternating between lift-based propulsion via pelvic undulations, including tail and hind limbs, and drag-based propulsion via independent strokes of the hind limbs.”
Here’s the distal part of the front leg:
Here’s the rear foot with flattened digits (I believe “3” are the hooflets):
Why do we think it could walk? I’ll quote the authors here: “The fore- and hind-limb proportions roughly similar to geologically older quadrupedal whales from India and Pakistan, the pelvis being firmly attached to the sacrum, an insertion fossa for the round ligament on the femur, and the retention of small hooves with a flat anteroventral tip at fingers and toes indicate that Peregocetus was still capable of standing and even walking on land.”
What else is interesting about this fossil? It had sharp and robust teeth, some shearing teeth, and a long snout, indicating that it probably ate large bony fish. Here is a mandible with teeth:
Further, this fossil suggests to the author a route for migration of ancestral whales from their origin in the Pacific to the Atlantic Oceans and into the New World.
Based on recent finds of whale fossils in West Africa, as well as this find from Peru, the authors suggest that whales made their way through the Tethys sea (the Mediterranean, which was contiguous with the Indian Ocean then), south along the West African coast, then hopped over the South Atlantic (much narrower at that time due to continental drift) along the coast of South America and then south through the Isthmus of Panama (open to the sea then) to get back to the eastern Pacific. It also is thought to have spread north along the east coast of North America. Below you can see the route of migration, with the authors’ hypothesis denoted by the black arrows below. (A trans-Pacific route can’t be ruled out, but the Pacific was much wider then and there are no early whale fossils on the Pacific coasts.)
Finally, here’s a reconstruction of the beast on land and see from the paper’s Supplemental Information:
(From paper): Figure S4. Artistic reconstruction of the middle Eocene (about 42.6 Ma) protocetid whale Peregocetus pacificus gen. et sp. nov., Related to Figure 2 and Data S2. Life reconstruction of two individuals of P. pacificus, one standing on the rocky shore and the other hunting sparid fish, along the coast of nowadays Peru. The presence of a caudal fluke in P. pacificus remains hypothetical and should be tested with the discovery of a more complete specimen, including posteriormost caudal vertebrae. Reconstruction by A. Gennari.
Here we have a true intermediate form, a transitional species which occurs when it’s supposed to: after the earliest whales but before the appearance of modern whales. Given this find, as well as the panoply of other fossil whales showing progression from ancestral to modern forms, creationists will be hard pressed to explain this.
I want to call your attention to a remarkable new fossil find in southern China; a rich group of soft-bodied animals, the “Qingjiang Fauna”, from about 518 million years ago. It’s comparable in importance to the Burgess Shale fauna popularized by Steve Gould’s book Wonderful Life, and to the Chengjiang fauna China from China. They’re of comparable age, too: The Burgess fauna is 508 million years old, and the new Qingjiang finds are 518 mya, the same age as the Chenjiang fauna. Yet the animals of the two later finds, despite being similar in age and marine habitat, are very different (see below).
You might be able to read this new paper if you have the legal Unpayall application, or you can try getting the pdf here. (I’m in the Netherlands and unable to send pdfs.) There’s also a News and Views piece by Allison Daley, “A treasure trove of Cambrian fossils,” that gives a one-page overview.
The paper:
The fossils are soft-bodied, preserved as exquisitely detailed carbon films on gray “claystone”: sediments interspersed between non-fossil-bearing black claystone. The fossil layers are seen as the light gray bands on the right side of the following picture (site of fossil finds in China are on the left).
From paper: Fig. 1 Locality map and early Cambrian stratigraphy of the study area. (A) Lithofacies map of the Yangtze Platform during Cambrian Stage 3, with type localities of the Qingjiang and Chengjiang biotas. (B) Geological map of the study area, showing the distribution of Cambrian outcrops and the location of studied sections with characteristic couplets of background and event claystone beds within the middle member of the Shuijingtuo Formation. (C) Composite stratigraphic column for the study area. (D) Stratigraphic column at the Jinyangkou type locality.
The fossils were formed in a manner similar to those of the Burgess-Shale fauna. They were one-off gravity flows”, or avalanches, of shallow sea sediments that carried animals and algae to a deeper, oxygen-poor location where they were rapidly covered up and then remained undisturbed by waves above. The animals’ body impressions weren’t replaced by minerals, but were turned into carbonized films that show exquisite detail.
And what detail, and what a variety of animals! Sponges, coelenterates (cnidarians), algae, ctenophores, kinorhynchs (“mud dragons”, found only once before as an early fossil), arthropods, brachiopods, priapulid worms, and even a chordate, a member of our own phylum, resembling the remarkable Pikaia fossil found in the Burgess shale. There are also a number of tiny larvae, and these aren’t easily placed as to the group because they metamorphose into something that looks very different.
Remarkably, 53% of these fossils belong to previously unknown taxa, making this truly the “treasure trove” described by Daley. Look at these fossils! Note the chordate, possibly along the lineage to our own ancestry, in F.
Fig 2 (from paper). Fig. 2 New soft-bodied taxa from the Qingjiang biota. (A) Medusoid cnidarian, showing radially symmetrical body plan, exumbrellar/subumbrellar surfaces (Eu/Su), manubrium (Ma), and tentacles (Te). (B) Polypoid cnidarian, showing oral disc and mouth (Mo), tentacles, column, and pedal disc (Pd). (C) Ctenophore, showing that comb rows and oral-aboral body axis have a biradial symmetry resulting from sheathed tentacles. (D) Branched alga, showing quadripartite thallus. (E) Sponge Leptomitella sp. (F) New chordate. (G) Yunnanozoon sp.
Look at the remarkable arthropod Leanchoilia in (A) to see the detail of these fossils. Every leg and feeler is preserved.
More details of the above; you can see every bristle on the kinorhynch (right). The cnidarian is on the left and the ctenophore’s in the middle.
Curiously, the fauna isn’t all that similar to that of the Chenjiang fauna that was preserved about the same time. This new fauna, for example, has far more cnidarians (jellyfish, anemones, etc.) and far more kinorhynchs than does the Chengjian samples. As the authors note, “Only a small number of species (n = 8) are shared with Chengjiang (materials and methods), and the most abundant taxa, Kunmingella and Maotianshania, as well as the iconic Fuxianhuia, are absent from the Qingjiang assemblage.”
The authors suggest that these differences represent not different artifacts of preservation but a real difference in the nature of the fauna between the two areas. They could be from different depths in the ocean.
Here’s a reconstruction of the fauna from the original paper:
Fig. 4 from paper. Artist’s rendering of the Qingjiang biota showing characteristic early Cambrian taxa from the Lagerstätte.
The potential of this find is immense, but of course not yet tapped. It could help reconstruct the history of life and the potential of various suggested group—like the ctenophores—to be part of our own ancestry. And of course there’s the new fishlike chordate above. Daley sums up this potential in her news and views piece:
One of the most remarkable findings reported by Fu et al. is that 53% of the animals and algae in the Qingjiang biota represent previously unknown taxa. When these taxa are described in detail, the Qingjiang biota will help to illuminate the reasons for faunal variation between localities. The Burgess Shale and the Chengjiang biota, for example, have some similarities in the overall type and abundance of animals found at each site, but only 15% of genera are found at both localities.
. . . Fu et al. convincingly demonstrate that the Qingjiang biota represents an assemblage of organisms that was preserved nearly in place, providing a snapshot of a real animal community 518 million years ago. The treasure trove of the Qingjiang biota provides an exciting opportunity to explore how paleoenvironmental conditions influenced ecological structuring and evolutionary drivers during the Cambrian Explosion.
Fu, D., G. Tong, T. Dai, W. Liu, Y. Yang, Y. Zhang, L. Cui, L. Li, H. Yun, Y. Wu, A. Sun, C. Liu, W. Pei, R. R. Gaines, and X. Zhang. 2019. The Qingjiang biota—A Burgess Shale–type fossil Lagerstätte from the early Cambrian of South China. Science 363:1338-1342.
Note: This has been slightly updated after I ran it by Davorka, who caught a few errors.
Over the years we’ve had a number of posts about Neanderthals and their genetic legacy in “modern humans” (see here for a collection), many of them written by Matthew Cobb. Croatia—in particular a hill near the small town of Krapina—is famous for its large collection of Neanderthal skeletons and relics, first discovered during quarrying in 1899. Because there were so many bones, this site afforded a unique look into a population of Neanderthals that lived about 130,000 years ago.
I reported a few days ago on my visit to the Neanderthal Museum in Krapina, which has nice dioramas of Neanderthal life, a cool movie (which, I’m told, was as accurate as possible given what we know about the subspecies), and casts of the bones.
But the bones themselves, and the Neanderthal relics, are carefully sequestered at the Croatia Natural History Museum, where they’re curated by Dr. Davorka Radovčić. My hosts here arranged for me and two of them to visit the Museum. There Dr. Radovčić spent several hours showing us the bones and artifacts, and explaining what they meant and what mysteries still remain (there are many). This required special permission from the Museum, and the visit was one of the high spots of my trip to Croatia. How often do you get to be a few inches away from Neanderthal skulls and teeth, and to hold a spearpoint chipped by one of them so long ago?
You can read more about the Krapina website here. As that article says (I’ve tweaked the English a bit):
. . . a total of 876 single fossil Neanderthal fossil remains were found, placing Krapina in the world”s scientific heritage as the world”s richest Neanderthal finding site.
The Krapina proto-human, scientifically known as Homo sapiens neanderthalensis was discovered in 1899, at the time of geological and panteological explorations at the Hušnjak hill in Krapina started. The excavations lasted for six years, supervised by Professor Dragutin Gorjanović-Kramberger, a famous Croatian geologist, paleonthologist and paleoanthropologost. His works contributed significantly to the European and world science about the fossil man. The half-cave in Krapina was soon listed among the world”s science localities as a rich fossil finding site, where the largest and richest collection of the Neanderthal man had ever been found.
In the sandy deposits of the cave about nine hundred remains of fossilised human bones were found – the fossil remains belonged to several dozen different individuals, of different sex, from 2 to 40 years of age. Numerous fossil remnants of the cave bear, wolf, moose, large deer, warm climate rhinoceros, wild cattle and many other animals were also found. Over a thousand pieces of various stone tools and weapons from the Paleolithic era were found, all witnessing to the material culture of the Krapina proto-human. This rich locality is approximately 130.000 years old.
And the site is here (the dots are other Neanderthal sites):
I’m going to show some of the bones and stones we saw, and explain as best I can remember what they mean.
The collection is stored in several locked metal cabinets, each containing wooden drawers with foam inserts holding the relics. Each drawer is labeled with its contents: “teeth”, “mandibles”, “patellas” (kneecaps), and so on. Here’s Davorka removing a drawer:
The first thing we saw were the crania (skulls), some of which were very well preserved. Notice the labeling of the drawer in the second photo:
This is a particularly interesting skull for a reason I’ll explain in a minute. It’s very well preserved but also has a feature unique among Neanderthal skulls known to science:
Davorka explains some of the features of the skull that set it apart from modern H. sapiens sapiens, and also identify it as a female skull:
You can see the prominent brow ridges and the upper part of the skull, which bears the cool feature:
This skull, of a young adult female (probably in her 20s or early 30s; you can tell the sex from the way the skull is shaped), has a series of 40 horizontal incisions made in the forehead at or soon after death (they aren’t healed). Their purpose isn’t known, but it seems likely it was involved with some kind of postmortem ritual, perhaps indicating a respect for the dead or even something associated with an idea of the afterlife. We simply don’t know, as Davorka emphasized. Below are two photos of the incisions and a brief video of Davorka explaining them:
Davorka explains the cuts in this video: they weren’t made to butcher or scalp the woman:
Neanderthal DNA is extracted from the middle ear capsule, as it is tough and well insulated from the environment. I erred in an earlier post in saying that DNA has been extracted from Krapina Neanderthals; Davorka tells me that Svante Pääbo and his colleagues extracted it from another Croatian Neanderthal site called Vindija.
We now know that Neanderthals interbred with “modern” humans (H. sapiens sapiens), and that the average non-African human carries about 3% of their genome from Neanderthals, including genes now used in the immune response. Although the offspring in at least one direction of the cross must have been fertile—for that’s the only way Neanderthal DNA could get into H. sapiens sapiens—we don’t know if offspring from both directions of the cross were fertile. For example, we haven’t found mitochondrial DNA from Neanderthals in modern humans. That could reflect either accidental loss of mitochondria, selection against mitochondrial DNA that did infiltrate modern human populations, or the sterility of offspring between Neanderthals mothers and H. sapiens sapiens fathers.
The middle ear capsule is at the upper left here, just above the red lettering that reads “88.11”. That’s the precious bit for paleogeneticists:
Mandibles! The teeth are relatively larger than ours, and the jaw has more space to accommodate all the molars, so the “wisdom teeth” are not crowded as they are in modern humans.
Two lower jaws (mandibles); note the rotation of one tooth in the left row of teeth:
The “rotated” tooth between the two white-ish ones. I can’t remember what the significance of this was, but I wrote to Davorka who said that some feel it’s due to genetic relationship and possibly inbreeding:
The scientists who worked on this concluded that they rotate due to “biological origin, an inherited condition common in the Krapina people. . . The sample is too small to for the observation to have significance, but we believe a hypothesis of biological relationship among the individuals found in Krapina levels 3 and 5 can be proposed to explain our results. Such a hypothesis is supported by the unusual superior deflection of the internasal suture in the only three Krapina specimens to preserve the suture” (Rougier et al. 2006; you can see the whole article in the book New insights on the Krapina Neandertals, pp. 43).
The jaw of a young (probably 6-7 year-old) Neanderthal, showing the deciduous teeth (“milk teeth”) and the three adult teeth that haven’t yet erupted. Neanderthals didn’t live very long: a 40-year-old individual was old:
Unfortunately, some of the mandibles were cleaned, removing the precious calculus (hardened plaque that the dentist scrapes off of your teeth at cleaning time). Davorka explains in the video how that cleaning caused the loss of precious biological information. Note the “retromolar space” giving ample room for all the molars.
Teeth, including a “shovel shaped” incisor, different from the shovel-shaped incisors found in Asian specimens of modern H. sapiens.
A well preserved molar:
A shovel-shaped incisor.
The wear patterns of these front teeth indicate that the Neanderthals held items in their teeth while processing them, like holding a skin in your mouth while scraping it with your hand. The position of the wear marks also shows that about 80% of Neanderthals were right-handed, scraping with their right arms while holding the item in the left side of their mouth. Isn’t that cool? In fact, this is about the same proportion of right-handers in Croatia today:
Arm bones. A drawer full of humerus (upper arm) bones:
This is an ulna (one of the two lower arm bones) that has been chopped off and then healed, indicating that the individual lost part of his or her arm. Then it healed after the injury, so the individual survived missing a hand:
A drawer full of kneecaps. They are lighter than kneecaps that are “fossilized”, as the sandstone has probably leached out many of the bone constituents:
A smashed leg bone (tibia), either trod on soon after death or smashed during death, perhaps during hunting or warfare. (Neanderthal bones show much less frequency of “warfare” damage than do the bones of earlier hominins like australopithecines. They seem to have been a peaceful subspecies.)
This Neanderthal shows a healed bash in the head (the dent in the center, which didn’t penetrate the skull), along with lines surrounding the wound. Life was tough for these hominins!
Here Davorka explains that we’re not sure what the lines are: they could have been deliberately incised (trephination) to relieve pressure on the wound coming from pus, or perhaps the lines could be just a taphonomic (preservation) artifact.
Neanderthals were largely carnivores, though we know they also used medicinal plants. They ate bears, beavers, and even rhinos. Here’s an adult rhino that I believe was killed by the Krapina Neanderthals. They would of course have had to hunt in groups, and it must have been very dangerous to spear a bear or a rhino to death.
They apparently killed birds, too, as bits of bird skeletons, with some of the parts modified, are found in association with the Neanderthal bones. Here are some talons and foot bones from the white-tailed eagle, Haliaeetus albicilla, a species that is still around.
There are cut marks in the talons and foot bones to which they were attached, suggesting that Neanderthals were using the talons and bones as jewelry. This is supported by recent findings of gut “fiber” tied around part of a talon. Here are a foot bone and a talon that have been modified by having grooves cut in them.
This is a toe bone to which the talon was attached. See the cut groove at the lower end?
Modified eagle talons:
Davorka is pointing to the human-cut groove:
Here’s a paper (click on screenshot to read) in which Davorka and her co-authors suggest the use of talons as jewelry:
A bowl full of Neanderthal tools:
I got to hold a beautiful 130,000 year old Neanderthal spear point, chipped out of flint:
I previously described the tool below as a “scraper”, but I remembered wrong. As Davorka tells me, it’s not a tool, but something even more interesting. It’s a piece of “mudstone” that was probably picked up and brought to the Krapina site because it is a curiosity: it has “ichnofossils” in it (traces of living organisms, like worms, that have modified the sediments). Of course the Neanderthals didn’t know what these were, but might have been so impressed by the unusual patterns of this rock that they decided to keep it.
And Davorka and I after our visit. It truly was one of the great experiences of my life, and I’m immensely grateful to Davorka for her instruction and kindness, and to my hosts, Igor, Damjan, Darko, and Pavel, for arranging this visit. (We all went to lunch after this, but more on that in another post.)
The Ediacaran fauna, a group of extinct species that lived between 571 and 541 million years ago, has been an evolutionary anomaly. Its fossil record contains multicellular organisms, but they are just plain weird, bearing little resemblance to present-day metazoan (multicellular) animals.
The two species of “dickinsoniids” shown below, for example, lack a mouth or gut (possibly having external digestion instead), are bilaterally asymmetrical, and bear a pattern of body “quilting” that isn’t seen in present-day animals or definite early metazoans like worms:
Here’s Dickinsonia, studied in the paper we’re discussing today. It’s about 7.5 cm long, or three inches, so it’s fairly large:
Here’s a dickinsoniid in the genus Andiva, also studied in the present paper:
What are these things? Controversy has centered on whether they were a whole kingdom of life different from any that we know today (a group that went wholly extinct), or, in contrast, perhaps the ancestors of modern day animals—or at least the relatives of modern day animals. Scientists have guessed that they might be either lichens or giant protozoans. (Yes, protozoans can get this large; some are nearly ten inches long!) This is important to resolve because the “Cambrian explosion” that gave rise to many modern groups of animals began about 541 million years ago, and we want to know if there were animal precursors before that, and what they were. We also want to know whether the Ediacaran fauna really does represent an entire group of creatures that disappeared without issue.
A new paper in Science by Ilya Bobrovskiy et al. (reference at bottom with free Unpaywall link, free pdf here) establishes fairly securely that Dickinsonia and related anomalous species do indeed seem to be metazoan animals rather than members of a separate large group that went extinct entirely. The telling data involves biochemical analysis of the thin films of organic matter that cover the fossils and was presumably produced by the fossils. These fossils were 558 million years old, which, if they were animals, would make them the oldest known metazoans.
To make a long story short, Bobrovskiy and colleagues collected specimens of dickinsoniids from sandstones of the White Sea region of Russia and removed the small (three micron thick) organic mat covering the fossils. Great care was taken to avoid contamination, and they also analyzed the sandstone around, above, and below the fossil to see if the peculiar organic profile they found was associated with the fossil itself. It was, and it also suggested that the fossils were animals, not lichens or giant protozoans.
The telling chemicals were 27 carbon steroids—cholesteroids—which were present as 93% of chemicals in the mat atop the fossils, but only 11-12% of the surrounding sandstone (probably coming from algae or other plants). The fossils, moreover, were almost entirely missing a class of chemicals, ergosteroids, that characterize lichens. And the chemical signature of these fossils didn’t much resemble that of the modern giant protists, either.
There was, however, one twist to the findings: the “isomers” (chemically identical molecules of different handedness or arrangement) of the cholesteroids in these fossils were mostly of a single handedness (the “5β” form), while that of more recent and genuine animal fossils have a more even mixture of right- and left-handedness. This is puzzling, and the authors have no real explanation. This may suggest that even if these fossils were related to modern animals, they were distantly related, having a unique metabolism. They may, then, have branched off from modern animals, with the dickinsoniids and other Ediacaran fauna having gone extinct without descendants. Further, analysis of Andiva doesn’t show the same elevation of cholesteroids, though it does show a preponderance of 5β forms.
So this isn’t as compelling a demonstration as I had wished, but it still shows that these things were probably metazoans and not creatures related to modern lichens or protozoans. The authors conclude this:
Molecular fossils firmly place dickinsoniids within the animal kingdom, establishing Dickinsonia as the oldest confirmed macroscopic animals in the fossil record (558 million years ago) next to marginally younger Kimberella from Zimnie Gory (555 million years ago). However alien they looked, the presence of large dickinsoniid animals, reaching 1.4 m in size, reveals that the appearance of the Ediacara biota in the fossil record is not an independent experiment in large body size but indeed a prelude to the Cambrian explosion of animal life.
“Prelude” is a bit ambiguous, but I’ll grant that these are animals. I asked my friend Latha Menon, who has a Ph.D. from Oxford in early life studies, whether this paper was important, and I give her answer (with permission):
I do think it is an important paper. I’ve recently seen a lovely Dickinsonia specimen in a collection. You can really see how it’s like a very thin flatworm like form (or like a giant Trichoplax?) draped over the uneven ground below. Much has been written about its morphology but to find a specimen with associated organic matter and demonstrate that it is an animal from the biomarkers is very solid evidence. We have indications of simple animals earlier, from the traces found by Alex Liu and myself, and the remarkable squished Haootia specimen Martin Brasier discovered, which seems to show muscle bands. This specimen is younger and part of a more complex assemblage, but it is good to have solid evidence of animals well before the Cambrian boundary and some 40 Ma before the Cambrian explosion. Animals did not burst on the scene then; there was, as most of us have suspected, a long fuse.
Of course this does not mean that all the large forms of the Ediacaran were animals; there were probably several kinds of forms. But it does show that one ‘quilted’ form was animal and that suggests a number of other enigmatic forms in the biota were too.
Just for grins, here’s a photo, courtesy of Australian National University, showing Bobrovskiy collecting fossils in Russia. It ain’t easy!
As you probably know, my colleague Neil Shubin was on the team of biologists and paleontologists who uncovered the fossil Tiktaalik, a lobe-finned fish that lived about 375 million years ago. Three skeletons of this species are now known, all found on Ellesmere Island, part of the Canadian territory of Nunavet. Below in red is the place these guys had to schlep to when searching for the skeleton. (The cool part, as recounted by Neil in his book Your Inner Fish, is that they chose this area because it had sediments of the right age: the age when fish started coming ashore to begin the evolution of tetrapods. That shows that evolution is indeed predictable: you can posit when such transitional forms might have lived, and then see if they really were around then.)
Tiktaalik has features that make it look as if it were on the line going from lobe-finned fish to tetrapods (terrestrial four-legged creatures), including a neck, fingerlike bones that could have been the precursors of terrestrial digits, a shoulder and a wrist, spiracles on the head, (which might have indicated lungs), and a robust ribcage and pectoral girdle that could have helped it move onto land. Now it’s not clear that this creature really did venture on land. As Neil surmises, it may have just lurked in shallow water near the shore, searching for prey. But perhaps its descendants turned into tetrapods: early land-dwelling amphibians.
But did they? Since Tiktaalik was discovered in 2004, paleontologists have reported finding tracks on land that pre-dated the Tiktaalik fossils, meaning that there were already terrestrial tetrapods before Tiktaalik (ergo it couldn’t have been the ancestor of tetrapods). Here are the fossil tracks from the Nature paper, along with the paper’s caption:
Figure 2 | Trackways. a, Muz. PGI 1728.II.16. (Geological Museum of the Polish Geological Institute). Trackway showing manus and pes prints in diagonal stride pattern, presumed direction of travel from bottom to top. A larger print (vertical hatching) may represent a swimming animal moving from top to bottom. b, On the left is a generic Devonian tetrapod based on Ichthyostega and Acanthostega (from ref. 18) fitted to the trackway. On the right, Tiktaalik (from ref. 29 with tail reconstructed from Panderichthys 23) is drawn to the same shoulder–hip length. Positions of pectoral fins show approximate maximum ‘stride length’. c, Muz. PGI 1728.II.15. Trackway showing alternating diagonal and parallel stride patterns. In a and c, photographs are on the left, interpretative drawings are on the right. Thin lines linking prints indicate stride pattern. Dotted outlines indicate indistinct margins and wavy lines show the edge of the displacement rim. Scale bars, 10 cm.
In response, Neil and others have questioned whether these really are tracks, or that they could have been made by Tiktaalik or other “walking fish.” Right now the issue is unsettled, but some people claimed that tetrapods were already on land 425 million years ago: 50 million years before Tiktaalik.
The issue isn’t resolved, but it’s clear that Tiktaalik was some kind of “transitional form” that could have been related to the real ancestors of tetrapods, even if it wasn’t. In that sense it’s like the feathered dinosaurs that appear in the fossil record when flying birds were already around. These feathered dinos weren’t on the direct line to birds, but showed that there were feathered reptiles around at roughly the times birds began appearing.
But I digress. Matthew found this tweet from Neil and sent it to me with the note, “This is a very cool photo. I would be a complete failure as a paleontologist.” It’s the very first view of Tiktaalik, and was almost missed as it’s hard to see (it was found after several unsuccessful expeditions to the area). Are you a good paleontologist—could you have spotted it?
This was the first view of Tiktaalik, announced 12 years ago today. The tip of the snout is poking out. Can you spot it? pic.twitter.com/vXtzw9w9dc
She’s gone. I was at the Field Museum on Wednesday for the first time since the previous month, and the removal of Sue the Tyrannosaurus rex has been completed.
Stanley Field Hall, where Sue used to be.
Viewed from the balcony above, visitors walk through Stanley Field Hall, seemingly unaware of the ghostly white outline of Sue’s now departed plinth.
Where Sue used to be, from above.
A sign explained where Sue will eventually show up.
Sue’s actually not gone away entirely, for the second floor balcony display, featuring Sue’s real skull, remains in place. [JAC: the skull was always up there as it was too heavy to mount on the skeleton downstairs.]
The second floor display also includes touchable, life-size, bronze models of various parts of Sue, including the (relatively) tiny forearm. Devotees of the concept of unity of type, and Neil Shubin‘s Your Inner Fish in particular, will recognize the “one bone, two bones, many bones” pattern found throughout the tetrapod vertebrates and their piscine forebears.
A bronze model of Sue’s forearm.
A closeup of the digits; the two distalmost phalanges of the outer (lower, in this photo) digit were among the few bones missing from Sue’s skeleton, and the ones in the model are based on Albertosaurus, a related theropod dinosaur.
Sue’s fingers.
From up on the balcony, I could also get a better look at the model of Pteranodon longiceps hanging from the ceiling.
Pteranodon longiceps in the Field Museum.
And zooming in a bit.
Does the position of this model mean that Pteranodon is Ceiling Reptile?
