A 520-Million-Year-Old Brain Scan news.sciencemag.org/evolution/2013/10/scienceshot-520-million-year-old-brain-scan Some of the most unusual creatures scuttling across the sea floor 520 million years ago were the “great-appendage” arthropods, which had scissorlike projections sprouting from their heads. Looking only at their general crustaceanlike body plan (inset), paleontologists have long debated where these animals fit within the arthropod family tree, a diverse group that includes insects, spiders, millipedes, and long-extinct trilobites. Now, researchers have gained clues about the creatures’ closest evolutionary kin by blasting fossils of one group of the animals with high-energy x-rays, which caused various elements in the fossils to fluoresce. They also took CT scans. The most useful images, the researchers found, were those produced by fluorescing iron (depicted in magenta, main image) and the CT scans (green). Together, these images denote the creature’s optic nerves (its eyes are the four dark circles at the top of the main image), brain, and the nerve tissue serving eight of its 11 body segments. That arrangement is most like the one seen in a group of modern-day arthropods known as chelicerates, the researchers report today in Nature (1). Living members of that group include spiders, scorpions, and horseshoe crabs. The finding clears up the picture of the arthropod family tree, which is particularly important because some of these creatures’ features were so unlike those of their presumed kin. If used more broadly, the technique used to analyze these fossils could help paleontologists gain insights into evolutionary relationships among other enigmatic, long-gone species, including many of the unusual animals strolling the sea floor during the same time period. - Extinct Mega Claw Creature Had Spider-Like Brain uanews.org/story/extinct-mega-claw-creature-had-spider-like-brain UA Regents Professor Nicholas Strausfeld and an international team of researchers have discovered the earliest known complete nervous system exquisitely preserved in the fossilized remains of a never-before described creature that crawled or swam in the ocean 520 million years ago. A team of researchers led by University of Arizona Regents Professor Nick Strausfeld and London Natural History Museums Greg Edgecombe have discovered the earliest known complete nervous system, exquisitely preserved in the fossilized remains of a never-before described creature that crawled or swam in the ocean 520 million years ago. The find suggests that the ancestors of chelicerates – spiders, scorpions and their kin – branched off from the family tree of other arthropods – including insects, crustaceans and millipedes – more than half a billion years ago. Described in the current issue of the journal Nature (1), the specimen belongs to an extinct group of marine arthropods known as megacheirans (Greek for large claws) and solves the long-standing mystery of where this group fits in the tree of life. We now know that the megacheirans had central nervous systems very similar to todays horseshoe crabs and scorpions, said Strausfeld, the senior author of the study and a Regents Professor in the UAs Department of Neuroscience. This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian. The scientists identified the 3-centimeter-long creature unearthed from the famous Chengjiang formation near Kunming in southwest China, as a representative of the extinct genus Alalcomenaeus. Animals in this group had an elongated, segmented body equipped with about a dozen pairs of body appendages enabling the animal to swim or crawl or both. All featured a pair of long, scissor-like appendages attached to the head, most likely for grasping or sensory purposes, which gave them their collective name, megacheirans. Co-author Greg Edgecombe said that some paleontologists had used the external appearance of the so-called great appendage to infer that the megacheirans were related to chelicerates, based on the fact that the great appendage and the fangs of a spider or scorpion both have an elbow joint between their basal part and their pincer-like tip. However, this wasnt rock solid because others lined up the great appendage either a segment in front of spider fangs or one segment behind them, Edgecombe said. We have now managed to add direct evidence from which segment the brain sends nerves into the great appendage. Its the second one, the same as in the fangs, or chelicerae. For the first time we can analyze how the segments of these fossil arthropods line up with each other the same way as we do with living species – using their nervous systems. The team analyzed the fossil by applying different imaging and image processing techniques, taking advantage of iron deposits that had selectively accumulated in the nervous system during fossilization. To make the neural structures visible, the researchers used computed tomography (CT), a technique that reconstructs 3-D features within in the specimen. However, the CT scan didnt show the outline of the nervous systems unambiguously enough, Strausfeld said, while a scanning laser technique mapping the distribution of chemical elements showed iron deposits outlining the nervous system almost as convincingly but with minor differences. Next, the group applied advanced imaging techniques to the scans, first overlaying the magenta color of the iron deposit scan with the green color of the CT scan, then subtracting the two. We discarded any image data that were not present in both scans, Strausfeld explained. Where the two overlapped, the magenta and the green added to each other, revealing the preserved nervous system as a white structure, which we then inverted. This resulted in what resembled a negative X-ray photograph of the fossil. The white structures now showed up as black, Strausfeld said, and out popped this beautiful nervous system in startling detail. Comparing the outline of the fossil nervous system to nervous systems of horseshoe crabs and scorpions left no doubt that 520-million-year–old Alalcomenaeus was a member of the chelicerates. Specifically, the fossil shows the typical hallmarks of the brains found in scorpions and spiders: Three clusters of nerve cells known as ganglia fused together as a brain also fused with some of the animals body ganglia. This differs from crustaceans where ganglia are further apart and connected by long nerves, like the rungs of a rope ladder. Other diagnostic features include the forward position of the gut opening in the brain and the arrangement of optic centers outside and inside the brain supplied by two pairs of eyes, just like in horseshoe crabs. To make the analysis more robust, the researchers then added these features to an existing catalog of about 150 characteristics used in constructing evolutionary relationships among arthropods based on neuroanatomical features. Greg plugged these characteristics into a computer-based cladistic analysis to ask, where does this fossil appear in a relational tree? Strausfeld said. Our fossil of Alalcomenaeus came out with the modern chelicerates. But according to Strausfeld, the story doesnt end there. The prominent appendages that gave the megacheirans their name were clearly used for grasping and holding and probably for sensory inputs. The parts of the brain that provide the wiring for where these large appendages arise are very large in this fossil. Based on their location, we can now say that the biting mouthparts in spiders and their relatives evolved from these appendages. Less than a year ago, the same research team published the discovery of a fossilized brain in the 520-million-year-old fossil Fuxianhuia protensa, showing unexpected similarity to the complex brain of a modern crustacean (Cambrian Fossil Pushes Back Evolution of Complex Brains - uanews.org/story/cambrian-fossil-pushes-back-evolution-complex-brains (*) - Ref.: Complex brain and optic lobes in an early Cambrian arthropod - Nature 490, 258–261 (11 October 2012) doi:10.1038/nature11495 - nature/nature/journal/v490/n7419/full/nature11495.html) (2). Our new find is exciting because it shows that mandibulates (to which crustaceans belong) and chelicerates were already present as two distinct evolutionary trajectories 520 million years ago, which means their common ancestor must have existed much deeper in time, Strausfeld said. We expect to find fossils of animals that have persisted from more ancient times, and Im hopeful we will one day find the ancestral type of both the mandibulate and chelicerate nervous system ground patterns. They had to come from somewhere. Now the search is on. For this research project, Strausfeld teamed up with Gengo Tanaka of the Japan Agency for Marine-Earth Science and Technology in Yokosuka, Japan; Xianguang Hou, director of the Yunnan Key Laboratory for Paleobiology at Yunnan University in Kunming, China; and Hous colleague Xiaoya Ma who is presently working with Gregory Edgecombe in the paleontology department of the Natural History Museum, London. The work was supported by grants from the Natural Science Foundation of China (no. 40730211), Research in Education and Science from the Government of Japan (no. 21740370) and a Leverhulme Trust Research Project Grant (F/00 696/T) and also by the UA Center for Insect Science and a grant from the AFRL (FA86511010001) to Strausfeld. (*) Cambrian Fossil Pushes Back Evolution of Complex Brains uanews.org/story/cambrian-fossil-pushes-back-evolution-complex-brains Complex brains evolved much earlier than previously thought, as evidenced by a 520-million-year-old fossilized arthropod with remarkably well-preserved brain structures. The remarkably well-preserved fossil of an extinct arthropod shows that anatomically complex brains evolved earlier than previously thought and have changed little over the course of evolution. According to University of Arizona neurobiologist Nicholas Strausfeld, who co-authored the study describing the specimen, the fossil is the earliest known to show a brain. The discovery will be published in the Oct. 11 issue of the journal Nature. Embedded in mudstones deposited during the Cambrian period 520 million years ago in what today is the Yunnan Province in China, the approximately 3-inch-long fossil, which belongs to the species Fuxianhuia protensa, represents an extinct lineage of arthropods combining an advanced brain anatomy with a primitive body plan. The fossil provides a “missing link” that sheds light on the evolutionary history of arthropods, the taxonomic group that comprises crustaceans, arachnids and insects. The researchers call their find “a transformative discovery” that could resolve a long-standing debate about how and when complex brains evolved. “No one expected such an advanced brain would have evolved so early in the history of multicellular animals,” said Strausfeld, a Regents Professor in the UA department of neuroscience. According to Strausfeld, paleontologists and evolutionary biologists have yet to agree on exactly how arthropods evolved, especially on what the common ancestor looked like that gave rise to insects. “There has been a very long debate about the origin of insects,” Strausfeld said, adding that until now, scientists have favored one of two scenarios. Some believe that insects evolved from an ancestor that gave rise to the malacostracans, a group of crustaceans that include crabs and shrimp, while others point to a lineage of less commonly known crustaceans called branchiopods, which include, for example, brine shrimp. Because the brain anatomy of branchiopods is much simpler than that of malacostracans, they have been regarded as the more likely ancestors of the arthropod lineage that would give rise to insects. However, the discovery of a complex brain anatomy in an otherwise primitive organism such as Fuxianhuia makes this scenario unlikely. “The shape [of the fossilized brain] matches that of a comparable sized modern malacostracan,” the authors write in Nature. They argue the fossil supports the hypothesis that branchiopod brains evolved from a previously complex to a more simple architecture instead of the other way around. This hypothesis arose from neurocladistics, a field pioneered by Strausfeld that attempts to reconstruct the evolutionary relationships among organisms based on the anatomy of their nervous system. Conventional cladistics, on the other hand, usually look to an organism’s overall morphology or molecular data such as DNA sequences. Strausfeld, who holds appointments in other UA departments including evolutionary biology and entomology, has catalogued about 140 character traits detailing the neural anatomies of almost 40 arthropod groups. “There have been all sorts of implications why branchiopods shouldn’t be the ancestors of insects,” he said. “Many of us thought the proof in the pudding would be a fossil that would show a malacostracan-like brain in a creature that lived long before the origin of the branchiopods; and bingo! – this is what this is.” Strausfeld traveled to the Yunnan Key Laboratory for Palaeobiology at Yunnan University in Kunming, China, to join his collaborator, Xiaoya Ma, a postdoctoral fellow at London’s Natural History Museum, in studying the brain anatomies of various fossil specimens. In the institute’s collection, they came across the fossil of Fuxianhuia protensa described in the paper. “I spent a frenetic five hours at the dissecting microscope, the last hours of my visit there, photographing, photographing, photographing,” he said. “And I realized that this brain actually comprises three successive neuropils in the optic regions, which is a trait of malacostracans, not branchiopods.” Neuropils are portions of the arthropod brain that serve particular functions, such as collecting and processing input from sensory organs. For example, scent receptors in the antennae are wired to the olfactory neuropils, while the eyes connect to neuropils in the optic lobes. When Strausfeld traced the fossilized outlines of Fuxianhuia’s brain, he realized it had three optic neuropils on each side that once were probably connected by nerve fibers in crosswise pattern as occurs in insects and malacostracans. The brain was also composed of three fused segments, whereas in branchiopods only two segments are fused. “In branchiopods, there are always only two visual neuropils and they are not linked by crossing fibers,” Strausfeld said. “In principle, Fuxianhuia’s is a very modern brain in an ancient animal.” The fossil supports the idea that once a basic brain design had evolved, it changed little over time, he explained. Instead, peripheral components such as the eyes, the antennae and other appendages, sensory organs, etc., underwent great diversification and specialized in different tasks but all plugged into the same basic circuitry. “It is remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years,” Strausfeld added. “The basic organization of the computational circuitry that deals, say, with smelling, appears to be the same as the one that deals with vision, or mechanical sensation.” Co-authors on the study are Xiaoya Ma and Gregory Edgecombe from the paleontology department of the Natural History Museum, London, and Xianguang Hou, director of the Yunnan Key Laboratory for Paleobiology at Yunnan University. References 1. Chelicerate neural ground pattern in a Cambrian great appendage arthropod Nature 502, 364–367 (17 October 2013) doi:10.1038/nature12520 nature/nature/journal/v502/n7471/full/nature12520.html Editors summary Great appendage arthropods are extinct jointed-legged creatures from the Cambrian period equipped with often large claw-like appendages of an arrangement not seen in modern arthropods. Their evolutionary relationships are much debated. Gregory Edgecombe and colleagues use micro-computed tomography to reconstruct the neuroanatomy of Alalcomenaeus, an exquisitely preserved great-appendage arthropod from China. Several characters of the nervous system are uniquely shared with the chelicerates — spiders, scorpions, mites and horseshoe crabs — placing the fossils firmly on the arthropod tree and demonstrating that chelicerate neuroanatomy had evolved by 520 million years ago. Abstract Preservation of neural tissue in early Cambrian arthropods has recently been demonstrated, to a degree that segmental structures of the head can be associated with individual brain neuromeres. This association provides novel data for addressing long-standing controversies about the segmental identities of specialized head appendages in fossil taxa. Here we document neuroanatomy in the head and trunk of a ‘great appendage’ arthropod, Alalcomenaeus sp., from the Chengjiang biota, southwest China, providing the most complete neuroanatomical profile known from a Cambrian animal. Micro-computed tomography reveals a configuration of one optic neuropil separate from a protocerebrum contiguous with four head ganglia, succeeded by eight contiguous ganglia in an eleven-segment trunk. Arrangements of optic neuropils, the brain and ganglia correspond most closely to the nervous system of Chelicerata of all extant arthropods, supporting the assignment of ‘great appendage’ arthropods to the chelicerate total group. The position of the deutocerebral neuromere aligns with the insertion of the great appendage, indicating its deutocerebral innervation and corroborating a homology between the ‘great appendage’ and chelicera indicated by morphological similarities. Alalcomenaeus and Fuxianhuia protensa demonstrate that the two main configurations of the brain observed in modern arthropods, those of Chelicerata and Mandibulata, respectively, had evolved by the early Cambrian. Supplementary information nature/nature/journal/v502/n7471/extref/nature12520-s1.pdf 2. Complex brain and optic lobes in an early Cambrian arthropod Nature 490, 258–261 (11 October 2012) doi:10.1038/nature11495 nature/nature/journal/v490/n7419/full/nature11495.html Editors summary The Cambrian explosion refers to a time around 530 million years ago, when animals with modern features first appeared in the fossil record. The fossils of Cambrian arthropods reveal sophisticated sense organs such as compound eyes, but other parts of the nervous system are usually lost to decay before fossilization. This paper describes an exquisitely preserved brain in an early arthropod from China, complete with antennal nerves, optic tract and optic neuropils very much like those of modern insects and crustaceans. This suggests that if insects evolved from quite simple creatures such as branchiopod shrimps, then modern branchiopods have undergone a drastic reduction in the complexity of their nervous systems. Abstract The nervous system provides a fundamental source of data for understanding the evolutionary relationships between major arthropod groups. Fossil arthropods rarely preserve neural tissue. As a result, inferring sensory and motor attributes of Cambrian taxa has been limited to interpreting external features, such as compound eyes or sensilla decorating appendages, and early-diverging arthropods have scarcely been analysed in the context of nervous system evolution. Here we report exceptional preservation of the brain and optic lobes of a stem-group arthropod from 520 million years ago (Myr ago), Fuxianhuia protensa, exhibiting the most compelling neuroanatomy known from the Cambrian. The protocerebrum of Fuxianhuia is supplied by optic lobes evidencing traces of three nested optic centres serving forward-viewing eyes. Nerves from uniramous antennae define the deutocerebrum, and a stout pair of more caudal nerves indicates a contiguous tritocerebral component. Fuxianhuia shares a tripartite pre-stomodeal brain and nested optic neuropils with extant Malacostraca and Insecta, demonstrating that these characters were present in some of the earliest derived arthropods. The brain of Fuxianhuia impacts molecular analyses that advocate either a branchiopod-like ancestor of Hexapoda or remipedes and possibly cephalocarids as sister groups of Hexapoda. Resolving arguments about whether the simple brain of a branchiopod approximates an ancestral insect brain or whether it is the result of secondary simplification has until now been hindered by lack of fossil evidence. The complex brain of Fuxianhuia accords with cladistic analyses on the basis of neural characters, suggesting that Branchiopoda derive from a malacostracan-like ancestor but underwent evolutionary reduction and character reversal of brain centres that are common to hexapods and malacostracans. The early origin of sophisticated brains provides a probable driver for versatile visual behaviours, a view that accords with compound eyes from the early Cambrian that were, in size and resolution, equal to those of modern insects and malacostracans. Supplementary information nature/nature/journal/v490/n7419/extref/nature11495-s1.pdf - Palaeontology: Cambrian nervous wrecks Nature 490, 180–181 (11 October 2012) doi:10.1038/490180a nature/nature/journal/v490/n7419/full/490180a.html News and Views Fossilized remains of an arthropod from the Cambrian period provide an unusual example of preservation of the brain and nervous system, and shed new light on when and how these tissues evolved. See Letter p.258 (2) -
Posted on: Sun, 20 Oct 2013 00:11:43 +0000
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