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New discoveries regarding the evolution of arthropods

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  • New discoveries regarding the evolution of arthropods

    Two discovering from the Great White North (i.e., Canada or Canuckistan), have added to our understanding about the phylum that contains such organisms as insects, arachnids, and crustaceans. The first comes from the Cambrian age Burgess Shale formation while the second is from a deposit near Ontario dating back to the Ordovician (which immediately followed the Cambrian).

    Source: Three-eyed Cambrian fossils shed new light on arthropod head evolution

    The Burgess Shale is located in British Columbia and is a site of marine fossils that are 506 million years old. It is celebrated for its “weird wonders,” containing a treasure trove of astonishingly well-preserved fossils.

    These organisms harken back to the Cambrian explosion, a time in Earth’s history when major animal groups were diverging from each other in a burst of evolutionary innovation. In addition to the first representatives of most surviving animal groups, Cambrian deposits preserve a menagerie of sea-dwelling invertebrates unlike anything alive today.

    One of these bizarre animals is Stanleycaris hirpex, a distant arthropod cousin of insects and spiders. As a PhD candidate at the University of Toronto, I had the privilege of working with a previously unstudied collection of Stanleycaris fossils from the Burgess Shale. Before this, Stanleycaris was only known from fragmentary bits and pieces. Our study is the first to reveal intact specimens. The amazing preserved details give us insight into the evolution of the brain and head in the most diverse group of animals.

    Famous fossils

    Stanleycaris is a relative of one of the most iconic animals of the Cambrian, Anomalocaris. Both were predators with bulging compound eyes, round, tooth-lined jaws, swimming flaps and a pair of jointed frontal claws used to snare prey. These and other species were members of a diverse group called radiodonts, which were among the first to branch off from the arthropod group. This happened prior to the evolutionary divergence of major living subgroups like insects, arachnids and millipedes.

    Smaller than the size of a human hand, Stanleycaris is shorter than the metre-long Anomalocaris, but no less odd-looking. The new fossils are also much better preserved, showing surprising features.

    For example, Stanleycaris sports a large third eye in the middle of its head, between the two compound eyes. This has never been seen before in a radiodont, and emphasizes that these early arthropods had already evolved a complex array of different visual organs to help them navigate the ocean depths. This can also be seen in many of their distant modern kin.

    However, perhaps the most exciting discovery is the preservation of much of the central nervous system of Stanleycaris in stunning detail. The fossils show that the brain of Stanleycaris surrounds part of the digestive tract. The brain is likely composed of two segments connected with the eyes and the frontal claws, respectively. Behind the brain are a pair of filamentous nerve cords that run along the belly of the organism.

    A head-scratcher

    For decades, an academic dispute raged over the “arthropod head problem.” This debate has far-reaching implications: The bodies of arthropods are made up of a repeated series of segments, and understanding how head segments line up is key to unlocking an understanding of nearly every aspect of their anatomy and evolution.

    Since arthropods make up roughly 85 per cent of living animal species, this is important for understanding the origin of much biodiversity.

    Much progress has been made on the problem, particularly for living arthropods, in which recognition of a ubiquitous brain composed of three segments — a protocerebrum, deutocerebrum and tritocerebrum — was critical.

    Fossils have proven more difficult to interpret, as only limited information is preserved. Information on brain anatomy is extremely rare in the fossil record.

    However, over the last decade this has begun to change with the discovery that some Cambrian deposits can occasionally preserve remains of nervous systems. Although so-called neuropaleontology is not without controversy, these discoveries have shed some light on the evolution of arthropod heads in early fossil groups.

    Turning arthropod heads

    Owing to their early divergence in the arthropod group, radiodonts are well-positioned to help inform the ancestral traits of arthropods. However, the alignment of their head segments with other extinct and living groups has been unresolved.

    Prior to our study, the only information on brain anatomy came from a single specimen from China showing only partial preservation, the interpretation of which was contested. Based on new Stanleycaris fossils, we can now say with confidence that the radiodont brain already included both the protocerebrum and deutocerebrum. The protocerebrum was connected with the eyes of Stanleycaris, while the deutocerebrum innervated the large frontal claws.

    Other fossil groups — such as certain worm-like animals called lobopodians, taco-shaped arthropods called isoxyids, and megacheirans, which look similar to radiodonts but have jointed limbs instead of flaps — share similar-looking frontal appendages.

    Based on our discoveries in Stanleycaris, we think all of these structures share a common origin. Ultimately, these grasping frontal appendages were transformed into the sensory antennae of insects, the fangs of spiders, and their equivalents in other living groups.

    While our new research is by no means the end of discussion about the arthropod head, it represents a key leap forward in understanding the evolution of this diverse and significant group of animals.


    © Copyright Original Source

    The paper describing the discovery, is A three-eyed radiodont with fossilized neuroanatomy informs the origin of the arthropod head and segmentation with the abstract from it provided below


    In addition to being among the most iconic and bizarre-looking Cambrian animals, radiodonts are a group that offers key insight into the acquisition of the arthropod body plan by virtue of their phylogenetic divergence prior to all living members of the phylum. Nonetheless, radiodont fossils are rare and often fragmentary, and contentions over their interpretation have hindered resolution of important evolutionary conundrums. Here, we describe 268 specimens of Stanleycaris hirpex from the Cambrian Burgess Shale, including many exceptionally preserved whole-body specimens, informing the most complete reconstruction of a radiodont to date. The trunk region of Stanleycaris has up to 17 segments plus two pairs of filiform caudal blades. The recognition of dorsal sclerotic segmentation of the trunk cuticle and putative unganglionated nerve cords provides new insight into the relative timing of acquisition of segmental traits, the epitome of the arthropod body plan. In addition to the pair of stalked lateral eyes, the short head unexpectedly bears a large median eye situated behind a preocular sclerite on an anteriorly projecting head lobe. Upon re-evaluation, similar median eyes can be identified in other Cambrian panarthropods demonstrating a deep evolutionary continuity. The exquisitely preserved brain of Stanleycaris is consistent with the hypothesized deutocerebral innervation of the frontal appendages, reconciling neuroanatomical evidence with external morphology in support of an ancestrally bipartite head and brain for arthropods. We propose that the integration of this bipartite head prior to the acquisition of most segmental characters exclusively in the arthropod trunk may help explain its developmental differentiation.

    Source: A new fossil discovery reveals a little more about arthropod evolution

    Paleontology is undergoing a new renaissance. Since the mid-20th century, genomics has become the main focus of evolutionary biology. But the last few decades have shown how the study of fossils can complement genomic data and improve our understanding of the history of life on Earth.

    Every fossil site provides information about the ecology and evolution of ancient life, but a handful of fossil sites are providing unique, critical data. These sites of exceptional preservation are known as laggerstätte. These sites can contain fossils with soft tissues, which are unmineralized structures that would normally decay is most conditions, thus usually becoming absent from the fossil record.

    The preservation of fossils consisting of soft tissue requires elusive conditions, like low-oxygen conditions that halt decay. Our research team found soft-tissue fossils in a well-known site in Ontario for the first time, among them a rare arthropod fossil.

    This surprising find sheds light on what the Ordovician Period (the second period of the Paleozoic Era) may have looked like. It also adds a new puzzle piece to one of the most enigmatic arthropod groups in the fossil record.

    Rare find

    The Royal Ontario Museum holds fossils from a plethora of sites around Canada. One of them is the Kirkfield Formation of Southern Ontario. Around 450 million years old, the Kirkfield was a shallow tropical ocean at the end of the Ordovician Period. The formation is well-known for its hard-shelled fauna, which includes trilobites, brachiopods and complete, exquisitely preserved sea lilies (crinoids).

    Fossil collector George Kampouris and his team were working a Kirkfield site near Brechin and discovered, for the first time, evidence of soft tissues. Later, paleontologists at the University of Toronto under the direction of Jean-Bernard Caron analyzed the new fossil specimens.

    Despite the fairly limited number of specimens, the findings show organisms that, in normal conditions, would not have been preserved. Among them were macroscopic branched algae, which confirmed that this was indeed a shallow-water environment; a carapace that could be the remains of a shrimp-like animal; and, most importantly, a marrellomorph. With less than a dozen species known worldwide, marrellomorphs are one of the rarest arthropod groups in the world.

    Identifying fossils

    Arthropods are the most diverse group of animals in the world. They can be easily classified into a handful of groups: chelicerates (which includes spiders and mites), myriapods (like centipedes and millipedes), crustaceans (like crabs and shrimps) and insects.

    Some arthropod groups, like the trilobites, have gone extinct, but their mineralized exoskeletons have left us an excellent fossil record.

    The evolutionary success of arthropods is an intriguing question that traces back to the beginnings of the Cambrian period, more than 520 million years ago. Sites of exceptional preservation include the 506-million-year-old Burgess Shale in British Columbia. There, the fossil record shows a “burst” of innovation in the first steps of arthropod evolution, with the emergence of many groups that later became extinct.

    One of the most common and well-known species at the Burgess Shale is Marrella. Marrellomorphs are characterized by a big spiny shield, one or two pairs of large appendages, and a short multisegmented thorax. Despite the abundance of Marrella fossils, there is no consensus on where marrellomorphs place in the evolution of arthropods.

    The segmentation of the head is usually considered a good tool to recognize and reconstruct the evolution of arthropods, but the head of marrellomorphs is nothing like any other arthropods we know of. We know marrellomorphs survived until the Devonian, around 410 million years ago, but only a handful of species have been discovered, with fossils found in Argentina, Morocco and Germany. Finding any new fossil of this group of arthropods, then, is critical to understand what they really were.

    The new marrellomorph from Ontario, named Tomlinsonus dimitrii gives us a new piece in the puzzle of this group. While Tomlinsonus dimitrii does not solve the identity of this group, it shows certain morphological and functional similarities with sea spiders.

    These present-day relatives of spiders and horseshoe crabs live on seafloors and use their elongated limbs to propel themselves. These limbs are particularly similar to those in Tomlinsonus, and so it is possible that Tomlinsonus followed a similar strategy.

    The fossil of the marrellomorph Tomlinsonus dimitri. Tomlinsonus had a shield with four elongated
    spines bearing smaller spines. This shield covered its head, which had one pair of large legs that resembled stilts

    Whether this has evolutionary implications will depend on further marrellomorph discoveries, as the puzzle of their evolutionary history is still far from its first stages.

    Revisiting archives

    Soft-tissue fossils require particular environmental conditions for their preservation. As we continue to explore sites of exceptional preservation, we will reveal additional information about the fossil record and fill in some of the evolutionary gaps.

    One may think that we already know all of these sites. However, areas with remote access, or in countries where paleontological efforts are being strengthened, have a strong potential for discovery. Several fossil sites can be rediscovered after being lost after decades of unexploration. Well-known sites like the Burgess Shale can be also expanded through adjacent outcrops, revealing vastly different animal communities.

    The discovery of Tomlinsonus dimitrii at Brechin shows that even previously known sites can still produce new discoveries. With time and more hands, new discoveries are guaranteed.


    © Copyright Original Source

    The paper describing the discovery, is A new marrellomorph arthropod from southern Ontario: a rare case of soft-tissue preservation on a Late Ordovician open marine shelf with the abstract from it provided below


    Ordovician open marine Lagerstätten are relatively rare and widely dispersed, producing a patchy picture of the diversity and biogeography of nonmineralized marine organisms and challenging our understanding of the fate of Cambrian groups. Here, for the first time, we report soft-bodied fossils, including a well-preserved marrellomorph arthropod, fragmentary carapaces, and macroalgae, from the Late Ordovician (Katian) Upper Member of the Kirkfield Formation near Brechin, Ontario. The unmineralized elements and associated exceptionally preserved shelly biota were entombed rapidly in storm deposits that smothered the shallow, carbonate-dominated shelf. The marrellomorph, Tomlinsonus dimitrii n. gen. n. sp., is remarkable for its ornate, curving cephalic spines and pair of hypertrophied appendages, suggesting a slow-moving, benthic lifestyle. Reevaluation of marrellomorph phylogeny using new data favors an arachnomorph affinity, although internal relationships are robust to differing outgroup selection. Clades Marrellida and Acercostraca are recovered, but the monophyly of Marrellomorpha is uncertain. The new taxon is recovered as sister to the Devonian Mimetaster and, as the second-youngest known marrellid, bridges an important gap in the evolution of this clade. More generally, the Brechin biota represents a rare window into Ordovician open marine shelf environments in Laurentia, representing an important point of comparison with contemporaneous Lagerstätten from other paleocontinents, with great potential for further discoveries.

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  • #2
    Fantastic Reference! Thank for the detailed post!
    Glendower: I can call spirits from the vasty deep.
    Hotspur: Why, so can I, or so can any man;
    But will they come when you do call for them? Shakespeare’s Henry IV, Part 1, Act III:

    go with the flow the river knows . . .


    I do not know, therefore everything is in pencil.


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