The Cambrian Explosion was marked by a sudden radiation of numerous Animal taxa, and the simultaneous expansion into numerous ecological roles. Exactly why this started remains unclear, but the evolution of predatory behaviour and the subsequent arms race between predators and prey is considered to be a significant part of the subsequent rapid evolutionary divergence. The 'explosion' is now recognised to have had three distinct steps; the appearance of Worm-like organisms in the terminal Ediacaran, the appearance of hard parts in the earliest Cambrian, and an rapid evolutionary diversification which produced the majority of modern phyla in Cambrian Stage 3.
Chaetognaths, or Arrow Worms, may have been the first group of Bilaterian Animals to specialise in hunting within the water column, with the Paraconadonts of the Earliest Cambrian now recognised as the grasping spines of early Chaetognaths. Amiskwia sagittiformis, a Burgess Shale fossil first described by Charles Doolittle Walcott in 1911, shows a strong similarity to modern Chaetognaths, possessing a roughly tubular body with paired lateral and tail fins. Despite this, it was for a long time thought unlikely to be a Chaetognath, as no specimen had ever been found with grasping spines, considered to be a key feature of the group. However, recent studies of these fossils have shown that they have an internal jaw apparatus, similar to that seen in Gnathiferans, a group which phylogenetic studies have suggested are closely related to the Chaetognaths, with the two forming a single clade, the Chaetognathifera. This combination of Chaetognath-like paired lateral and tail fins with an Gnathiferan-like internal jaw apparatus, raises interesting questions about the origin of these groups; where the two traits present in the ancestors of all Chaetognathiferans? Or was Amiskwia sagittiformis a member of one lineage that had had convergently gained a trait associated with the other? Understanding the answer to this question could have implications for our understanding of not just the origins of the four modern phyla, the Chaetognatha, Gnathostomulida, Micrognathozoa, and Rotifera.
In a paper published in the journal Science Advances on 3 January 2023, a team of scientists led by Tae-Yoon Park of the Division of Earth Sciences at the Korea Polar Research Institute, and the University of Science and Technology, describe a new, giant, stem-group Chaetognath from the Early Cambrian Sirius Passet Lagerstätte of Peary Land, North Greenland, and discuss its implications for the origin of the group.
The new species is named Timorebestia koprii, where 'Timorebestia' means 'fear-inducing beast' (it also appears to be a reference to Robert Burn's line 'timorous beastie', from the poem 'To a Mouse', though this is not stated), and 'koprii' derives from KOPRI, the acronym for the Korea Polar Research Institute. The species is descibed from 13 specimens collected from several horizons at the main Sirius Passet Lagerstätte locality; specimens were found over 12 m of exposure, although most were from within a particular fossiliferous interval of between 5 m and 7 m.
Timorebestia koprii is a wide bodied 'Amiskwiiform' with lateral fins running along the majority of the length of its trunk and a well-developed rounded caudal fin; these fins are rayed, with no division between the trunk and caudal fins. It's fore-end has a distinct tapering 'head' with paired antennae. An internal jaw apparatus is preserved as a pair of blunt anterior elements, and a single anterior plate. longitudinal bands of discrete muscles can be seen in the trunk, as well as additional outer circular or transverse muscles, although these are much more sparse. A digestive tract can be seen running from the head to just in front of the caudal fin.
Timorebestia koprii shows considerable variation in size, with the smallest being about 22 mm in length, while the largest is an incomplete specimen with a preserved bodylength of 206 mm, plus 92 mm antennae, giving a total preserved length of 298 mm, and an estimated original length in excess of over 300 mm. This is remarkable, as most extant Chaetognaths are only a few millimetres in length, while the largest living species, Pseudosagitta gazellae, reaches about 10 cm in length, similar to the largest described fossil species to date, Capinatator praetermissus, from the Burgess Shale of British Columbia.
Timorebestia koprii shares a general bodyplan with Amiskwia sagittiformis, which is not seen in any extant group of Gnathiferans. Since Amiskwia sagittiformis was the first discovered organism with this bodyplan, Park et al. refer to these Animals as 'Amiskwiiforms'. They also note the presence of another, as yet unnamed stem-Chaetognath in the Sirius Passet Lagerstätte, which does have external grasping spines, and is interpreted as being closer to the crown group than either Timorebestia koprii or Amiskwia sagittiformis. Thus it is assumed that an internal jaw apparatus is the ancestral state in Chaetognaths and Gnathiferans. The accessory pair of transverse elements within these internal jaws resembles the uncus elements of Rotifers (which are closely related to Chaetognaths and Gnathiferans). The external transverse or circular muscles seen in Amiskwiiforms are absent in modern Chaetognaths, however, Rotifers and Mictognanozoans have strong circular muscles and Gnathostomulids have numerous, but much thinner circular muscles, similar to those seen in Timorebestia koprii, suggesting that this might be the ancestral trait in Chaetognathiferans. Both Timorebestia koprii and the unnamed Sirius Passet species show phosphatized structures which appear to be a ventral ganglion with lateral neuron somata, something which strongly suggests they are more closely related to Chaetognaths than to other groups. A phylogenetic analysis carried out by Park et al. suggests that Timorebestia koprii and Amiskwia sagittiformis are the earliest (known) branching taxa on the Chaetognath branch of the Chaetognathiferan tree.
The large size of Timorebestia koprii is surprising; the largest known specimen of Amiskwia sagittiformis is only about 35 mm long, while the previous largest known fossil Chaetognath, Capinatator praetermissus from the Burgess Shale reaches only about 100 mm, comparable to the size of the largest living species, Pseudosagitta gazellae. The large size of Timorebestia koprii makes it one of the largest Early Cambrian pelagic species, which in combination with its complex swimming apparatus and long antennae, would probably have made it a top predator in its environment, an interpretation which is supported by the presence of the Bivalved Arthropod Isoxys volucris within the digestive tract of many specimens.
This is surprising, as Chaetognaths are close to the bottom of the food chain in modern oceans, feeding on tiny zooplankton and, in the case of the largest species, very small Fish. However, they are one of (if not the) earliest pelagic predatory groups to appear, with Paraconadonts such as Protohertzina appearing in the oldest Cambrian deposits, so it is perhaps unsurprising that they were for a time the top predators in the Cambrian oceans. The first Panarthropods are thought to have colonised the water column around the transition from Cambrian Stage 2 to Cambrian Stage 3, or roughly 525 to 522 million years ago. The Sirius Passet Lagerstätte is thought to be between 523 and 518 million years old, making it a window into a time when one ecological realm was replacing another, including the replacement of Chaetognaths as top pelagic predators by the emerging Panarthropods.
See also...
Online courses in Palaeontology.
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