A giant stem-group chaetognath

Systematic paleontology

Unranked clade Spiralia Scleip, 1929

Unranked clade Chaetognathifera Bekkouche and Gąsiorowski, 2022

Phylum Chaetognatha Leuckart, 1854 (stem group)

T. koprii gen. et sp. nov.

LSID

urn:lsid:zoobank.org:act:E5E117F9-E6A9-489B-B5C1-668326BC8BCC.

Holotype

MGUH 34286 (Fig. 1 and figs. S1 and S2), Natural History Museum of Denmark, University of Copenhagen.

Referred material

MGUH 34287 (Fig. 2, A and B, and fig. S3), MGUH 34288 (Fig. 2, C to E, and fig. S4), MGUH 34289 (Fig. 2, F and G), MGUH 34290 (Fig. 5, A and B, and fig. S8, A to E), MGUH 34291 (Fig. 6, A to C, and fig. S15, A to D and H to M), MGUH 34292 (fig. S5), MGUH 34293 (fig. S6), MGUH 34294 (fig. S7), MGUH 34295 (Fig. 3 and fig. S8, K to M), MGUH 34296 (fig. S8, F to J), MGUH 34297 (fig. S15, E to G and N to Q), MGUH 34298 (fig. S8, N to P).

Locality and horizon

T. koprii occurs in several horizons at the main Lagerstätte locality (17) in Sirius Passet, 82°47.6′N, 42°13.7′W, Peary Land, North Greenland. Collections from which material is derived have been focused on an exposed section that totals 12 m, with collection focused on a particular fossiliferous interval between 5 and 7 m (21).

Etymology

Genus name: Timor, (Latin) for causing fear or dread and bestia (Latin), meaning beast. The species name is after Korea Polar Research Institute (KOPRI) for their support of the past, and ongoing, field expeditions to Sirius Passet.

Diagnosis for genus and species

Wide-bodied “amiskwiiform” with lateral fins along most of the trunk length. The body and the lateral fins taper distally and terminate in a well-developed rounded caudal fin. Distinct fin rays are present with no rayless zone separating the trunk and tail fins. Anterior region of trunk tapers markedly into a short head bearing a pair of long antennae, which are about half the length of the body. An internal jaw apparatus in the anterior trunk region consists of a pair of larger subtriangular elements connected by a symphysis, a pair of blunt anterior elements, and a single anterior, presumed ventral/basal, plate. Longitudinal muscles occurring in discrete bands are present in the trunk with additional sparse and delicate outer circular/transverse muscles. The digestive tract extends from the trunk-head transition and terminates anterior to the caudal fin with no septum separating the caudal region from the trunk.

Description

The study included 13 specimens attributed to T. koprii, ranging in size from ~22 mm (fig. S15, E to G) to 200 mm in length, or ~290 mm (Fig. 2A) when antennae are included. Variation in degree and mode of preservation due to different levels of decay and secondary mineralization by phosphatization between available specimens result in variable expressions of body extremities and internal organs. Consistent features across specimens allow for identifying and characterizing the body outline (including lateral view; Fig. 2, E and F), fin rays, alimentary tract, jaw apparatus, longitudinal/transverse musculature, and a ventral ganglion. The body is divided into four distinct regions (Fig. 2, B and F): a pair of anterior antennae about one-third of the total body length (Fig. 2, A, B, E, and F, and fig. S3, A to C), a broad head lacking a neck region, and a dorsoventrally flattened trunk divided into a wide midbody that tapers into a caudal region separated by a longer, constricted margin (Figs. 1 and 2). The head region is often poorly preserved, but is present as a rounded lobe in the best preserved specimens (Fig. 2, A, C, and E, and figs. S3, S6, S7, and S8, F to J). The fin region shows well-developed continuous fin rays along the midbody and caudal region (Fig. 1, A to C, and figs. S1, S5, S6, and S8, A to E), contrasting with A. sagittiformis and living chaetognaths in which the lateral and caudal fins are separate. The gut was a tubular structure, widest in the mid body and inferred, based on multiple specimens, to have extended from the rear of the head region and into the caudal region (figs. S1, S3, S4, and S7), terminating anterior to the caudal fin. The position of the mouth opening is unclear.

The holotype (Fig. 1 and figs. S1 and S2) is a medium-sized individual with a complete trunk. The head region is poorly preserved. The preserved body length is 88 mm, while the widest part of the mid body is 33 mm. The trunk widens markedly about 17 mm from the anterior margin of the preserved head and tapers markedly again about 46 mm farther posteriorly into a caudal region. The caudal region (fig. S1G) is ~26 mm long (including the tail fin) and ~13 mm and 18 mm wide at the narrowest and widest part of the fin, respectively. Two specimens are exceptionally large. The largest individual (MGUH 34287; Fig. 2, A and B, and fig. S3) has a body length of ~206 mm and well-defined antennae that are additionally 92 mm long (total length ~298 mm). The posterior region is poorly preserved due to decay, but based on relative proportions between the trunk and the caudal region, as observed in the holotype, the total body length is calculated to slightly surpass 300 mm and is therefore nonetheless largely complete. A second specimen, MGUH 34288 (Fig. 2, C and D), preserves similar body proportions and paired, arcuate phosphate mineralizations in the midbody after musculature and an underlying organ (ventral ganglion, see below). It lacks antennae but has a body length of 178 mm and so are slightly smaller than MGUH 34287.

Several specimens reveal distinct fin rays preserved along the entire trunk (Fig. 1 and figs. S1, A, B, E, and H, S5, A and B, S6, A, C, E, and F, S7, A, B, E, and J, and S8, B, C, and E). Fin rays are stout along the midbody (~350 μm wide in the holotype) becoming longer and narrower in the caudal region (~100 μm wide) (compare fig. S1, G and H). Fin rays are present along most of the trunk (commencing about 10% from the head/trunk junction) and are uninterrupted at the transition into the caudal region (Fig. 1 and figs. S1, A, B, E, and G, and S8, C and E).

Sirius Passet has experienced a higher degree of metamorphic recrystallization (22) and volatilization of less recalcitrant organic phases (23) than other classic Burgess Shale-type localities limiting some features to those expressed mainly through their relief. We identify faintly impressed structures attributable to a jaw apparatus in the holotype as well as other specimens (Fig. 1, D to F, and fig. S2) traced using multiple, stacked images with different illumination angles (demonstrated in movie S1). At least five elements can be identified: a pair of larger triangular plates that connect posteriorly in a symphysis; a pair of anteriorly placed, transverse, elements that taper laterally with a flattened margin medially and a median anterior plate with anteriorly bulging lateral lobes (Fig. 1, D to F, and fig. S2). The digestive tract is narrow and best observed in the midbody, but it is typically poorly exposed in the caudal and head regions (Fig. 1 and figs. S1F and S3F). Sometimes the digestive tract is locally expanded (fig. S1, D to F) or widened along most of its length (Fig. 6, A to C, and figs. S7, A to G, and S15, A to G), often accompanied by food content. The digestive tract can be traced further into the caudal region where it terminates anterior of the caudal fin (figs. S1, D and E, S3, S4, S6, S7, S8E, and S15, A to G). Anteriorly, it extends into the base of the head and widens into a region containing sclerotized jaw elements, preserved as faint impressions with little reflectivity (Fig. 1D). The digestive tract continues along the midline, preserved sometimes as a reflective film, with some relief and/or as occasional three-dimensional phosphate-derived mineralization (fig. S3F) (22). Some specimens preserve food content, dominated by carapaces of the nektonic arthropod Isoxys (Fig. 6, A to C, and figs. S4, D and E, S7, C to E, and S15). Their outline is often faint due to preservation within the gut tract of T. koprii, which often is preserved with higher reflective carbon. In one specimen (fig. S15, A to D), over five individual Isoxys specimens are present. One small specimen contains an Isoxys in the jaw region (fig. S15, E to G and N to Q).

The musculature is partly preserved through phosphatization (22) (Figs. 3 and 5A and figs. S1, I and J, S3, D and E, and S4F) and as organic strands (Fig. 1A and figs. S1, B and K, S5, B and G, and S6, C and F). The longitudinal muscles form separate ribbons that are densely arranged as observed when phosphatized but are splayed from each other when preserved organically (fig. S6). This variation is likely a consequence of preservation timing, with phosphatization occurring early during post-mortem decay, while organically preserved elements experienced further decay that enabled displacement before compaction. Discrete, transverse musculature is sometimes present as imprints in the mineralized tissues (Fig. 3 and figs. S1I, S3D, S8P). In one specimen, the association between the longitudinal and transverse muscles on both the dorsal and ventral side (Fig. 3, D and E) can be surmised due to muscle preservation by extensive phosphatization in the midbody in which the digestive tract can be seen as an impression, embedded within the two layers (Fig. 3, B and C). The uneven splitting of the rock in which portions of the mineralized material adhered to the counterpart reveals impressions of longitudinal muscles and underlying circular muscles from the opposite surface (Fig. 3E), demonstrating the presence of circular muscles on both sides.

The mineralized musculature is concentrated in the mid-body and varies in extent from a single mass (fig. S6) into a restricted set of bilateral arcuate structures, which appear to be due to conflation with some other organ containing phosphate having this morphology (Figs. 2A and 5A and figs. S3, S4, and S8). Phosphatization of anatomy is common in several taxa from Sirius Passet (22), and mostly mineralizes tissues enriched in phosphate in vivo, such as adenosine triphosphate (ATP)–rich musculature (24) or the digestive systems of carnivorous taxa (25). Neither the nature of the arcuate lateral structures corresponds to the distribution of the more extensive musculature nor do they connect to the digestive tract. They are therefore considered to be distinct organs that phosphatized in association with adjacent musculature, which is evident in some specimens where the arcuate mineralizations can be discerned from the musculature by a more featureless texture (fig. S8). The structures resemble the lateral neuron somata of the ventral ganglion of extant chaetognaths (Fig. 5, E to G) (19). This organ is composed of serially arranged neurons in which each cell nucleus is localized to the lateral somata (19, 20), apparent when immunohistochemically stained (for DNA) with bisbenzimide (0.5%, Hoechst H 33258) (Fig. 5, E to G). Its unique structure likely aided phosphatization. Cell nuclei contain DNA with a phosphate backbone and could therefore act as a source of phosphate for mineralization when concentrated enough in a tissue such as the lateral neuron somata observed here (fig. S9). This is additionally corroborated by a chaetognath from Sirius Passet with external grasping spines (Fig. 5, C and D, and figs. S10 and S11) shown here for the first time that more closely resembles members of the crown group. Many specimens preserve discrete phosphate mineralizations in their midbody, ranging from a decayed patch (fig. S11) into a set of discrete and paired mineralizations (Fig. 5C and fig. S10, A to D) similar to T. koprii. While previous studies have proposed that the pedal ganglion of rotifers is a potential homolog of the chaetognath ventral ganglion (13), similar small ganglia are also present in Gnathostomulida (26) and Gastrotricha (27), suggesting that they are widely distributed among early branching spiralians. These structures differ vastly in their relative size to that present in chaetognaths, and therefore, these massive nervous structures are restricted to members of the chaetognath total group.

Nervous systems typically preserve as carbon-rich reflective structures (28, 29). We note that the reflective structures labeled as “indeterminate organ” in some specimens of A. sagittiformis (11) conform in appearance and localization to the phosphatized lateral neuron somata observed in T. koprii (fig. S12), hence corroborating the identity of these structures as derived from the nervous system and having an affinity with chaetognaths.

Material studied

Thirteen specimens of Timorebestia gen. nov. were included in this study along with five specimens of a currently undescribed chaetognath. While one specimen of T. koprii (MGUH 34289; Fig. 2F) is preserved in lateral view, all other specimens are in dorso-ventral aspect. Apart from the three-dimensional mineralization in some specimens of the lateral somata in the ventral ganglion and neighboring muscles, fossils are usually preserved as thin reflective films, with some relief defining the sclerotized jaw elements and the outline of the gut. All specimens are deposited in the Geological Museum, Natural History Museum of Denmark, University of Copenhagen, prefixed with MGUH. Specimens were cleaned with water and detergents. Some specimens were furthermore cleaned with dilute hydrochloric acid to remove rust on the surface. Mechanical preparation of the fossils was done with an AUTOMEL Electric Engraver (Dong Yang Electric Co.).

Methods

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