Sunday, 20 December 2020

Gyaltsenglossus senis: A Hemichordate from the Middle Cambrian Burgess Shale.

Hemichordate relationships remain contentious due to conflicting molecular results and the high degree of morphological disparity between the two Hemichordate classes, Enteropneusta and Pterobranchia. Additionally, Hemichordates have a poor fossil record outside of the Cambrian, with the exception of the collagenous tubes of the Pterobranchs (which include Graptolites). By the Middle Cambrian, tube-dwelling colonial Pterobranchs and tube-dwelling enteropneusts coexisted, supporting the origin of the Hemichordate body plan earlier in the Cambrian without clarifying the morphology of their last common ancestor.

In a paper published in the journal Current Biology on 2 November 2020, Karma Nanglu of the Department of Paleobiology at the Smithsonian National Museum of Natural History, Jean-Bernard Caron of the Department of Natural History at the Royal Ontario Museum, and the Department of Ecology and Evolutionary Biology and Department of Earth Sciences at the University of Toronto, and Christopher Cameron of the Departement de sciences biologiques at the Universite de Montreal, describe a new Hemichordate, based on 33 specimens from the 506-million-year-old Burgess Shale (Odaray Mountain, British Columbia).

The new species is named Gyaltsenglossus senis, where 'Gyaltsenglossus' derives from Gyaltsen (pronounced ‘Gen-zay’, emphasis on the first syllable) for Karma Nanglu’s father, who inspired his interest in biology as a child, and glossus from the Greek glossa, meaning tongue, a common generic suffix for Hemichordates, and 'senis' derives from the Latin senex, meaning old.

Gyaltsenglossus senis is described from 33 specimens (all at the Royal Ontario Museum), collected from Odaray Mountain, Yoho National Park, British Columbia, within the upper part of the ‘thick’ Stephen Formation (Odaray Shale Member). With the exception of the holotype specimen (ROMIP 65606.1), all the other specimens are either fragmentary or poorly preserved, and none show the tentacles or proboscis as clearly as the holotype specimen.

Gyaltsenglossus senis is vermiform, with a maximum length of approximately 2 cm. Body consists of an elongate, ovoid proboscis, a crown of six feeding arms, a cylindrical trunk, and a round posterior structure. The feeding arms possess roughly 15 pairs of symmetrical tentacles. Gut is straight, anus terminates immediately before the posterior structure.

From the anterior-most point of the oblong structure, interpreted as a proboscis to the posterior end of the fossil, Gyaltsenglossus is between 1.5 and 2 cm in length, although few specimens preserve the entire morphology clearly. The inside of the proboscis is preserved slightly three dimensionally, beginning at the medial base and following a slightly darker area and projecting forward roughly 40% of its total length. A thickened mass of tissues is present at the base of the proboscis, although it is not well preserved. Six feeding arms radiate from the dorsal area directly posterior to the proboscis. The arms are thin and roughly one-and-a-half times the length of the proboscis, with the most complete arm being ca. 1 cm long. They have a foliose appearance due to roughly 15 pairs of tentacles extending from each arm, although these tentacles are not clearly observable in most specimens. Only the most proximal portion of each tentacle, which adjoins the feeding arm, is preserved clearly on the holotype, often presenting as a sub-triangular structure. In most specimens, the feeding arms are either damaged or retracted such that they appear as a rounded mass, in some specimens, obscuring the proboscis. Posterior to the crown of feeding arms, a row of thin appendages projects from a small elevated platform. These thin appendages are considerably shorter than the feeding arms (about 0.25 cm), devoid of tentacles, and are most clearly observable on the holotype. The trunk is vermiform, roughly twice as wide at the anterior margin as at the posterior end. The trunk appears relatively rigid, with the most curvature observed in the posterior half. A dark, thin, axial structure is frequently preserved in the trunk, consistent with the size and position of a gut. A wider, more thickly walled area at the anterior-most region of the gut may represent a muscular pharynx. The gut does not appear to extend to the end of the body, but terminates before a posterior bulbous structure, which appears darkly preserved. This structure (width: length ratio ranging from 1:1 to 1.4:1) is usually wider than the posterior-most margin of the trunk. A darker band is frequently preserved at the posteriormost rounded margin of this structure and may represent the carbonaceous remnants of thickened tissues.

 
Morphology of Gyaltsenglossus senis from the Burgess Shale. (A) ROMIP 65606. Four specimens of Gyaltsenglossus senis preserved on the same surface (an additional three specimens are out of frame on the same surface). The two best-preserved specimens are in the dashed boxes. (B) Holotype (ROMIP 65606.1). Close-up of boxed specimen on the left in (A). Most complete specimen, showing full length of the proboscis, six feeding arms, a cluster of thin appendages located on the dorsal surface of the trunk, and the posterior structure with dark internal structures. (C) Close-up of the thin appendages on the counterpart of ROMIP 65606.1. The thin appendages are unlike the feeding arms; they are much shorter and entirely bare. (D) Line drawing of (B). (E) ROMIP 65606.2. Close-up of boxed specimen on right in (A). The individual arms cannot be clearly differentiated, but the tentacles surrounding them give the area a tufted appearance. A dark, oblong structure may be the proboscis recurved over the trunk, and the posterior structure is bulbous. (F) Close-up of boxed area in (B). The proboscis coelom is indicated by arrowheads. The thin appendages are filamentous and without tentacles. (G) ROMIP 65607.2. The arms appear retracted and cannot be clearly differentiated. The anterior portion of the gut appears more thickly walled and may represent a pharynx, but the internal organization is unclear. (H) ROMIP 65607.1. The terminal point of the gut is most clearly preserved in this specimen, ending before the posterior-most point of the animal. The feeding arms are significantly degraded, as is sub-triangular structure, which may be a decayed proboscis. Abbreviations: an, anus; fa, feeding arms; gu, gut; is, internal structures; ps, posterior structure; pc, proboscis coelom; ph?, possible pharynx; pr, proboscis; tr, trunk; ta, thin appendages. Scale bars (A) 5 mm; (B), (E), (G), and (H) 2 mm; (C) and (F) 1 mm. Nanglu et al. (2020).

Identification of a proboscis is based on three primary factors. First, it is highly similar in morphology to that of the extant Hemichordate class Enteropneusta, particularly to that of the genus Saccoglossus because it is significantly longer than wide (roughly 4:1). Second, it is the anterior-most structure within the tripartite, Hemichordate body plan of Gyaltsenglossus. Finally, it is preserved similarly to the proboscises of the Cambrian Hemichordates Spartobranchus and Oesia. In particular, it is preserved darkly in comparison with the rest of the body, presumably due to the density of muscle tissue that characterizes the Hemichordate proboscis.

 
Feeding Arms and Thin Appendages of Gyaltsenglossus senis. (A) Close-up of four feeding arms on the holotype specimen, ROMIP 65606. (B) Line drawing of (A). Only the base of the tentacles adjoining the feeding arm is usually visible. The arms and tentacles are surrounded by apparently degraded tissue, likely representing decayed tentacles. (C) ROMIP66000. A poorly preserved specimen, showing two thin appendages and degraded feeding arms. (D) Counterpart of (C), showing feeding arms more closely. Abbreviations: fa, feeding arms; ps, posterior structure; ta, thin appendages. Scale bars 2 mm. Naglu et al. (2020).

Although several external characters are well preserved, other features, such as the darker mass within the proboscis and the tissues basal to the proboscis, are less well preserved. These may represent remnants of the heart-kidney-stomochord complex and of the collar, respectively. Although not preserved, Naglu et al. infer the position of the mouth to be on the ventral side of the Animal between the base of the proboscis and the putative collar, as it is in all modern Hemichordates and as has been inferred in Cambrian Hemichordates previously. In addition, Naglu et al. find no evidence of gill bars or gill pores in any of the 33 specimens of Gyaltsenglossus observed. Such features, however, especially the gill pores, were likely affected by taphonomic bias. The gill bars are internal structures that may be obscured by the ectoderm in cases of pristine preservation. However, Naglu et al. do find abundant evidence of gill bars preserved in the similarly sized and approximately coeval Burgess Shale Hemichordates Spartobranchus tenuis and Oesia disjuncta, which share a common preservation pathway with Gyaltsenglossus. Additionally, the collagenous gill bars are the most decay-resistant feature of the Hemichordate body plan, making it unlikely that they would have been preferentially lost before soft tissue characters such as the proboscis. While the relatively small sample size of Gyaltsenglossus in comparison to Spartobranchus and Oesia does not preclude the possibility of finding new specimens with gill bars preserved, Naglu et al. consider this character to be absent in Gyaltsenglossus. With regard to ectodermal gill pores, these are among the Hemichordate features most susceptible to decay, and they have not been found in any Hemichordate fossils described thus far even though the gill bars of Spartobranchus and Oesia are clearly preserved; gill bars would presumably require gill pores to function. Gill pores are therefore regarded as equivocal in all Cambrian enteropneusts, including Gyaltsenglossus.

Another Hemichordate character that warrants additional discussion based on potential taphonomic biases is the collagenous tube, or tubarium. The tubarium is a feature of all Pterobranchs (except for the genus Atubaria), including Graptolites, where it is secreted by the cephalic shield, a homolog of the Enteropneust proboscis. Spartobranchus and Oesia, despite their Enteropneust-like gross morphology, also produced tubes, which Naglu consider to be homologs of the Pterobranch tubarium. Notably, only a subset of the total number of Worm specimens are found inside of a tube, and many tubes have been found lacking a Worm. A variety of factors may contribute to this phenomenon; for instance, the Animals may have abandoned or been dislodged from their tubes prior to fossilisation, or tube dwelling may have been facultative. Though it is possible that Gyaltsenglossus was tubicolous even though its tubes were not found, Naglu et al. consider the character of the tubarium as absent in their study.

Both Bayesian and strict consensus parsimony analyses resolve the Enteropneusta and Pterobranchia classes as reciprocally monophyletic (posterior probabilities of 0.91 and 1.0, respectively). This result is supported by the most recent molecular phylogenies, though it is not without debate. Spartobranchus and Oesia are resolved as stem-group Enteropneusts, while Gyaltsenglossus is resolved as a stem-group Hemichordate in Naglu et al.'s Bayesian analysis. Support for Gyaltsenglossus being a total-group Hemichordate is high (posterior probability of 0.96 in Naglu et al.'s Bayesian analysis), disproving an affinity with other tentaculate taxa included in these analyses, including Annelids, Cnidarians, and Echinoderms. However, support for its exact position within this phylum is lower (posterior probability of 0.56 in Naglu et al.'s Bayesian analysis, and found in a polytomy with Enteropneusts and Pterobranchs in Naglu et al.'s parsimony analysis, owing to its shared morphological characteristics with both Enteropneusta and Pterobranchia.

 
Ambulacrarian Phylogeny Incorporating Cambrian Fossils and a Schematic of Hemichordate Homologies (A) Phylogenetic position of Gyaltsenglossus senis and other Cambrian Ambulacrarians using majority-rules Bayesian analysis of 113 characters and 32 taxa. The tube-building Oesia and Spartobranchus are stem-Enteropneusts, while Gyaltsenglossus senis is stem to Hemichordata. Numbers at nodes represent posterior probabilities. (B) Homologous structures among fossil and extant Hemichordates. (i): 33 h larva of Rhabdopleura normani with homologous structures in the adult Rhabdopleurid body plan shown in (ii). The cells of the adhesive organ attach the larva to the benthos and develop into a stalk. (iii): Cephalodiscus. (iv): While Gyaltsenglossus senis has feeding arms derived from the collar, as in Rhabdopleura and Cephalodiscus, it lacks a U-shaped gut and distinctive cephalic shield, instead having an Enteropneust-like proboscis. (v): Oesia has an Enteropneust body plan, although with a unique and distinctive posterior bilobed attachment structure and gill bars that extend the length of the trunk. (vi): Spartobranchus has a Harrimaniid-like anatomy, with the exception of a pronounced post-anal bulbous structure. (vii): A 4-week-old juvenile of Saccoglossus horsti, with a distinct post-anal tail used for crawling and adhering to the benthos. Naglu et al. (2020).

The unique morphology and phylogenetic placement of Gyaltsenglossus suggest that paired feeding arms derived from the dorsal collar were present in the Hemichordate last common ancestor. As in modern Pterobranchs, the arms and tentacles were likely used to capture and transport suspended particles from the water column to the mouth. Feeding arms provide an advantage to exploit food sources in the water column by epibenthic organisms across a wide range of body sizes, from Ciliates to Crinoids. Notably, the long proboscis also suggests that Gyaltsenglossus was a deposit feeder like the extant Acorn Worm, Saccoglossus. Gyaltsenglossus provides the clearest example of the morphology of the Hemichordate last common ancestor and suggests that a bimodal feeding mechanism may have been an early feature of the phylum. In this context, feeding arms were lost in the lineage leading to Spartobranchus, Oesia, and the crown-group Enteropneusts, concomitant with a gradual specialisation away from tentaculate filter feeding and, eventually, toward infaunal niches.

While gill bars were likely present in the Hemichordate last common ancestor (and are indeed most likely to be a Deuterostome plesiomorphy), Naglu et al. found no evidence of them in the Gyaltsenglossus specimens studied. Computational fluid dynamics analyses of Hemichordate pharynx models showed that Pterobranchs, which also lack gill bars, may have abandoned their gill pores for feeding arms because very small gill pores impose a functional constraint on pharyngeal filter feeding as the energetic cost of pumping fluids becomes prohibitive. Gill pores appear to be non-functional in the Pterobranch Cephalodiscus and were lost in the diminutive Rhabdopleura. Gyaltsenglossus possibly represents a second example of gill loss in the Hemichordates, but the organism is sufficiently large that this energetic constraint would not have applied. Instead, pharyngeal filter feeding may have been simply redundant with the tentaculate suspension feeding and proboscis deposit feeding modes of Gyaltsenglossus, allowing for the loss of gill pores.

The gut of Gyaltsenglossus is straight like that of an Enteropneust, rather than U-shaped like that of a Pterobranch, and it ends just before the muscular post-anal attachment structure. A straight gut has been inferred in some stem-Echinoderms such as Ctenoimbricata, suggesting that it may be a plesiomorphic ambulacrarian trait. The bulbous structure at the posterior end of Gyaltsenglossus is consistent with the posterior attachment structures of the Burgess Shale Enteropneusts Spartobranchus tenuis and Oesia disjuncta. This bulbous structure appears to be a ubiquitous feature of Cambrian Hemichordates and is likely plesiomorphic for the phylum. Morphologically, it is most similar to that of Spartobranchus, being approximately round to ovoid and without the bifurcating attachment form that Oesia used to grasp its secreted tube. Considered in conjunction with its morphology and position, Naglu et al. hypothesise that Gyaltsenglossus was able to use this structure for attachment as an anchor to access the water column to feed on plankton and possibly for locomotion. The most comparable structure in extant Acorn Worms is the postanal tail of juvenile Harrimanids, which are glandular, ciliated, and used for attachment and locomotion. This tail was lost in derived crown-group Enteropneusts (Spengelids, Ptychoderids, and Torquaratorids), and is believed to be a homolog of the Pterobranch stalk. If this is true, then the Pterobranch stalk is an exaptation of the posterior bulbous structure seen in Gyaltsenglossus, Oesia, and Spartobranchus. This exaptation was a major event in Pterobranch evolution because it permitted coloniality, making possible the abundance of Graptolites that characterised Palaeozoic seas.

Artistic Reconstruction of Gyaltsenglossus senis. The image in the foreground shows Gyaltsenglossus as it would appear while motile. The background illustrates the posture of the animal while attached to the seafloor and suspension feeding. Emily Damstra in Naglu et al. (2020).

The arms of Pterobranchs, and presumably of Gyaltsenglossus, develop from paired extensions of the collar coeloms, or mesocoels. These mesocoels are homologous with those of Echinoderms, but only the left coelom (hydrocoel) is elaborated through metamorphosis of the living Echinoderm classes to become the water vascular system (and hyponeural ring sinus). The feeding arms of Crinoids, Asteroids, and Ophiuroids are then not homologous with the arms of Hemichordates, including Gyaltsenglossus. Support for this idea is the absence of feeding arms among the earliest diverging Echinoderm lineages, including the bilateral symmetric Ctenocystoids, which presumably would have had paired mesocoels. The thin appendages in Gyaltsenglossus have no clear homolog with any extant Hemichordate, although dorsal elaborations of the trunk are common in Acorn Worms (i.e. hepatic caeca and gonadal wings).

Further study of a number of putative fossil Ambulacrarians, including the Cambroernids Herpetogaster and Eldonia, may contribute to further disentangling of early Ambulacrarian evolution. However, their body plans are so diverse and bizarre that homologous characters are difficult to code, and their phylogenic positions remain uncertain. The impressive disparity of Cambrian body plans is thought to be due to a combination of open ecological niches to exploit and few developmental constraints, perhaps due to greater flexibility in early gene regulatory networks. While modern Hemichordates are not particularly diverse, Enteropneust and Pterobranch body plans are so different that the traditional literature largely regarded them as distant relatives. Gyaltsenglossus, representing a combination of both Enteropneust and Pterobranch morphologies, demonstrates starkly the unique insights the fossil record provides through direct evidence of long-extinct body plans.

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