Thursday, 1 August 2019

Hanusia sp. & Thoralicystis sp.: Soft tissue preservation in Stylophorans from the Ordovician of Morocco shed light on the affinities of this enigmatic Echinoderm group.

The Deuterostomes are the group of animals which include the Chordates (the wider group that includes the Vertebrates) and Echinoderms, and therefore are of great interest to palaeontologists and evolutionary biologists studying the origins of these groups. Unfortunately all living (and fossil) Chordates and Echinoderms are thought to be highly modified compared to the last common ancestor of the group, leading scientists to theorise that the earliest members of the group probably resembled most closely the third Chordate group, the Hemichordates. The Hemichordates are also highly derived, and comprise two major groups, the colonial, tube-dwelling Pterobranches and the burrowing Enteropneusts or Acorn Worms. Genetic studied have shown that the Echinoderms and Hemichordates are more closely related to one-another than either group is to the Chordates, which further supports the idea that the earliest Echinoderms may have been Hemichordate-like. The Stylophorans are an enigmatic group of Middle Cambrian-Late Carboniferous fossil interpreted as non-pentameral Echinoderms. They have a single, bilaterally symmetrical, appendage inserted into an asymmetric test made up of multiple calcitic plates. The nature of this appendage has important implications for the affinities of the Sytlophorans, and potentially the nature of the earliest Echinoderms. Two theories have emerged about the nature of this appendage, the first that it is a single feeding appendage similar to the five present in most modern Echinoderms, and the second, which suggests that it might be a segmented tail, containing a notochord and serially arranged muscle blocks, something which might have been seen in an ancestral Chordate- or Hemichordate-like Echinoderm (this has sometimes led to the group being called 'Calcichordates').

In a paper published in the journal Giobios on 26 November 2018, Bertrand Lefebvre of the Université Claude-Bernard Lyon, Thomas Guensburg of the Field Museum, Emmanuel Martin, also of the Université Claude-Bernard Lyon, Rich Mooi of the Department of Invertebrate Zoology and Geology at the California Academy of Sciences, Elise Nardin of Géosciences Environnement Toulouse at the Observatoire Midi-Pyrénées, Martina Nohejlova and Farid Saleh, again of the UFeniversité Claude-Bernard Lyon, Khaoula Kouraïss and Khadija El Hariri of the Département des Sciences de la Terre at the Université Cadi-Ayyad, and Bruno David of Biogéosciences at the Université de Bourgogne Franche-Comté, and the Muséum National d’Histoire naturelle, describe a series of Sytlophorans from the Ordovician Fezouata Lagerstãtte of Morocco, which show preservation of the soft tissue of the appendage, shedding light on its nature and therefore the affinities of the Stylophorans.

The two current interpretations (H1 and H2) of Stylophoran morphology and their implications for soft tissue anatomy, based on the Cornute Phyllocystis blayaci (Lower Ordovician, France). (A)–(C). H1, Stylophorans as typical Echinoderms with a single feeding arm. (D)–(F). H2, Stylophorans as pre-radial Echinoderms with a Hemichordate-like stalk.
(A), (D). Basic anatomical features deduced from skeletal morphology. (B), (E). Reconstructions of soft tissue anatomy along a longitudinal section of the Stylophoran appendage. (C), (F). Three-dimensional reconstructions of the soft tissue anatomy in the distal part of the Stylophoran appendage. Lefebvre et al. (2019).

The Early Ordovician Fezouata Lagerstãtte outcrops on the western flank of Bou Izargane hill, about 18 km north of Zagora in the central Anti-Atlas, Morocco. The Fezouata Shale comprises a 1000 m thick, repetitive succession of argillaceous siltstones, deposited unconformably over the middle Cambrian sandstones of the Tabanite Group (the term 'deposited unconformably' implies that there was a gap in deposition between the strata being laid down, generally with some evidence of erosion in between). The fossil-bearing beds are about 270 m above the base of this sequence, and comprise three separate beds, each one recording a different fauna with excellent soft-tissue preservation, the lowermost bed being dominated by Marrellomorphs, the middle by Trilobites, and the upper bed by Stylophorans belonging to two taxa, Hanusia sp. and Thoralicystis sp.. The living organisms are thought to have lived on a shallow seafloor, and to have been buried rapidly during storm events.

 Geographic and geologic context of Bou Izargane locality. On the maps (left), Ordovician outcrops in northwestern Africa and in the Zagora area are indicated in green. The red star shows the location of Bou Izargane hill. On the right, partial stratigraphic column of the Fezouata Shale, showing the position of the Bou Izargane excavation. The left column indicates sea level changes. Numbers along the column indicate the height (in m) above the Cambrian/Ordovician boundary. The colors used in the log correspond to those of outcropping rocks. Lefebvre et al. (2019).

At least one specimen of Thoralicystis nov. sp. shows clear evidence of soft-bodied structures in the distal part of its appendage. Preserved soft tissues consist of two superimposed longitudinal canals housed in the continuous, median groove on the upper surface of ossicles, and extending from the stylocone cavity (proximally) to the distal end of the appendage, and numerous small, lateral tube-like extensions branching regularly from the external longitudinal canal and protruding outward from between widely open, hinged plates.

Thoralicystis sp. from the Fezouata Shale, Lower Ordovician, Morocco. (A)–(C) Complete specimen in lateral view. (A) Composite photographic reconstruction. (B) Composite SEM elemental map of Fe. (C) Composite reconstruction of soft part anatomy based on camera lucida drawings, SEM elemental maps (Fe) and back-scattered electron micrographs. (D)–(F) Magnified views of parts of the distal appendage outlined in pictures (A)–(C). (D) Back-scattered electron micrograph. The arrows indicate the position of tube feet. (E) SEM elemental map showing the distribution of Fe. (F) Reconstruction of the soft parts based on camera lucida drawings, SEM elemental maps (Fe) and back-scattered electron micrographs. Abbreviations: pr. cavity: proximal cavity; pr. rings: proximal rings; styl.: stylocone; wvs: water vascular system. Scale bars: 5 mm (A)–(C), 1 mm (D)–(F). Lefebvre et al. (2019).

Several specimens of Thoralicystis sp. and Hanusia sp. exhibit strong evidence of soft tissues preserved within the proximal part of their appendage. All these specimens display a spindle-shaped, elongate cavity extending throughout the proximal rings of the appendage from the stylocone cavity (distally), to the main body cavity (proximally).

Soft tissue preservation in the test and proximal appendage of the cornute Stylophoran Hanusia sp.; late Tremadocian (Lower Ordovician), Bou Izargane, Zagora area (Morocco). (A) Composite photographic reconstruction of the specimen in lateral view. (B) Composite reconstruction based on back-scattered electron micrographs. (C) Reconstruction of the soft parts based on camera lucida drawings and back-scattered electron micrographs. Abbreviation: bis. platelets: biserial platelets. Scale bars: 5 mm. Lefebvre et al. (2019).

Lefebvre et al. reject the idea that the appendage of these Stylophorans might represent a muscular tail on the basis that the presence of open plates that precludes a coelom, and the presence of lateral extensions that branch regularly from the external longitudinal canal and protrude externally in between the open plates. Instead the external longitudinal canal is interpreted as an ambulacral canal of the water vascular system, and the lateral, tube-like structures as ambulacral tube feet the morphology of which is strikingly similar to that of exceptionally preserved tube feet described from Paleozoic arm-bearing forms.

Soft tissue preservation in the proximal appendage of the cornute stylophoran Hanusia sp.; late Tremadocian (Lower Ordovician), Bou Izargane, Zagora area (Morocco). (A) Specimen in upper view. (B) Reconstruction of (A) based on camera lucida drawings. (C) Closer view of the proximal region of the appendage outlined in (A) (red rectangle). (D) Reconstruction of (C) based on camera lucida drawings. Abbreviations: pr. cavity: proximal cavity; pr. rings: proximal rings. Scale bars: 1 mm. Lefebvre et al. (2019).

Lefebvre et al. also reject the idea that the idea that the Stylophorans mighr represent an early group of Echinoderms that appeared before pentameral symetry evolved within the group, on the basis that feeding appendages similar to that of the Stylophorans seem to have appeared in other Echinoderm groups after pentameral symmetry, which would require such a structure to evolve twice, Lefebvre et al. therefore consider it more likely that the Sylophorans are descended from a pentameral ancestor with five feeding appendages, and have somehow lost their pentametal symmetry secondarily.

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