Biologically controlled biomineralisation is a fundamental process in the evolution of the animal kingdom. Though it was sporadically achieved already in the latest Precambrian, the so-called 'Cambrian Explosion' around 540 million years ago, marks the general advent of Animals with hard parts in the fossil record. It is well known, that this event was not an instantaneous process, but that it was accomplished in a time span of about 20–25 million years. The acquisition and evolution of hard skeletons within the great majority of phyla and classes are the basic prerequisites for the reconstruction of animal form and function through time. Once hard parts were created in order to protect and stabilize soft tissues and organs, this evolutionary advantage indeed developed a great variety of morphologies, but in spite of this diversity and modification, solid skeletons were generally maintained as such until extinction of the equivalent groups.
In a paper published in the Journal of Paleontology on 24 January 2020, Jobst Wendt of the Fachbereich Geowissenschaften der Universität Tübingen, describes and decrypts an unusual case of biomineralisation in fossils of the rather poorly known class Ascidiacea (Sea Squirts) of the subphylum Tunicata.
The reduction of a hard skeleton into a mere softbodied stage seems to have been achieved in some Coleoid Cephalopods and Opisthobranch Sea Slugs, but this accordance is based on notional palaeontological evidence. An alleged late acquisition of a hard skeleton inherited from soft-bodied ancestors is suspected in Bryozoans and Scleractinian Corals, but in contrast to Ascidians, once these groups had acquired calcified skeletons, they maintained this protective and stabilizing advantage until Recent. Thus, the very late acquirement of a solid calcareous skeleton among Tunicates and its later loss in favor of an almost soft-bodied stage is a very rare phenomenon in the geological record.
From their form and mode of life, Ascidiacea (Sea Squirts) appear as rather simple-structured organisms that superficially and functionally are evocative of Porifera. But the presence of a tubular dorsal nerve cord and a notochord in their larval stage and pharyngeal clefts allocates them amuch higher position within the Animal Kingdom as a subphylum of the Chordata. Ascidians are an artificial, polyphyletic group that comprises three orders (Aplousobranchiata, Stolidobranchiata, Phlebobranchiata) with a total of about 2940 living species. They generally lack hard parts, thus minimizing the possibility of preservation. Only a few genera of the orders Aplousobranchiata and Stolidobranchiata segregate tiny isolated spicules, embedded in the mantle (tunica) or other organs, but their origin remains unknown. Tunica is the Latin word for mantle. Therefore, many zoologists regard both terms as synonymous. Others, however, distinguish an inner layer (tunic), in which the spicules are secreted, from an outer one (mantle) In order to guard against misunderstandings, Wendt uses both terms as synonyms because, with regard to fossils, the differential application of both terms would appear arbitrary.
Spicules of cpossible Tunicates have been found in rocks as old as lower Liassic. Generally, ascidian spicules consist of aragonite, more rarely of vaterite or other minerals. Unlike some siliceous sponge spicules, they are never fused together and thus could not form solid skeletons. The discovery of complete soft-bodied ascidians (lacking spicules, however) in the lower Cambrian of China has revealed that ascidians existed already at an early stage of Metazoan evolution, but this finding does not allow to reconstruct their relationships to Mesozoic to recent representatives of this class. A new evolutionary branch among Ascidians is the recent discovery of calcareous Tunicate exoskeletons in the Upper Triassic and their ancestors in the Permian.
Wendt suggests that these very unusual skeletons, which are composed of irregular aragonitic plates, can be assigned to no phylum other than the Tunicates. This systematic position of these previously unknown organisms to the class Ascidiacea has been the fundamental clue for the systematic attribution of the new discoveries described by Wendt. The fact, however, that they are endoskeletons, opens an unusual insight into a new aspect of the evolution of this poorly known class which has no counterpart among their living representatives. Tunicate exoskeletons have long been known from Permian deposits in east Asia and Europe, but until recently they were erroneously attributed to Rugose Corals. These skeletons are composed of a varying number (2 to about 35) of irregular plates that consist of acicular aragonite. This very unusual construction and mineralogy were crucial for their systematic attribution, though similar living representatives of this class are unknown. Comparably organized endoskeletons now add a new aspect to the fossil record of this largely ignored subphylum of the Chordata.
Endoskeletons are widespread in the Animal Kingdom and display a great variety of shape, function, and mineralogical composition. They are typical for the major phyla of Deuterostomia (Echinodermata and Vertebrata, including Conodontophorida) and many Protista (Foraminifera and Radiolaria), which exhibit a stunning variety of shapes and geometries. Endoskeletal spicules (sclerites) of different mineralogical composition evolved independently among several phyla and classes (Porifera, Octocorallia, Vermes, Holothuroidea, Tunicata). Apart from the calcareous octocoral Tubipora musica, only some Porifera only some Porifera (Hexactinellida and Lithistida), produced siliceous spicules, which are fused or articulated into compound endoskeletons forming cubic or irregular meshworks, thus giving more stability to the soft body. Among other phyla, compact solid calcareous endoskeletons occur only among the Mollusca. Composite calcareous endoskeletons consisting of numerous plates with flexible boundaries are characteristic for Echinodermata and for the newly discovered fossil Tunicata.
Wendt describes two incomplete specimens from the the Cassian Formation (lower Carnian) of the Dolomites (northern Italy). Extensive searches for additional, either biostratigraphically older, contemporaneous, or younger specimens from European and North American reef specialists and collections were unsuccessful. The two specimens were discovered among far over a million of skeletal remains from the the Cassian Formation. The number of taxa and the diversity of skeletal remains collected from the Cassian Formation during almost two centuries are indeed impressive and assign this rock unit a singular rank among the Fossil-Lagerstätten in the geological record. As of 2019, 1429 species have been described so far from this formation. A special feature of this unrivaled fauna is their often excellent state of preservation, exemplified by diagenetically almost unaltered aragonitic microstructures, which are among the oldest in earth history. In this state of perfection and considering its age, the Cassian Fauna can even be regarded as unique.
Fossil record of Tunicates. Black dot, soft-bodied; black triangles, calcareous exoskeletons; black square, calcareous endoskeletons; asterisks, spicules. Wendt (2020).
The fossils are placed in a new order and family of Ascidians, named the Cassianosomidae and Cassianomorpha respectively; both names refer to the Cassian Formation. Both are placed in the new genus Toscanisoma, which is named in honour of Maria Luigia Toscani, who collected the fossils upon which the study is based. Each of the two fossils is described as a separate species within the new genus.
The first species described is named Toscanisoma multipartitum, where 'multipartitum' means 'consisting of several parts'. This is a sessile colonial species consisting of seven branching tubes (zooids) composed of irregular plates that are joined by straight or curved boundaries on the outer and zigzag ones on the inner side. The only complete zooid of the colony is closed at the top by six plates that are smaller than the remaining ones. The other zooids are incomplete, but were probably slightly larger. Spicules have not been observed.
Toscanisoma multipartitum holotype (GPIT/TU 82). (1) Lateral view (drawing); (2) opposite side; (3) view from top; (4) base. Asterisks mark complete zooids, arrows indicate zigzag sutures on inner surfaces. Scale bar 10 mm. Wendt (2020).
The only specimen of Toscanisoma multipartitum available is not complete, lacking the basal portion, which probably served as a holdfast. The seven zooids, which bud from the flattened base, have different shapes ranging from almost circular to elongate or flattened in cross section. Only the smallest zooid is complete, 25 mm long and consists of about 10 plates with very faint and barely recognisable outer boundaries. The inner plate boundaries of the incomplete zooids show zigzag sutures similar to contemporaneous representatives of the order Khmeriamorpha. The zooids fit closely together at the base and are separated by small open interspaces higher up.
The second new species is named Toscanisoma triplicatum, meaning 'triplicate'. as the only known specimen consists of three zooids. This species is distinguished from Toscanisoma multipartitum by the presence of spicules, the different kind of budding, in which the separation into three individual zooids starts at a distance of about 2 cm above the base. Thus, in their early growth stage, the individual zooids share the outer walls of the adjacent ones, but higher up than in Toscanisoma multipartitum.
Toscanisoma triplicatum holotype (GPIT/TU 83). (3) Lateral view; (4) view from base; (5) view from top; 1, 2, 3 mark cavities of three incomplete zooids. Scale bar 15 mm. Wendt (2020).
Because of the fragileness of the specimen, the inner side of the basal cup and the individual zooids could not be sufficiently prepared, but it is assumed that the inner plate boundaries show zigzag sutures similar to Toscanisoma multipartitum. Due to the incomplete state of preservation and the lack of comparable material, speculations about the final growth form and the number of zooids are premature.
The morphological and mineralogical
features described by Wendt raise the pivotal question of the systematic
attribution and the functional morphology of these enigmatic remains.
Solid skeletons (whether exo- or endo-) composed of irregular plates
that consist of acicular aragonite crystals are unknown in the Animal
Kingdom, with one exception: Permian/Triassic Ascidian Tunicates with a
compound calcareous skeleton, which appeared in the early(?) Permian and
became extinct during the Late Triassic. These remains reveal a certain
relationship to some living Ascidians (e.g. Chelyosoma and Forbesella),
although these lack any hard parts. But they are partially composed of
irregular soft plates that can be moved by muscles. The hinge-like
sutures on the inner plate surfaces of the newly described fossil
endoskeletons also suggest a certain flexibility of the compound
skeleton, which is indispensable for their here proposed assignment to
Ascidian Tunicates. The latter are filter-feeders in which a steady flow
of seawater moves through an atrial and a branchial siphon. If we
assign a similar function to the fossil counterparts described by Wendt,
it must be required that one or two of the top plates could be opened
by muscles during the life-time of the organism because this is the case
in the genera Chelyosoma and Forbesella. Unfortunately, the upper part of the examined specimens generally is not preserved. But one zooid of Toscanisoma multipartitum
is complete and exhibits a mosaic of tiny plates, which probably could
be opened by muscles to allowaccess for the protruding siphons.
At
a first glance, the presence of a solid calcareous endoskeleton in
Cassianomorpha might be surprising. However, one should bear in mind
that, apart from Arthropoda (which have a totally different kind of
growth), all invertebrate classes with a calcareous exoskeleton show
well-developed growth lines reflecting an intermittent growth at the
edge of the skeleton-secreting tissue. This is not the case in the
Cassianomorpha in which the skeleton is formed within the mantle
(tunica) starting more or less simultaneously at several nucleation
points. Faced with these observations, it is less surprising that the
extinct Cassianomorpha developed an endoskeleton, as did the other main
Deuterostome phyla or classes (Echinodermata, Vertebrata, Jurassic to
recent Ascidiacea), than the fact that the other contemporaneous fossil
Ascidian order Khmeriamorpha strangely developed an exoskeleton.
By mid-Cambrian times, representatives of almost all Metazoan phyla and subphyla had reached a high degree of biomineralisation in such away to enable a reliable reconstruction of their evolution. Why is this not the case with Tunicates, which must have existed contemporaneously? It is possible to speculate about a change in seawater chemistry or a possible global perturbation of the carbon cycle near the Carboniferous/Permian boundary, but such speculations about the appearance of mineralised (aragonitic) tunicate skeletons at this interval appear rather theoretical, because the time-span of an 'aragonitic ocean' (Mississippian to Middle Jurassic) is not consistent with the existence of aragonitic Ascician skeletons.
A possible answer to this fundamental question is that compound pre-Permian tunicate skeletons might have existed in earlier Paleozoic times, but they have not yet been discovered or recognized as such. It can also not be totally dismissed that they are hidden among the great number of previously described fossil Problematica or those of incertae sedis. The discovery of rare spicules embedded in the solid skeleton clearly points to a new and successful attempt of Tunicate biomineralisation in the Late Triassic, which persisted until recent. However, post-Triassic survivors of Tunicates with compound calcareous, either endo- or exoskeletons, are unknown from the fossil record. Wendt speculates that post-Triassic Ascidians developed other protective strategies that served as a defensive function (e.g., secretion of indigestible chemicals), which, of course, cannot be detected in fossil remains. Not taking into account these theoretical considerations, the total loss of a compound calcareous skeleton in Ascidians during the Late Triassic in favor of a much less stable one consisting of isolated spicules only remains an unresolved question.
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