Primary description
Specimens presented here are from the latest Ediacaran (> ca. 541 Ma) Gaojiashan biota of the Dengying Formation, southern Shaanxi Province, South China (Fig. 1), well known for documenting the radiation of skeletal tubular fossils, such as Cloudina and Sinotubulites [19], but with little comparative work on diversified, well-preserved spherical fossils assemblages, in particular with respect to their possible affinities with animal embryos or algae.
The five specimens are spherical, 506.76 ± 34.22 μm in diameter (Fig. 2, Additional file 1: Dataset S1, Additional file 2: Fig. S1). These spheres are composed of an outer layer of tightly juxtaposed, club-shaped tubes with rounded tips 112.00 ± 11.60 μm in length and connected via a central mass of intertwined filaments 261.76 ± 29.18 μm in width (Figs. 2 and 3). Tomographic sections show crossing lines and vesicle-shaped structures inside the tubes (Fig. 3b). The small, intertwined filaments of the inner medullar mesh show notable variations in their shape and size (average width 3.06 ± 1.13 μm; Fig. 3d, Additional file 1: Dataset S1). In the peripheral layer, the differentiated reproductive finger-like swells (Figs. 2 and 3a–e) are tightly packed and clavate to cylindrical, with tips rounded and smooth, lacking hairs (Figs. 2 and 3a–c). The external tube tip morphology is consistent within and across both specimens, and the contact between these tips is roughly pentagonal to hexagonal (Figs. 2 and 3a). At their base, tubes connect to the central mesh via a single medial filament branching basally into two other filaments (Fig. 3c, e). Fossils are preserved via phosphatization (Additional file 3: Fig. S2), but the original composition of the organic material could not be resolved.
Interpretation
Tomographic renderings showing multiple layers (Fig. 2c, d) confirm the integrity of the tubes as undivided spaces connected at their base to a filamentous mesh. Outer-layer cells of fossilized metazoan embryos can superficially resemble our fossils [13], but the regular juxtaposition of tube tips, the constant unit number across our specimens, and details of the internal organization in Protocodium all rule out an embryonic identity. Although individual nuclei are not preserved, the connection of the outer layer structures to an undivided filamentous core suggests coenocytic unicellularity. No sexual structures such as gametangia are identifiable.
The irregular lines inside the tubes are artifacts of the computed tomography caused by slight differences of alignment of the tubes onto the transversal plane, which sometimes reveal part of the tubes’ wall membrane or even the cavity of overlapping tubes. Single tomographic slices show a continuous variation of this artifact, with apparent tube deformations corresponding to the superimposition of several tubes (Figs. 2b and 3b). By comparison, fossilized septa expectedly display regular width, alignment and position distinct from irregular and partial preservations of cell walls (e.g., [22]).
We therefore identify our fossil specimens as single, spherical, coenocytic siphonous cells (Fig. 2, Additional file 2: Fig. S1), as they are found specifically in bryopsidales green algae, without a larger branching or mat-shaped thallus.
Affinity
The presence of a medullar core of small intertwined siphonous tubes surrounded by a uniform layer of elongate bulb-shaped structures is diagnostic of the genus Codium among siphonous green algae [23]. Our fossils are virtually indistinguishable from modern Codium representatives in both external and internal structure (Fig. 3e, f). A comparable organization is found in the bryopsidales relative Halimeda (e.g., [24]), but, to our knowledge, no member in this genus typically forms spherical thalli; in addition, in Halimeda utricles are short and bulbous, and siphons are stout, each branch bearing several utricles. The species is also not a dasyclad or cyclocrinitid, otherwise well-represented in the fossil record, including some problematic forms in the early Cambrian [25]. Despite some structural similarities, thalli of these algae possess a holdfast, which is an extension of the small siphonous core, while a large part of the volume remains hollow, because the outer layer is composed of small bulbous heads borne by well-spaced-out, thin primary branches [26].
However, the size of siphons and utricles in our specimen are almost an order of magnitude smaller than the average in Codium [27] and is also about half the size of medullar siphons and utricles of smaller individuals known within this genus, including in the well-known and invasive C. fragile [28]. Resolving further the affinity of our fossil species within the genus Codium based on morphology is made difficult by a challenging trait-based systematics confronted by considerable intraspecific variations [28] and wide-reaching convergences [23], thus still relying on identification keys [29], as well as by the significance among extant species of characters here either inapplicable, unseen, or absent (e.g., gametangia, thallus branches and utricle hairs). Morphology alone is otherwise largely insufficient to constrain phylogenetic relationships in this group [23].
For this reason, we undertook a phenetic approach integrated to molecular results for extant species instead (Fig. 4), with the intent to estimate the morphological distance between our fossils and comparable Codium morphogroups—specifically C. bursa, C. minus, and their relatives. Among hierarchical clustering methods, weighted pair group method with arithmetic mean (WPGMA) is the ordination closest to the principal coordinate analysis (PCOA) (Additional file 4: Fig. S3), whose axis 1 is influenced by thallus habit (mat-forming, spherical, erect, repent) and traits further describing branching patterns (Fig. 4). Although thallus type is also significant in explaining the variance of axis 2, this dimension is most conspicuously ordinated by the presence and type of utricle hairs, distinctly separating upper and lower part of the two-dimensional graph (Fig. 4). The phylomorphospace shows that these axes are also broadly consistent with high-level phylogeny, insofar as thallus habit is reflected in the separation between the main clades, especially branching versus non-branching forms [23] (Fig. 4). K-means clustering does not converge on a significant solution, but a sub-division into seven optimal clusters closely matches the PCOA ordination on axes 1 and 2 and hence major clades.
C. bursa resolves in its own separate cluster with C. ovale, while our fossils resolve in a cluster including C. minus and other taxa with spherical thalli (Fig. 4). The ordination therefore suggests that, based on the set of characters used to set apart different Codium species, our fossils are well nested within the C. minus cluster of spherical taxa, as opposed to lying apart from all other Codium species. It follows that the morphology of Protocodium is generally consistent with the rest of the genus and particularly the C. minus subgroup.
Based on these results, as well as the difference in size (by a factor of at least 2 compared to extant species) and the presence of short, single medullar connections at the base of utricles, we propose that our fossil species constitutes a stem taxon of the Codium genus. The lack of gametangia may be explained by the fact that our fossils are sexually immature gametophytes, which would seem more likely than to invoke a very different form of germination. The presence of a short single medullar filament at the base of utricles is not documented among spherical forms [30] but is known to be a possible developmental feature of certain mat-forming taxa, such as C. picturatum (H. Verbruggen, pers. comm.). As a fundamental difference, most mat-forming species possess composite utricles, with some exceptions, such as C. coralloides [23]. Interestingly, the thallus of mat-forming is broadly irregular but also commonly produces spherical extensions. There is therefore arguably in our fossil taxon a heterogeneous combination of traits, which associates morphologies of both spherical and mat-forming lineages.
Hence, we devise the systematic paleontology as follows:
Systematic paleontology
Chlorophyta Reichenbach, 1828.
Ulvophyceae Stewart & Mattox, 1978.
Bryopsidales J.H. Schaffner, 1922.
Codiaceae Kützing, 1843.
Protocodium sinense gen. et sp. nov.
(Figs. 2 and 3, Supp. Figures 2 and 3).
LSID: urn:lsid:zoobank.org:act:397AB8FD-143C-481B-A9B7-F6F034A2E4E7.
Material. Holotype NWULJG 10,034; Paratypes NWULJG 10,015, 10,021, 10,026, 10,042.
Occurrence. Beiwan Member, Dengying Formation at the Lijiagou section in southern Shaanxi Province, China.
Derivation of name. From the Greek πρῶτος, “first”, owing to the arguably ancestral nature of the type compared to other modern Codium algae. The species is named after the discovery of the fossil in China.
Diagnosis. Cells (thalli) 506.76 ± 34.22 μm in diameter, with siphonous medulla 261.76 ± 29.18 μm and length of utricles 112.00 ± 11.60 μm.
Description. Individuals are single spherical siphonous, coenocytic cells. Cells 506.76 ± 34.22 μm in diameter, divided into an inner medullar mesh of small, intertwined siphons (diameter 261.76 ± 29.18 μm), and utricles (length 112.00 ± 11.60 μm). Medullar siphons width of 3.06 ± 1.13 μm. Utricles tightly packed and clavate to cylindrical, with tips rounded and smooth, lacking hairs. External utricle tip morphology consistent within and across specimens. Tip wall thickness (12.38 ± 2.87 μm) about four times that of utricle lateral wall (2.90 ± 0.59 μm). Contact between utricle tips is roughly pentagonal to hexagonal. No gametangia can be observed. At their base, utricles connect to the medulla via a single medial siphon branching basally into two other siphons.