Putative Proterozoic sponge spicules reinterpreted as microburrows

Mark McMenamin

Putative Proterozoic sponge spicules reinterpreted as microburrows

2021, Academia Letters

ACADEMIA Letters Putative Proterozoic sponge spicules reinterpreted as microburrows Alice Kris, Mount Holyoke College Mark McMenamin, Mount Holyoke College Abstract Putative sponge fossils occurring in deep Proterozoic rocks are reinterpreted here as metazoan burrows made by animals that we infer to be ancestral to the graphoglyptid tracemakers. The co-occurrence of metazoan microburrows plus the oldest known thrombolitic textures indicate that the earliest evidence for metazoans is approximately 890 Ma (Little Dal Group, northwestern Canada). This date is in accord with the estimated molecular phylogeny age for animal origins. Introduction “Vermiform microstructure[s]” in ancient carbonates were defined by Malcolm Walter in an unpublished draft glossary originally part of Walter (1972) as structures “in which narrow, sinuous, pale-colored areas (usually of sparry carbonate) are surrounded by darker, usually fine-grained areas (usually carbonate).” Walter (1972, p. 19) interpreted these structures as ‘algal filaments’ (p. 87) or ‘algal boring[s]’ (p. 160). Vermiform structures described by Turner (2021) occur in the approximately 890 Ma Little Dal reefs of the Stone Knife Formation in northwestern Canada. Turner (2021) interpreted these structures as the spongin fiber networks of keratosan sponges. Turner (2021) also mistakenly identified these putative sponges as metazoans; in fact, if they are sponges, they would be parazoans. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Mark McMenamin, mmcmenam@mtholyoke.edu Citation: Kris, A., McMenamin, M. (2021). Putative Proterozoic sponge spicules reinterpreted as microburrows. Academia Letters, Article 3800. https://doi.org/10.20935/AL3800. 1 If Turner’s (2021) interpretation is correct, this would be among the earliest evidence for parazoans in the fossil record. We agree that these structures are animalian in origin, but that they represent ichnofossils, rather than body fossils. This means that Turner’s ‘vermiform structures’ are, like their informal name suggests, burrows formed by tiny and very ancient worm-like animals. Microburrows Microburrow nests were described from Lower Cambrian rocks of Sonora, México and Nevada, USA by McMenamin (2016). These nests were formed by collapse of fecal pellets packed into cryptic cavities in archaeocyath reefs to form a microenvironment that served as a nursery for juvenile metazoa. Microburrowing activity by the metazoan fry eventually disaggregated the pellets. Ventilation was achieved in the cryptic space by perimeter burrows on the margins of the micrite-filled cavity. Comparable perimeter burrows are seen in microburrow nests in carbonate rocks of Devonian and Triassic carbonates described by Luo and Reitner (2014, their fig. 2a [lower left of image] and 2f). Microburrow nests are known to occur inside archaeocyath central cavities (McMenamin, 2016) and inside articulated brachiopod shells (Lee et al., 2014; Luo and Reitner, 2014). We propose here that Turner’s (2021) vermiform structures are not sponges, but rather microburrow networks. As a case in point, observe Turner’s (2021) Fig. 2e, where the vermiform structures are gravitationally collapsed in the lower part of the cavity fill, as one would expect for cylindrical burrows. The polygonal shapes of the burrows suggest that they are in fact ancestral to the graphoglyptid trace fossils of later times (Seilacher, 1986). Lee et al. (2014) describe comparable structures as siliceous sponges, but the presence in these of ‘peloids’ (which we interpret as pellets), the absence of any convincing spongocoel in these structures, and the difficulty of passage of seawater in sufficient quantities for parazoan filter-feeding suggests that these structures also represent microburrow assemblages. Conclusion If our reinterpretation is correct, Turner’s (2021) discovery represents the oldest evidence for complex animal life (animals that had reached the organ grade of evolutionary development). This makes the Little Dal structures of even greater significance than is argued in Turner’s (2021) original report. This finding helps to eliminate the notorious mismatch between the molecular phylogeny age for animal origins and their earliest appearance as recorded in the fossil record. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Mark McMenamin, mmcmenam@mtholyoke.edu Citation: Kris, A., McMenamin, M. (2021). Putative Proterozoic sponge spicules reinterpreted as microburrows. Academia Letters, Article 3800. https://doi.org/10.20935/AL3800. 2 Further evidence in support of our hypothesis occurs in the Little Dal Group. Thrombolites are clotted microbialites associated with extensive metazoan burrowing (Shapiro, 2004; McMenamin, 2016). Burrow margins in thrombolites can be diffuse; burrow margins in the microburrows discussed here are usually sharp but can also be diffuse, probably due to carbonate solution-related diagenetic effects on the burrow walls. Interestingly, the earliest known thrombolites are known form the Little Dal Group (Shapiro, 2004). Thus the co-occurrence of microburrow nests and thrombolitic textures is not mere coincidence, but rather represents the first appearance of metazoan burrowing activity in the fossil record. We propose here that these microburrows were ancestral to the tracemakers of later supposed ‘mushroom farmer’ graphoglyptid burrows, a feeding strategy (Seilacher, 1986) that would eventually appear on land in the galleries of leaf cutter ants (Mueller et al., 2005). This is very much in accord with a microbially-dominated Proterozoic world, where animal activity often took place beneath the sediment-water interface, as in the case of under-mat miners (Seilacher, 1986). Furthermore, our new interpretation helps to explain the extensive time lag between the origin of animals and the Cambrian Explosion. Before the Cambrian, metazoan lifestyles were too intrinsically linked with the ubiquitous microbial mats, leading to long-term stable Proterozoic ecosystems. These ecosystems were not induced to change until mat grazers began to destroy the microbial mat communities near the basal Cambrian boundary. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Mark McMenamin, mmcmenam@mtholyoke.edu Citation: Kris, A., McMenamin, M. (2021). Putative Proterozoic sponge spicules reinterpreted as microburrows. Academia Letters, Article 3800. https://doi.org/10.20935/AL3800. 3 References Lee J-H et al. (2014) Furongian (Late Cambrian) sponge-microbial maze-like reefs in the North China Platform. Palaios 29:2727-37. Luo C, Reitner J (2014) First report of fossil ’keratose’ demosponges in Phanerozoic carbonates: preservation and 3-D reconstruction. Naturwissenschaften 101:467-477. McMenamin MAS (2016) Dynamic Paleontology. Springer, Cham, Switzerland. Mueller UG et al. (2005) The evolution of agriculture in insects. Annual Review of Ecology, Evolution, and Systematics. 36:563-595. Seilacher A (1986) Evolution of behavior as expressed in marine trace fossils. In M. H. Nitecki and J. Kitchell, eds., Evolution of Behavior, Oxford University Press, Oxford, pp. 62-87. Shapiro RS (2004) Neoproterozoic-Cambrian microbialite record. Paleontol Soc Pap 10:515. Turner EC (2021) Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature 596:87-91. Walter MR (1972) Stromatolites and the Biostratigraphy of the Australian Precambrian and Cambrian. Palaeontological Association, London. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Mark McMenamin, mmcmenam@mtholyoke.edu Citation: Kris, A., McMenamin, M. (2021). Putative Proterozoic sponge spicules reinterpreted as microburrows. Academia Letters, Article 3800. https://doi.org/10.20935/AL3800. 4

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Putative Proterozoic sponge spicules reinterpreted as microburrows

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