An early Cambrian mackenziid reveals links to modular Ediacaran macro‐organisms

Abstract

Many Ediacaran macrofossils are apparently composed of unique pneus (similar, repetitive tubular modules) forming a variety of body shapes by combinations of modular growth regimes. The large‐scale disappearance of Ediacaran macro‐organisms at the end of the Precambrian leaves this now extinct modular body plan only rarely reported from the Cambrian onwards. Mackenzia costalis is an enigmatic and poorly understood soft‐bodied organism from the middle Cambrian, and is a possible surviving member of the Ediacaran macrobiota. Here, we describe a new mackenziid, Paramackenzia canalifera, from the early Cambrian Chengjiang biota, Yunnan Province, south‐west China. It has a body plan comprising longitudinal tubular modules that are radially arranged to form a cylindrical body with a basal attachment. Each tubular module is enclosed by an inner and outer concentric membrane, separated from adjacent modules by a thin lamellar sheet. Infilling sediment and preservation of dark, basal stains at the inner and outer membranes reveal a pore–canal system within the lamellae, connecting the internal cavity to the exterior. This arrangement of tubular modules is similar to that of several macroscopic Ediacaran species, including erniettomorphs and dickinsoniomorphs, which were constructed of serial hydrostatic modules referred to as ‘pneus’. The mackenziid pore system is similar to that of sponges and archaeocyathids in function and feeding strategy, but the overall anatomy precludes affiliation to this phylum. We suggest that mackenziids are most likely to be a blastic or diploblastic anatomical grade of stem eumetazoans, similar to other modular Ediacaran macro‐organisms.

Decline and fall of the Ediacarans: late‐Neoproterozoic extinctions and the rise of the modern biosphere

“…In this assemblage erniettomorphs, rangeomorphs, and arboreomorphs co‐occur with an expanded range of tubicolous metazoan biomineralisers and bilaterian ichnofossils (Boag, Darroch & Laflamme, 2016; Darroch et al ., 2018). Finally, although most of the Nama Ediacarans disappear near the Cambrian boundary some frondose and modular body plans have been suggested to persist until at least 518 Ma (Zhao et al ., 2022; Hoyal Cuthill, 2022), attesting to late‐surviving Ediacaran‐grade animals in the midst of the Cambrian Explosion (Fig. 2).…”

“…1n is equivalent to the basal Cambrian carbon isotope excursion (BACE) (Bowyer et al, 2022). The pre-Shuram BAINCE is the Baiguoyuan negative carbon isotope excursion (Zhu et al, 2013). Ma, million years ago.…”

“…Similarly, ecospace takeover by Nama eumetazoans may be taken to imply a competitive advantage over the comparatively 'ineffectual' and 'static' niche occupancy of Ediacarans (Schiffbauer et al, 2016). However, recent fossil discoveries demonstrate the coexistence of Ediacarans and crown-group eumetazoans as early as the Avalon (Dunn et al, 2022) and perhaps as late as the mid-Cambrian (Zhao et al, 2022;Hoyal Cuthill, 2022). Meanwhile, other findings dramatically expand their known range of interactions in the White Sea assemblage, at the ecological apex of the Ediacaran Biota (Tarhan et al, 2018(Tarhan et al, , 2015Droser, Tarhan & Gehling, 2017;Bowyer et al, 2022).…”

“…Frondose or modular architectures directly comparable to those of bona fide Ediacarans occur in Paramackenzia from the ~518 Ma Chengjiang Lagerstätte (Zhao et al ., 2022) and a suite of fragmentary remains from lower Cambrian strata worldwide (reviewed in Hoyal Cuthill, 2022). Tentatively, phylogenetic analyses (Hoyal‐Cuthill & Han, 2018) have also recovered the frondose Stromatoveris from Chengjiang (Shu et al ., 2006) as a close relative of rangeomorphs.…”

“…In addition, described Cambrian Ediacarans typically show a highly modular construction (Zhao et al ., 2022) or carry multiple, radially arranged fronds or petaloids (Hoyal Cuthill & Han, 2018; Hoyal Cuthill, 2022). This architecture is shared by many Nama Ediacarans, such as Swartpuntia , Rangea , and Pteridinium (Ivantsov & Zakrevskaya, 2021 a ).…”