Revisiting the phosphorite deposit of Fontanarejo (central Spain): new window into the early Cambrian evolution of sponges and the microbial origin of phosphorites

Abstract

Fossils within early Cambrian phosphorites worldwide are often well preserved due to early diagenetic permineralization. Here, we examine the fossil record contained within phosphorites of the Lower Cambrian Pusa Formation (late Fortunian to Cambrian Stage 2) in Fontanarejo, central Spain. The sedimentology and age of these phosphorites have been controversial and are here reviewed and discussed, providing also an updated geological map. The Pusa Formation is composed of fine clastic sediments that are partly turbiditic, with channels of quartz-rich conglomerates and abundant phosphorites in the upper part of the succession. The microfacies and mineralogy of these channel deposits are studied here for the first time in detail, showing that they are mainly composed of subspherical apatite clasts, with minor mudstone intraclasts, quartzite and mica grains. Numerous sponge spicules, as well as entirely preserved hexactinellid sponges and demosponges, were collected within these phosphorites and likely represent stem groups. In addition to sponges, other fossils, such as small shelly fossils (SSF) of the mollusc Anabarella sp., were found. The phosphorites exhibit multiple evidence of intense microbial activity, including diverse fabrics (phosphatic oncoidal-like microbialites, thrombolites, stromatolites and cements) and abundant fossils of filamentous microbes that strongly resemble extant sulphur-oxidizing bacteria. Our findings strongly suggest that microbial processes mediated the rapid formation of most of the Fontanarejo apatite, probably accounting for the exceptional preservation of fragile fossils such as sponge skeletons. The apparent presence of taxonomically diverse hexactinellid and demosponge communities at the lowermost Cambrian further corroborates a Precambrian origin of the phylum Porifera.

Deep origin of the long root tuft: the oldest stalk-bearing sponge from the Cambrian Stage 3 black shale of South China

“…Since most of the Precambrian sponge-like fossils are more or less questionable (Gehling & Rigby 1996;Brasier et al 1997;Li et al 1998;Reitner & Wörheide 2002;Clapham et al 2004;Clites et al 2012;Antcliffe et al 2014;Turner 2021), the earliest sponges (spicule fossils) have been dated back to the beginning of Cambrian, ca. 535 Ma (Chang et al 2017(Chang et al , 2019Reitner et al 2022). Probable stem-group hexactinellids and demosponges were revealed by skeletal frames from the Cambrian Stage 2 phosphorites of the basal Niutitang Formation in Hunan, China (Luo & Reitner 2019) and the upper Pusa Formation in Spain (Reitner et al 2022).…”

“…535 Ma (Chang et al 2017(Chang et al , 2019Reitner et al 2022). Probable stem-group hexactinellids and demosponges were revealed by skeletal frames from the Cambrian Stage 2 phosphorites of the basal Niutitang Formation in Hunan, China (Luo & Reitner 2019) and the upper Pusa Formation in Spain (Reitner et al 2022). The earliest diverse articulated body fossils of sponges, which could shed more light on the evolution of sponge morphology and adaptive strategies, were discovered from the Cambrian Stage 3 Burgess Shaletype Lagerstätten (e.g.…”

Adaptive specialization of a unique sponge body from the Cambrian Qingjiang biota

“…Thereafter, the sponge group, along with other metazoans, diversified rapidly during the Cambrian Explosion [11]. The Cambrian sponges are represented by numerous exceptionally preserved body fossils and isolated spicules from all over the world [12][13][14][15][16][17][18][19][20][21]. They occurred in a wide range of environmental settings [22,23] while were particularly dominant in the early Cambrian (Terreneuvian to Epoch 2, ca 540-510 Ma) black shales in South China compared with other animal groups [12,14,24,25].…”