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.2017 Feb 15;3(2):e1602159.
doi: 10.1126/sciadv.1602159. eCollection 2017 Feb.

Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna

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Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna

Arnaud Brayard et al. Sci Adv..

Abstract

In the wake of the end-Permian mass extinction, the Early Triassic (~251.9 to 247 million years ago) is portrayed as an environmentally unstable interval characterized by several biotic crises and heavily depauperate marine benthic ecosystems. We describe a new fossil assemblage-the Paris Biota-from the earliest Spathian (middle Olenekian, ~250.6 million years ago) of the Bear Lake area, southeastern Idaho, USA. This highly diversified assemblage documents a remarkably complex marine ecosystem including at least seven phyla and 20 distinct metazoan orders, along with algae. Most unexpectedly, it combines early Paleozoic and middle Mesozoic taxa previously unknown from the Triassic strata, among which are primitive Cambrian-Ordovician leptomitid sponges (a 200-million year Lazarus taxon) and gladius-bearing coleoid cephalopods, a poorly documented group before the Jurassic (~50 million years after the Early Triassic). Additionally, the crinoid and ophiuroid specimens show derived anatomical characters that were thought to have evolved much later. Unlike previous works that suggested a sluggish postcrisis recovery and a low diversity for the Early Triassic benthic organisms, the unexpected composition of this exceptional assemblage points toward an early and rapid post-Permian diversification for these clades. Overall, it illustrates a phylogenetically diverse, functionally complex, and trophically multileveled marine ecosystem, from primary producers up to top predators and potential scavengers. Hence, the Paris Biota highlights the key evolutionary position of Early Triassic fossil ecosystems in the transition from the Paleozoic to the Modern marine evolutionary fauna at the dawn of the Mesozoic era.

Keywords: Early Triassic; End-Permian mass extinction; Idaho; Paris Biota; biotic recovery; marine ecosystems.

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Figures

Fig. 1
Fig. 1. Chronostratigraphic subdivisions of the Early Triassic [radiometric ages from studies by Galfettiet al. (5), Ovtcharovaet al. (47), Ovtcharovaet al. (48), and Burgesset al. (49)] with simplified global geochemical trends [δ13Ccarb data from Galfettiet al. (5); anoxic episodes modified following studies by Grasbyet al. (6), Galfettiet al. (50), Wareet al. (51), and Hermannet al. (52)] and the relative temperature fluctuations in the Tethyan realm [adapted from studies by Sunet al. (2) and Romanoet al. (3)].
Subdivisions of the Smithian and Spathian follow the ammonoid zonation from studies by Brühwileret al. (53) and Guexet al. (54). The late Smithian extinction event is highlighted in red. Time correlations for the late Smithian and the Smithian-Spathian boundary are based on high-resolution ammonoid zonations (20,55,56). A,Anasibirites beds; X, Xenoceltitidae beds (both late Smithian); T,Tirolites beds (early Spathian); ea., early; mi., middle; l., late; VPBD, Vienna Pee Dee belemnite.
Fig. 2
Fig. 2. Maps showing the location of the newly discovered Paris Biota.
(A) Present-day map showing the location of southeastern Idaho in western United States. (B) Map of southeastern Idaho showing the location of the Paris Biota. (C) Early Triassic map with the location of the Paris Biota. (A) to (C) were modified following the study by Romanoet al. (57).
Fig. 3
Fig. 3. Simplified stratigraphic log showing main Early Triassic fossil-bearing levels currently known in the Bear Lake area.
Fossil occurrence is modified following the study by Romanoet al. (57), with additional data from studies by Massare and Callaway (58) (ichthyosaur remains), Tosk and Andersson (59) (foraminifers), and Hofmannet al. (60) and from our own field observations (black stars). Conodonts are also sparsely present throughout the section but are not represented here. Stratigraphy follows the main units defined by Kummel (61,62). Zonation is based on ammonoids (20,54,55,62,63). Position of the Paris Biota is highlighted in red. A U-Pb age of 250.55 ± 0.4 My is associated with theTirolites/Columbites beds from South China (5). LL, Lower Limestone; LS, Lower Shale; ML, Middle Limestone; MS, Middle Shale; UCS, Upper Calcareous Siltstone; Meek,Meekoceras beds; Anasib,Anasibirites beds; Tirol,Tirolites beds; Colum,Columbites beds.
Fig. 4
Fig. 4. Characteristic earliest Spathian organisms of the Paris Biota.
(A) Leptomitid protomonaxonid sponge (s) and ammonoids (a) (slab IMNH IP-026/777). (B) Leptomitid protomonaxonid sponge showing a twisted apex and minute epizoan brachiopods (arrows) (slab UBGD 30505). (C) Glyphidean lobsterLitogaster ?turnbullensis (UBGD 30548). (D) New thylacocephalan genus (UBGD 30561). (E) Penaeid shrimp (IMNH IP-026/778). (F) Caridean shrimp under ultraviolet (UV) light (365 nm) (UBGD 30558). (G) Gladius-bearing coleoid (UBGD 30545). (H) Holocrinid crinoid stem with cirri (UBGD 30563). (I) Ophiuroid (UBGD 30565). Scale bars, 5 mm (A and C to I) and 10 mm (B). [Photo credits: A. Brayard, Université Bourgogne Franche-Comté (A to G); T. Saucède, Université Bourgogne Franche-Comté (H); and B. Thuy, Natural History Museum Luxembourg (I).]
Fig. 5
Fig. 5. Specimens and characteristic features of leptomitid protomonaxonid sponges from the Paris Biota.
(A andB) General and closeup view of the specimen UBGD 30504 showing projected longitudinal spicules (ls) from the apex forming a fringe of marginalia (m) and transverse spicules (ts). (C andD) Closeup views of twisted apex (ta) of two specimens (UBGD 30505 and 30581) under natural and UV light (365 nm). Projecting spicules from the apex forming a fringe of marginalia are also visible. Fine transverse spicules appear mainly as wrinkles, perpendicular to the longitudinal spicules. An epizoan brachiopod (e) is attached to the sponge specimen C. (E andF) Closeup views of specimens UBGD 30506 and 30508, showing longitudinal and transverse spicules. (G andH) Large-sized specimens UBGD 30510 and 30511. Scale bars, 5 mm (A to D and G and H), 2 mm (E), and 1 mm (I). [Photo credits: A. Brayard, Université Bourgogne Franche-Comté.]
Fig. 6
Fig. 6. Characteristic earliest Spathian organisms of the Paris Biota.
(A andB) Belemnoid arm hooks (UBGD 30540 and 30544). (C) Penaeid shrimp (UBGD 30559). (D toF) Mass accumulations of caridean shrimps under UV light (365 nm) and natural light (slabs UBGD 30553 and 30554). (G) Rod-shaped, unbranched specimens of putative noncalcified dasycladales (slab UBGD 30576). (H) Isolated arms of a holocrinid crinoid (UBGD 30563). (I) Chondrichthyan tooth referable toAcrodus (slab IMNH 1143/46168). (J) Osteichthyan tooth plate (slab UBGD 30569). (K) Coprolite specimen (UBGD 30573). (L) Closeup view of belemnoid hooks accumulated in a coprolite (slab UBGD 30575). (M) Discinoid inarticulate brachiopodOrbiculoidea sp. (UBGD 30522). Scale bars, 5 mm (A to K and M) and 0.5 mm (L). [Photo credits: A. Brayard, Université Bourgogne Franche-Comté (A to H and J to M); L. J. Krumenacker, Montana State University (I).]
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