Is There Evidence of Bilateral Animals With Complex Behavior From The Latest Ediacaran?

(yes)

Trace fossil evidence for Ediacaran bilaterian animals with complex behaviors

Authors:

1. Zhe Chen (a)
2. Chuanming Zhou (a)
3. Mike Meyer (b)
4. Ke Xiang (c)
5. James D. Schiffbauer (d)
6. Xunlai Yuan (a)
7. Shuhai Xiao (b)

Affiliations:

a. State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China

b. Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

c. Hunan Institute of Geological Survey, Changsha, Hunan 410116, China

d. Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA

Abstract:

Distinguishing Ediacaran trace fossils from tubular body fossils can be a challenge, and several Ediacaran fossils previously interpreted as animal traces have been shown to be tubular body fossils. Nonetheless, true Ediacaran trace fossils are present, but they are relatively few and morphologically simple, dominated by horizontal trails and shallow burrows. Such simple morphologies have been taken as evidence for a modest behavioral complexity and limited geobiological impact of animal bioturbators before the Cambrian explosion. Here we report three types of trace fossils—horizontal tunnels, surface tracks/trails, and vertical traces—from the latest Ediacaran Dengying Formation (551–541 Ma) in the Yangtze Gorges area of South China. Cross-cutting tunnels and the presence of scratch marks indicate that these traces are unlikely tubular body fossils; instead, these three types of traces likely represent animal activities related to under-mat feeding, epibenthic locomotion, and temporary dwelling, respectively. We show that these three types of traces were constructed by the same bilaterian animals that had moderately complex interactions with microbial mats to exploit nutrients and O2 resources. These animals heralded a new age in ecosystem engineering, animal–sediment interaction, and biogeochemical cycling.

(However, Retallack says no (never thought him to be contrarian!))

Comment on “Trace fossil evidence for Ediacaran bilaterian animals with complex behaviors” by Chen et al.

Author:

1. Gregory J. Retallack (a)

Affiliation:

a. Department of Geological Sciences, University of Oregon Eugene, Oregon 97403

Abstract:

Trace fossils from the Shibantan Member of the Denying Formation (551-541 Ma) of south China, interpreted by Chen and colleagues as evidence of undermat feeding, epibenthic locomotion and temporary dwelling of a small worm-like animal (Metazoa, Bilateria), show four features incompatible with that interpretation: (1) bulbous ends, (2) interruptions, (3) variable width, (4) narrow ends, and (5) beaded levees. These features are better explained as due to formation of a sorocarp base, looping upward, shape changing, cell aggregation and collar placement, respectively, of the slug (grex or pseudoplasmodium) phase of a cellular slime mold comparable with living Dictyostelium discoideum (Amoebozoa, Mycetozoa). Another three features are found in worm as well as slime mold trails: (6) transverse markings, (7) central ridge, and (8) small size. Similar observations can be made, and mycetozoan affinities considered for comparable trace fossils from the 550 Ma Ediacaran Member of South Australia, 558 Ma Ust Pinega Formation of Russia, 585-600 Ma Tacuari Formation of Uruguay, 1100 Ma Chordat Sandstone of India and 2000-1800 Ma Stirling Range Sandstone of Western Australia. Slime molds are terrestrial eucaryotic protists, and may have a fossil record extending back as far as the likely 1800 Ma age of aquatic metazoans.

(yes, there is, reply Chen et al)

Reply to comment on “Trace fossil evidence for Ediacaran bilaterian animals with complex behaviors”


Authors:

1. Zhe Chen (a)
2. Chuanming Zhou (a)
3. Mike Meyer (b)
4. Ke Xiang (c)
5. James D. Schiffbauer (d)
6. Xunlai Yuan (a)
7. Shuhai Xiao (b)

Affiliations:

a. State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China

b. Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

c. Hunan Institute of Geological Survey, Changsha, Hunan 410116, China

d. Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA

Abstract:

Retallack (this issue) argues that Shibantan trace fossils described by Chen et al. (2013) were not made by bilaterian animals, but instead by a motile grex phase of cellular slime molds, such as Dictyostelium discoideum, a terrestrial eukaryote. Based on this reinterpretation, he further indicates that the Shibantan traces are terrestrial and the hosting limestone could be paleosols. The slime mold interpretation is interesting, but can be falsified on the ground of petrographic observations. The Shibantan trace makers were capable of excavating and displacing sediments (Figs. 3H, 5A-C of Chen et al., 2013), constructing persistent tunnels by splitting clay-rich microbial mats (Fig. 2C and D of Chen et al., 2013), and truncating and deforming microbial laminae (Fig. 6F of Chen et al., 2013). Although some slime molds can leave sub-millimeter-sized surface trails on agar gel, to the best of our knowledge no slime molds are known to be able to displace and excavate intraclastic sediments, to construct persistent millimeter-sized tunnels within sediments, and to truncate sedimentary laminae. Below we briefly respond to Retallack's specific comments.

Um.  Retallack is at least moderately respectable.  However, his increasingly...contrarian  POV and lack of coauthors has me scratching my head a bit.