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, 10 (4), e0125446
eCollection

A Diverse Tetrapod Fauna at the Base of 'Romer's Gap'

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A Diverse Tetrapod Fauna at the Base of 'Romer's Gap'

Jason S Anderson et al. PLoS One.

Abstract

The lack of fossil tetrapod bearing deposits in the earliest Carboniferous ('Romer's Gap') has provoked some recent discussions regarding the proximal cause, with three explanations being offered: environmental, taphonomic, and collection failure. One of the few, and earliest, windows into this time is the locality of Blue Beach exposed in the Tournaisian deposits at Horton Bluff lying along the Avon River near Hantsport, Nova Scotia, Canada. This locality has long been known but, because the fossils were deposited in high energy settings they are almost always disarticulated, so the fauna has not been described in detail. Recent intensive collection has revealed a diverse assemblage of material, including for the first time associated elements, which permits an evaluation of the faunal constituents at the locality. Although not diagnosable to a fine taxonomic level, sufficient apomorphies are present to identify representatives from numerous clades known from more complete specimens elsewhere. The evidence suggests a diverse fauna was present, including whatcheeriids and embolomeres. A single humerus previously had been attributed to a colosteid, but there is some uncertainty with this identification. Additional elements suggest the presence of taxa otherwise only known from the late Devonian. Depositional biases at the locality favor tetrapod fossils from larger individuals, but indirect evidence from trackways and tantalizing isolated bones evidences the presence of small taxa that remain to be discovered. The fossils from Blue Beach demonstrate that when windows into the fauna of 'Romer's Gap' are found a rich diversity of tetrapods will be shown to be present, contra arguments that suggested this hiatus in the fossil record was due to extrinsic factors such as atmospheric oxygen levels. They also show that the early tetrapod fauna is not easily divisible into Devonian and Carboniferous faunas, suggesting that some tetrapods passed through the end Devonian extinction event unaffected.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Location of Blue Beach.
A, map of the shore line cliff along the Avon River Estuary where Horton Bluff Formation strata are exposed (continuing into the intertidal zone); B, Oldest beds (Hurd Creek Member) at the base of the section showing the road termination on the beach; C, the cliff and intertidal zone at Lighthouse Point (Blue Beach Member), where the majority of tetrapod specimens were recovered.
Fig 2
Fig 2. Syncline at Lighthouse Point.
The resistant beds here include especially productive sandstones known informally as the ‘Theta Layer’ (Theta) and ‘Lighthouse Sandstone’ (LHSS).
Fig 3
Fig 3. Humerus Type 1.
A-F, YPM PU 23545, right humerus in A, oblique posterior, B, dorsal, C, ventral, D, oblique ventral, E, proximal, and F, distal views. G, NSM007GF004.634, small left humerus in ventral view. Abbreviations: dp, deltopectoral crest; ecc, ectepicondyle; ef, entepicondylar foramen; lr, longitudinal ridge.
Fig 4
Fig 4. Humerus Type 1.
NSM005GF045.037, right humerus in A, proximal, B, anterior, C, ventral, D, anterior, and E, dorsal views. Abbreviations: dp, deltopectoral crest; ef, entepicondylar foramen; lds, latissimus dorsi spike; md, m. deltoideus scar; mfu, m. flexor ulnaris scar; mer, m. extensor radialis scar; meu, m. extensor ulnaris scar; msl, m. supinator longus scar; rf, radial facet; sd, scapulhumeral depression; uf, ulnar facet.
Fig 5
Fig 5. Humerus Type 2.
RM 20.6707, right humerus in A, oblique posterior, B, dorsal, C, ventral, and D, oblique posterior views. Abbreviations: dp, deltopectoral crest; ec, ectepicondyle; ef, entepicondylar foramen; ld, latissimus dorsi scar; rf, radial facet; sul, distal sulcus; uf, ulnar facet.
Fig 6
Fig 6. Humerus Types 2 and 3.
YPM PU 20754, left humerus in A, dorsal; and B, ventral views, less areas still obscured by matrix. RM 20.6770 left? humerus in C, dorsal, and D, ventral, views. Abbreviations: dp, deltopectoral crest; ect, ectepicondyle; ef, entepicondylar foramen.
Fig 7
Fig 7. Scapulocoracoid Type 1.
NSM005GF045.005, left scapulocoracoid in A, lateral, and B, medial views. Reconstruction of full extent of this morphotype from multiple specimens in C, lateral, and D, medial views. Abbreviations: cf, coracoid foramina; gl, glenoid; sgb, supraglenoid buttress; sgf, supraglenoid foramen.
Fig 8
Fig 8. Femur Type 1.
YPM PU 23550, right femur in A, posterior, B, dorsal, C, anterior, and D, ventral, E, proximal, and F, distal views. Scale bar equals 10 mm. Abbreviations: ab, adductor blade; ac, adductor crest; ff, fibular facet; icf, intercondylar fossa; tf, tibial facet.
Fig 9
Fig 9. Femur Type 2 and tibia Type 1.
NSM004GF045.034A-C, right and left femora (Type 2) and left tibia (Type 1) found in association. A-D, I, J, left femur in A, posterior, B, ventral, C, anterior, D, dorsal I, proximal, and J, distal views. E-H right femur in E, dorsal, F, anterior, G, ventral, and H, posterior views. Left tibia in K, flexor (ventral), and L, extensor (dorsal) views. Abbreviations: ab, adductor blade; ac, adductor crest; cr, thin anterior crest; ff, fibular facet; fo, fibular fossa; ft, fourth trochanter; icf, intercondylar fossa; it, internal trochanter; itf, intertrochanteric fossa; tf, tibial facet.
Fig 10
Fig 10. Femur Type 3.
NSM007GF004.629, left femur (reversed for comparison) in A, posterior, B, dorsal, C, anterior, and D, ventral views. Abbreviations: ab, adductor blade; ac, adductor crest.
Fig 11
Fig 11. Femur Type 4.
NSM005GF045.048-A, right femur in A, anterior, B, ventral, C, posterior, and D, dorsal views. Abbreviations: ab, adductor blade; ac, adductor crest; it, internal trochanter; itf, intertrochanteric fossa; popb, popliteal butress.
Fig 12
Fig 12. Tibia Type 2.
NSM014GF036.003 (A) and NSM.014.GF.036.004 (B). Right tibiae each in anterior, flexor, posterior, and extensor views. Abbreviations: ap, anterior process; cc, cnemial crest.
Fig 13
Fig 13. Pelvis Type 1.
NSM005GF045.001, right pelvis in A, medial, and B, lateral views. Abbreviations: ap, anterior process; il, ilium; ish, ischium; ob, obturator foramen; pp, posterior process; pub, pubis; sac, sacral rib facet.
Fig 14
Fig 14. Pelvis Type 2.
NSM014GF036.001, left ilium in medial view. Abbreviations: ap, anterior process; pp, posterior process; rr, robust ridge; sac, sacral rib facet.
Fig 15
Fig 15. Interclavicle.
NSM.014.GF.036.005, undescribed interclavicle found in situ in the Theta Layer sandstone below the lighthouse in 2002. The morphology is consistent with a whatcheeriid and with similar elements from Dora, Scotland.
Fig 16
Fig 16. Eoherpeton humerus.
New outline drawings of humerus NMS 1993.56.21 (formerly NUZ 78.1.58) in the plane of the entepicondyle. A, ventral, and B, dorsal views.
Fig 17
Fig 17. Associated material from Blue Beach.
NSM005GF045.034, the ‘Sherm Block’ (for collector Sherman Williams) of associated front and hind limb material.

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Grant support

This work was supported by the Natural Environment Research Council Grant NE/J022713/1 (http://www.nerc.ac.uk/) to JC, and the Natural Sciences and Engineering Research Council Discovery Grant 327756-2011 (http://www.nserc-crsng.gc.ca/index_eng.asp) to JSA. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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