We here present an echinoderm assemblage based on microfossils retrieved from sediments drilled by the Ocean Drilling Program (ODP) Leg 171B on the Blake Nose escarpment in the western tropical Atlantic, off Florida [13] (Fig. 1). The echinoderm remains described herein originate from the chalk succession recovered at ODP site 1049 underlying the black shales of Oceanic Anoxic Event (OAE) 1b [13] and dated as middle/late Aptian to earliest Albian (Hedbergella trocoidea to Microhedbergella rischi planktonic zones, ca 114 Ma) [14]–[15]. The geologic history of site 1049 implies deposition of the Aptian–Albian chalk succession at lower bathyal depths (at least 1500 m), whereas benthic foraminiferal assemblages indicate middle bathyal (800–1000 m) or greater depths [13], [16]. There was no input of periplatform debris at the time the chalk was deposited, as indicated by the lack of shallow-water foraminifera [13], and the echinoderm plates are thus interpreted to represent an autochthonous bathyal assemblage.

Taxonomic Composition

The ophiuroid (brittle star) remains from the Blake Nose samples are composed mostly of the highly diverse, spine-bearing lateral arm plates [17]. Nearly half of these plates (48%) are assignable to the extant, predominantly deep-sea family Ophiacanthidae [18] on account of the ear-shaped spine articulations with a sigmoidal fold, the relatively thin aspect of the plates and a vertical series of perforations on the inner side (Figs 2A–F, 3A–D). Within this group, lateral arm plates with a conspicuous vertical striation including a minutely denticulate distal edge of the stripes, spine articulations sunken into the distal edge of the lateral arm plates, and well-developed connecting ridges between the spine articulations and the vertical stripes are assignable to the extant genus Ophiolimna (Fig. 2A–B). Lateral arm plates with a moderately to weakly developed, fine vertical striation and freestanding spine articulations on an elevated vertical ridge are commonly found in the extant genus Ophiacantha (Fig. 3D). Parasol-shaped arm spines similar to those found in the Blake Nose material are an exclusive feature of the modern deep-sea genera Ophiohelus and Ophiotholia (Fig. 2E–F). The co-occurrence of lateral arm plates with vertical and single (Fig. 2C–D) rather than oblique and multiple rows of spine articulations precludes assignment to Ophiohelus [19]. Most of the remaining lateral arm plates of the assemblage are assignable to the subfamily Ophioleucinae within the Ophiuridae (31%) (Fig. 2G–H), on account of the fragile plate architecture, the large tentacle notches, an outer surface ornamentation consisting of minute, scale-like granules and small rhombic spine articulations sunken into the distal edge of the plate, and to the extant deep-sea ophiolepidid genus Ophiomusium (8%) [18] (Fig. 2I–J). Lateral arm plates of the latter are typically rectangular in outline, with tentacle openings developed as within-plate perforations, and with a conspicuous large dorsal contact surface with the opposite lateral arm plate on the inner side. The composition at family level of the Blake Nose ophiuroid assemblage is closest to that of modern lower bathyal communities, and clearly differs from modern and Cretaceous shallow-water assemblages (Figs 4–5).

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larger image TIFF original image Download: Figure 4. Relative abundances of the most common ophiuroid families in present-day middle and lower bathyal settings, in comparison with the middle to lower bathyal ophiuroid assemblages from the upper Aptian–lowermost Albian of Blake Nose (ODP site 1049). Relative family-level abundances of the Blake Nose ophiuroid assemblage were inferred from lateral arm plate counts, assuming that the number of lateral arm plates serves as an approximation for the number of individuals. https://doi.org/10.1371/journal.pone.0046913.g004

The asteroid (starfish) material from the Blake Nose samples (Fig. 2K–N) is dominated by the remains of a single taxon. The ambulacrals are hourglass shaped, with equally expanded triangular head and base, a benthopectinid synapomorphy. Furthermore, the ambulacral-adambulacral contact is highly modified [20] and unique to the Benthopectinidae, representatives of which are common elements of modern deep-sea asteroid assemblages [21].

The Blake Nose holothuroid (sea cucumber) assemblage consists of body wall wheels assignable to the Laetmogonidae on account of the presence of 6 to 15 spokes, a perforated hub with a central primary cross and a smooth rim (Fig. 2O–P). Body wall wheels presenting 10 to 20 spokes, a flat hub and a rim with small to medium-sized teeth are typical of the Myriotrochidae, a group which is furthermore represented in our material by calcareous ring elements the radial ones of which typically display a perforation for the passage of the radial nerve and long anterior prolongations (Fig. 2Q–T). A third type of wheels is characterized by the presence of 6 or rarely 7 to 8 spokes, a non-perforated hub which is complex at both sides, and a circular and finely denticulate rim, which unquestionably places them in the Chiridotidae (Fig. 3P). Modern myriotrochids and laetmogonids are typically bathyal groups, and, although not restricted to deep-sea settings, chiridotids are also commonly found in modern bathyal habitats [22].

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larger image TIFF original image Download: Figure 5. Quantitative assessment of the Blake Nose ophiuroid assemblage. A: Detrended Correspondence Analysis (DCA) of modern ophiuroid assemblages in comparison with the Blake Nose ophiuroid fauna and Cretaceous shelf assemblages. The Blake Nose assemblage plots within the modern lower bathyal communities, and strongly differs from modern shallow-water communities and Cretaceous shelf assemblages, challenging the possibility of repeated deep-sea recolonization from shelf depths in the Cretaceous. The analysis is based on the relative abundances of all 17 extant ophiuroid families minus the Ophiuridae, which are abundant at all depths (see Table 1 for abundance data). B: Linear correlation between DCA scores (axis 1) and LOG depth. The relationship is very strong (r = −0.80, adjusted r-square = 0.64). The probability of such strong correlation occurring by chance is virtually zero (4.8899E−29). When the score of the Blake Nose fauna is projected onto this relationship, it would be assigned a depth of 1,486 m (1,218–1,864 m) (uncertainty based on 95% confidence interval for regression line). Remarkably, this is exactly within the range of paleodepth reconstructions for this site [13], [16]. This means the fauna is similar to present-day lower bathyal assemblages to such degree that even the faunal composition versus depth relationship appears to have remained the same. https://doi.org/10.1371/journal.pone.0046913.g005

The echinoid (sea urchin) assemblage studied herein includes spines with a smooth surface, a long collar, dorso-ventral flattening and ornamentation with coarse lateral thorns (Fig. 3K–L), a combination of characters known exclusively from the oral secondary spines of the extant family Histocidaridae. Strongly reduced ambulacral plates bearing a single large pore pair and lateral flanges bevelling under neighbor plates are assignable to the echinothurioids (Fig. 3H). Co-occurring secondary spines show morphologies consistent with those of extant echinothurioid secondary spines (Fig. 3I). Fragments of hollow, strongly verticillate spines were found in several samples, and clearly belong to a diadematoid echinoid (Fig. 3J), although attribution to any particular taxon within this large group is problematic. Almost all samples yield fragments and spines of thin-shelled atelostomate echinoids (Fig. 3M–O). Based on plate shape, ambulacral pore shape and position, as well as tuberculation, which consists of widely scattered primary tubercles with few distinctly smaller granules interspersed, an attribution to late stem-group holasteroids (tithoniids) [23] or early crown-group members seems likely. Although histocidarids, echinothurioids, diadematoids and holasteroids are typical components of modern deep-sea echinoid assemblages [6], the Blake Nose echinoid material fails to provide sufficient diagnostic elements for a detailed faunal analysis and comparison with modern equivalents. As predicted by phylogenetic analyses [24] our results indicate that echinothurioids and histocidarids evolved earlier than previously known, predating the oldest shallow-water occurrence of these groups. This may suggest that much of their evolution took place in deep-water settings.

The crinoid (sea lily) material from the Blake Nose assemblages includes columnals assignable to the isocrinid genus Balanocrinus on account of the short and uniform radiating crenulae along the margin, narrow radial ridges or ribbons of minute crenulae or granules, and large areolae (Fig. 3E). The assemblage furthermore comprises thin primibrachials 2 and a holdfast that are strongly reminiscent of the extant bourgueticrinid Bathycrinus (Fig. 3F–G). Modern isocrinids are found almost exclusively in bathyal settings, while Bathycrinus, and the modern bourgueticrinids in general, are among the deepest-dwelling of all extant crinoids, occurring from lower bathyal to hadal depths [25].