Geologic context

Extensive outcrops of largely shallow-marine, siliciclastic successions of the Argentine Cordillera Oriental are included in the Santa Victoria Group that encompass the Cambro-Ordovician Santa Rosita (Furongian-Tremadocian) and Acoite (Floian) formations (Bahlburg and Furlong, Reference Bahlburg and Furlong1996; Astini, Reference Astini2008). The crinoids described herein occur in the middle-upper part of the Acoite Formation, exposed at the Quebrada Los Baños, a tributary of the Quebrada Chamarra, Los Colorados area (23°34" S, 65°42" W), 30 km northwest of Purmamarca village, at the western foot of the Argentine Cordillera Oriental (Tumbaya Department, Jujuy Province).

The Acoite Formation reaches a thickness of ~2000 m in the Quebrada Chamarra and is interpreted as a shallow-marine succession displaying stacked, large-scale shallowing-upward cycles, encompassing shelf to shoreface settings (Astini and Waisfeld, Reference Astini and Waisfeld1993; Astini et al., Reference Astini, Waisfeld, Toro and Benedetto2004). The unit is biostratigraphically constrained within the Tetragraptus phyllograptoides (lower Floian) and the Dydimograptellus bifidus (upper Floian) graptolite zones (Toro, Reference Toro1997; Toro and Maletz, Reference Toro and Maletz2008). The new crinoids described herein are found in the lower part of the Dydimograptellus bifidus Zone (lowermost upper Floian).

The new crinoids occur in a 50-cm thick, fossiliferous, yellow-green shale bed. This and other shales alternate with fine to very fine-grained sandstone intervals that display hummocky cross-stratified beds below and climbing ripple cross-lamination above. Quiet-water, background suspension (“fall-out”) mud deposition, punctuated by oscillatory flows related to numerous storms and accompanying influx of coarser sediments, is indicated. A diverse trilobite fauna is also in the crinoid-bearing shales (Waisfeld and Vaccari, Reference Waisfeld and Vaccari2003).

Biogeographic context

An extensive Floian trilobite fauna reveals a complex biogeographic pattern, consistent with the South American margin at intermediate latitudes (McCormick and Fortey, Reference McCormick and Fortey1999; Fortey and Cocks Reference Fortey and Cocks2003; Benedetto et al. Reference Benedetto, Vaccari, Waisfeld, Sanchez and Folgia2009). Endemism supports semi-isolation where provincially significant elements exhibit a mixture of affinities either with warm water areas or with cooler western Gondwana (cf. Waisfeld and Vaccari, Reference Waisfeld and Vacarri2008). Likewise, Ramseyocrinus occurs in cool water Gondwanan settings, whereas Quechuacrinus is judged phylogenetically close to the warm water Laurentian Reteocrinus.

Phylogenetic position of Quechuacrinus

The cup of Quechuacrinus is characterized by prominent truss-like plate ridges with fenestra or deep pockets at plate corners, similar to the Middle and Late Ordovician Reteocrinus. The posterior morphology of Reteocrinus includes a prominent sagittal anal series of arm-like plates bisecting the CD interray, and extending from a basal plate. There is no articulation to ray plating. The posterior morphology of Quechuacrinus is incompletely known, but similar to Reteocrinus in expression of a prominent anal series, but not sagittal, instead articulating to the C-ray first primibrachial (Figs. 1, 2). Posterior plating more closely associated with the C-ray occurs in the early Floian putative reteocrinid Cnemacrinus, where the anal series extends from the basal below and C-radial. Pocket/fenestra expression in Quechuacrinus falls between Reteocrinus and Cnemacrinus.

Figure 1

Quechuacrinus ticsa n. gen. and sp., latex casts, all flattened by compaction; (1, 2) holotype CEGH-UNC 23541, partly flattened, stem fractures to resemble pentameres, 1, D ray view, cup folded under; (2) B ray view, C ray on left with projecting articulation from IBr₁ to longitudinal anal series, anal series folded under, depressed platelet field between and below arms; (3) paratype CEGH-UNC 23539, small interbasal gap; (4) paratype CEGH-UNC 23543, C ray view, irregular supernumerary plates below C radial; (5) paratype CEGH-UNC 23546, latex, much of stem, pentameric distal portion.

Figure 2

Quechuacrinus ticsa n. gen. and sp., line tracings of plating, expanded cup plate diagram; (1, 2) holotype CEGH-UNC 23541; 1, D ray view; (2) B ray view, projecting anal series ridge to left from C ray first primibrach, inset interbrachial plate field platelets, small interbasal gaps; (3) paratype CEGH-UNC 23543, irregular plating below C ray; (4) paratype CEGH-UNC 23539, interbasal gaps; (5) composite plate diagram, radials in black.

Reteocrinus rocktonensis Kolata, Reference Kolata1975, has a sagittal anal series articulating to the C-radial rather than the first primibrachial. Its anal series is narrower than arms. The taxonomic status of this form is unresolved.

Crown construction and phylogenetic position of Ramseyocrinus

Much has been written about Ramseyocrinus (see Donovan and Savill, Reference Donovan and Savill1988; Donovan, Reference Donovan1989) and for a time, this taxon was recognized as the oldest known crinoid, influencing thinking on early crinoid phylogeny and origin of the class itself. However, Ramseyocrinus has historically been difficult to decipher, with differing views of cup homologies and crown orientation (Donovan, Reference Donovan1984, Reference Donovan1989; Cope, Reference Cope1988). Six different interpretations are summarized in Donovan (Reference Donovan1989, fig. 1).

Earlier authors (Bates, Reference Bates1968; Ubaghs, Reference Ubaghs1983; Cope Reference Cope1988) considered the four-plate cup base circlet of Ramseyocrinus to be basals. Donovan (Reference Donovan1984; Reference Donovan1989) argued that these constitute radials, observing that cup base plates express prominent articular arm facets, so the articulating but not fixed plates above are first primibrachials. These putative primibrachs form in continuity with, and are similar to, subsequent primibrachs.

The radials of Ramseyocrinus are arranged in a rare pattern (Donovan, Reference Donovan1984, fig. 6). Uniquely, two cup plates support an arm and half of another appendage by sharing the arm facet. This is here termed a compound radial facet. The remaining two radials underlie and align with arms above in the normal arrangement. The four radials arrangement echoes the tetrameric stem of Ramseyocrinus, though not the symmetry.

Initially, the “co-supported” appendage of Ramseyocrinus was interpreted as an anal “tube” (Bates, Reference Bates1968; Ubaghs, Reference Ubaghs1983; Donovan, Reference Donovan1984), but later, after discovery of a specimen demonstrating isotomous branching, as an arm (Cope, Reference Cope1988; Donovan, Reference Donovan1989). The cup lacks any obvious posterior plating, and orientation was considered uncertain (Donovan, Reference Donovan1989, fig. 1.6).

Two pieces of evidence indicate the co-supported appendage is the C arm. First, it is bisected by the only symmetry plane in the cup (Fig. 3.4, 3.5), arguably the crinoidal plane. The cup of Ramseyocrinus expresses near perfect bilateral symmetry, with one large and one small plate per side. The cup CD interray is normally bisected by the crinoidal plane, but in this case, there is no CD interray. Second, the R. argentinus holotype preserves the lower portion of an anal sac curving and extending upward alongside the putative C ray arm (Fig. 4.4). We assume, then, the sac originates from the CD region of the tegmen. Previous authors illustrate disarticulated sac platelets associated with the putative C ray (this interpretation) of R. vizcainoi (Ubaghs, Reference Ubaghs1983, text fig. 11; fig. 3.2) and R. cambriensis (Cope, Reference Cope1988, text-figs. 2b, 3; fig. 5.4), documenting this expression across all known Ramseyocrinus species. The anal sac extends to at least 80% of the total crown height of R. cambriensis (Fig. 6.4)

Figure 3

Ramseyocrinus species; line drawings; (1, 3, 4) R. argentinus n. sp.; (1) paratype CEGH-UNC 23548; (3,4) holotype, CEGH-UNC 23549, tracing and cup plate diagram with radials in grey, respectively; (2, 5) R. vizcainoi; (1) tracing; (2) cup plate diagram (modified from Ubaghs, Reference Ubaghs1983).

Figure 4

Ramseyocrinus argentinus n. sp.; (1) paratype CEGH-UNC 23548, anal sac platelets left center, at level of secundibrachs; (2) paratype CEGH-UNC 24633, coarse preservation; (3–6) holotype CEGH-UNC 23549, D view of crown and stem segment, anal sac detail, C view (C on right) of cup, and D view of cup, respectively.

Identification of the C ray helps clarify the cup plate homologies of Ramseyocrinus. The unusual co-shared C radial facet supports the interpretation of no posterior cup plating or C radial; that functional role filled by expanded B and D radials.

Like Ramseyocrinus, the cup of Tetragonocrinus is built of just four plates with no anal plating; similar to, though not symmetrically disposed like Ramseyocrinus (Rozhnov, Reference Rozhnov1988, Fig. 1). This single cup circlet has been considered basals (Rozhnov, Reference Rozhnov1988), but extending the example from Ramseyocrinus, an alternative hypothesis posits radials. Synapomorphies include tetrameric stems and compound radial facets co-formed by adjacent radials, and lack of posterior plating. Tetragonocrinus expresses just three arms. Disparid ontogeny indicates A and D arms develop last (Sevastopulo and Lane, Reference Sevastopulo and Lane1988; see also Rozhnov, Reference Rozhnov1988). Following this reasoning, these arms are absent in Tetragonocrinus.

The phylogenetic position of a putative Ramseyocrinus-Tetragonocrinus clade is cloudy. Close association of the anal sac and C arm suggests disparids or camerates. Few plated cup construction suggests the former.

The lower Silurian camerate Stipatocrinus Eckert and Brett Reference Eckert and Brett1987, expresses a similar cup base circlet pattern to that of Ramseyocrinus, but this similarity is interpreted as homoplastic. Here the cup base is formed by either infrabasals (following the DRHS of Guensburg and Sprinkle, Reference Guensburg and Sprinkle2003) or basals (by the “Treatise,” Ubaghs, Reference Ubaghs1978).

Implications for the early radiation of crinoids

Taking a broad view, the new discoveries help document the great range of evolutionary tinkering with early diversification. This is apparent despite the relatively poor Floian crinoid record. These are not variations on a narrow theme, instead the range of morphologies points to differing adaptive strategies. The “minimalist” cup construction (single circlet, no posterior plating) in Ramseyocrinus contrasts sharply with coeval camerates (e.g. Proexenocrinus) and cladids (e. g. Elpasocrinus) with “usual” configurations and many more elements (see also Deline and Ausich, Reference Deline and Ausich2011).

Systematic paleontology

Terminology used here adopts both the Dual Reference Homology System following Guensburg and Sprinkle (Reference Guensburg and Sprinkle2003) and Guensburg (Reference Guensburg2012), a modification of the Ubaghs (Reference Ubaghs1978) terminology in the “Treatise.” Repositories for illustrated specimens are: Natural History Museum, London (NHMUK); Centro de Investigaciones Paleobiológicas, Universidad Nacional de Córdoba, Córdoba, Argentina (CEGH-UNC); Lyon University 1, Lyon, France (FSL); Národní Muzeum (Barrande Collection), Czech Republic (NM-Bar.); National Museum of Wales (NMW).

Class Crinoidea Miller, 1821

Subclass Camerata Wachsmuth and Springer, Reference Wachsmuth and Springer1885

Order Unassigned

Family Reteocrinidae Wachsmuth and Springer, Reference Wachsmuth and Springer1885

Remarks

Ramseyocrinus species differ in plate proportions but share similar plate arrangement. The new species expresses taller stem, cup, and arm plating relative to the other two known species. Ramseyocrinus argentinus (Figs. 3.1, 3.3, 3.4, 4.1–4.6) is closest to R. vizcainoi (Figs. 2.2, 4.3) from the Floian of the Montagne Noire region of France but the latter’s crown and stem plates are shorter. Specimens assigned to R. cambriensis exhibit great variation in overall size and plate proportions. Smaller specimens (compare Figs. 5.1, 5.2, 5.4, 6.1–6.3) more closely resemble R. argentinus. A well-preserved large specimen shows parts of a large anal sac formed of granular plating and shorter tetrameres.

Figure 5

(1, 2, 4) Ramseyocrinus cambriensis (Hicks, Reference Hicks1873); (1, 2) latex casts of holotype NMW 29.308.G318 in A and CD views, respectivley; (4) NMW 86.93G.1a, referred specimen by Cope, Reference Cope1988, CD view, coarse preservation; (3) Ramseyocrinus vizcainoi Ubaghs, Reference Ubaghs1983, FSL 712 076, referred specimen, proximal stem with disarticulated crown.

Figure 6

Ramseyocrinus cambriensis (Hicks, Reference Hicks1873); latex cast of referred specimen NHMUK E3, large crown and much of stem; (1) entire specimen; (2) distal stem segment; (3) crown and proximal stem; (4) mid-section of arms and well-preserved portions of granular anal sac.

Entrochus primus (Waagen and Jahn, Reference Waagen and Jahn1899) (Fig. 7) from the early Middle Ordovician of the Czech Republic has been assigned to Ramseyocrinus (Donovan, Reference Donovan1984). Type material consists of stem fragments alone. These are monomeric, circular in outline, with narrow but well-defined crenularium, and a disjunct tetrameric lumen. These expressions contrast from other fossils assigned to Ramseyocrinus. We exclude this taxon from Ramseyocrinus.

Figure 7

Entrochus primus, NM-Bar. 35050 L13 304, oblique view of stem segment with monomeres, circular cross section, disjunct tetrameral lumen, and narrow crenularium.