Misunithyris goyi, Baeza-Carratalá & Pérez-Valera & Pérez-Valera, 2018

Baeza-Carratalá, José Francisco, Pérez-Valera, Fernando & Pérez-Valera, Juan Alberto, 2018, The oldest post-Paleozoic (Ladinian, Triassic) brachiopods from the Betic Range, SE Spain, Acta Palaeontologica Polonica 63 (1), pp. 71-85 : 76-80

Misunithyris goyi sp. nov.

Fig. 3 .

Etymology: In a tribute to Antonio Goy (Complutense University of Madrid), prominent Triassic and Jurassic palaeontologist, to whom the authors are very indebted for long term teaching and collaboration.

Type material: Holotype: DCTMA-BQ-TA1.2 ( Fig. 3F ; Table 1 ) from the Talave section. Paratype: DCTMA-BQ-AH2.2 ( Figs. 3B , 4 , 5 ). Intraspecific variability is shown through the paratypes ( Fig. 3 ) and measured in Table 1 .

Type locality: Talave section (External Betic Zone, SE Spain).

Type horizon: Upper member of Siles Formation, Fassanian (early Ladinian); Gevanites epigonus Biochronozone.

Material.— Six specimens (BQ-CL1.1, BQ-CL1.2, BQ-TA1.1, BQ-TA1.2, BQ-AH2.1, and BQ-AH2.2), including type material, with diverse state of preservation. All individuals show articulated but generally fragmented and encrusted valves. Internal recrystallization is visible in two specimens.

Diagnosis.—Medium-sized and ventribiconvex zeilleriid, pyriform in outline with strong sub-labiate beak and epithyrid/permesothyrid foramen. The lateral commissure is straight and the anterior one is clearly sulcate. Sharp ribs (8–13) appearing only near the anterior margin (3–5 in the median sulcus), often bifurcate and occasionally intercalate. Deltidial plates present and well-developed striate cardinal process. Crural bases are given off dorsally and hinge plates initially fused and subparallel. Dorsal septum long, exceeding the length of the brachidium. Long descending branches, distally with ventral progression.

Description.— External features ( Fig. 3 ): Medium-sized ventribiconvex shells ( Table 1 ), pyriform to subovoidal in dorsal outline. W/L ratio is rather variable, but all specimens show L> W. Thickness is about 3/5 of the length. Maximum convexity lies in the posterior third while maximum width lies in the anterior third of the shell. Maximum thickness is observed near the mid-length. The beak is massive, sub-labiate, and strong, erect to slightly incurved, with a medium-sized epithyrid to permesothyrid foramen; the beak ridges are well-perceived and blunt, developing short and narrow interareas. The lateral commissure is straight and the anterior one is clearly sulcate, with a wide and arcuate dorsal median sinus. Sulcation is generally shallow, being more pronounced in the larger specimens.

Ribbing pattern is distinctive of this species. Ribs are absent in the posterior third of the shell; costation consists of 8 to 13 ribs on each valve, 3–5 of which can be present in the median sulcus. Ribs are wide, strong, and square in cross-section, becoming stronger and sharper near the anterior margin. They are often bifurcate; the thicker specimens possess a set of weaker, intercalated ribs. Costation is more evident in the ventral valve and on the flanks of the dorsal one. In the median dorsal sulcus, ribs are only noticed from the mid-length onwards. Concentric, dense and strong growth lines are present on the entire surface giving a reticulate aspect to the shell.

Internal structure ( Figs. 4 , 5 ): This species shows a suboctagonal to ovoidal delthyrial cavity in cross-section where the remains of a short pedicle collar and well-developed deltidial plates are visible. Dental plates are difficult to distinguish as they are enveloped in a thickened-shell wall ( Fig. 5 ). They are short and subparallel. Hinge teeth are massive with crenulations, inserted in broad, shallow, and also crenulated sockets; small denticula are also discernible. It has a noticeable cardinal process, striated and raised by a high cardinal platform where a central cavity is located under the myophore ( Fig. 5 ). Hinge plates are initially horizontal, fused and subparallel. The dorsal median septum is long and well-developed up to 2/3 of the shell. A short elliptical septalium is discernible. Crural bases are located in the ventral area of the hinge plates but an incipient dorsal thickening emerge clearly showing an early dorsal development ( Fig. 4 ), suggesting a Bakonyithyris - type hinge plate/crural base inter-relation (sensu Baeza-Carratalá and García Joral 2014). The descending branches of the brachidium show subparallell and vertical plates and they are developed in the commissural plane in the posterior third of the shell, acquiring a slight ventral development anteriorly. The transverse band is not visible.

Remarks.—In addition to the main biometric ratios ( Table 1 ), intraspecific variability mainly lies in the number of ribs present in each valve, depending on the bifurcation or intercalation. The smooth posterior stage is also variable between ½ and ¼ of the shell. A single specimen shows an exceptionally wide sulcus giving to the anterior commissure an almost rectimarginate aspect, but analyzing in detail this individual the sinus can be noticed from the early stages in the posterior areas of the shell. Thus, even width of sulcus is rather stable, it ranges from mid-width in the BQ-TA1.2 specimen up to wider developments (BQ-AH2.1 and BQ-CL1.1) occupying nearly the entire anterior margin. On the other hand, maximum width can be shifted toward the anterior third, resulting in a more trigonal dorsal outline.

As it is discussed above in the supraspecific assignment, this species shows high singularity in the external and internal traits, probably because it is an endemic taxon. The closer affinity is shown with the Lower Jurassic (mostly anteriorly) multicostulate zeilleriids Tauromenia polymorpha , T. brevicostata , and Fimbriothyris spp. ( Seguenza 1885; Dubar 1942; Elmi et al. 2003; Alméras et al. 2007; Baeza-Carratalá and García Joral 2012), mainly recorded in the southern part of Western Tethys and even with the Middle Jurassic Eudesiidae representatives recorded in both Tethyan margins and the Mid-East platforms (e.g., Alméras 1987; Cooper 1989; Mukherjee et al. 2000; Alméras et al. 2010b), having in common with these last taxa several internal features (cardinal process present in several taxa, long median septum) but not sharing the brachidium architecture and the ribs covering the whole surface of the shell in the Middle Jurassic taxa.

Among the Triassic representatives, the closer affinities are found in Menathyris wilsoni Feldman, 2013 from Israel, virtually contemporary with the Betic material, sharing the sulcate shape of the shell, the marginal ribbing pattern and comparable internal structure, except for the cardinal process, absent in M. wilsoni ; however, sulcus is shallower and narrower, anterior commissure is generally rectimarginate and ribs are simple without bifurcation or intercalation in such species. Curious similarity concerning the internal structure is observed in one specimen of Coenothyris with a central umbonal chamber in the cardinal process, similarly to the Betic material, and several Aulacothyroides species developing wide cardinal process, all of them depicted by Senkowiczowa and Popiel-Barczyk (1996) in the Anisian– Ladinian from Poland.

Stratigraphic and geographic range.—In the Betic Range, this new species has been recorded together with biostratigraphical marker of the Gevanites epigonus Biochronozone Pérez-Valera F. 2005 ; Pérez-Valera et al. 2011; Pérez-Valera 2015) of the Fassanian (early Ladinian).

Figures

Fig. 3. Intraspecific variability of zeillerid brachiopod Misunithyris goyi gen. et sp. nov. of the lower Ladinian from the South-Iberian Triassic platform, the Arroyo Hurtado (A, B), Calasparra (C, E), Talave (D, F) sections; in dorsal (A1–F1), anterior (A2–F2), and lateral (A3–F3) views. A. BQ-AH2.1. B. BQ-AH2.2, sectioned in the present work. C. BQ-CL1.1. D. BQ-TA1.1. E. BQ-CL1.2. F. BQ-TA1.2, holotype. All specimens were coated with magnesium oxide.

Fig. 4. Internal structure of zeillerid brachiopod Misunithyris goyi gen. et sp. nov. (BQ-AH2.2) from the lower Ladinian of the South-Iberian Triassic platform, the Arroyo Hurtado section. A. Transverse serial sections through the specimen (distance from the beak in mm). B. Microphotographs of acetate peels showing the hinge plates-crural bases relationship: crural bases are located in the ventral part of the hinge plates but a dorsal thickening emerges towards an early dorsal development. Section at 4.10 mm (B1) and 4.30 mm (B2) from the apex. Abbreviations: cb, crural bases; dp, deltidial plates; ld, dental lamellae; ms, median septum; pc, cardinal process; sp, septalium.

Fig. 5. Microphotographs of acetate peels from zeillerid brachiopod Misunithyris goyi gen. et sp. nov. (BQ-AH2.2) from the lower Ladinian of the South- Iberian Triassic platform, the Arroyo Hurtado section. A. Section at 1.00 mm from the apex showing dental lamellae enveloped in a thick-shelled wall. B. Section at 2.60 mm from the apex showing the disposition of deltidial plates.C, D. Sections at 3.30 and 3.50 mm from the apex, respectively, showing the first stages of hinge plates and earlier cardinalia and the evolution of the cardinal process, clearly striated and raised by a high cardinal platform. E, F. Sections at 4.10 and 4.30 mm from the apex, respectively, showing the cardinal area with the position of crural bases and the articulation system.G–I. Partial sections at 4.90, 5.70, and 7.10 mm from the apex, respectively, showing the evolution of the crural architecture and dorsal median septum development.