Briveichthys chantepieorum, Štamberg & Steyer, 2021

Štamberg, Stanislav & Steyer, Jean-Sébastien, 2021, New Actinopterygians From The Permian Of The Brive Basin, And The Ichthyofaunas Of The French Massif Central, Fossil Imprint 77 (1), pp. 145-165 : 147-154

Briveichthys chantepieorum gen. et sp. nov.

Text-figs 2–6

H o l o t y p e. Specimen GMC 15 with a partially disarticulated skull ( Text-figs 2a , 4a–c, e, f , 5d, g ), housed in Museum of Noailhac, Noailhac, France.

P a r a t y p e s. Specimens GMC 18, GMC 90 and

GMC 126.

E t y m o l o g y. In honour of the collectors Maryse and

Guy Chantepie.

D i a g n o s i s. An actinopterygian with predatory dentition, with a very long frontal, three times longer than wide, anteriorly conspicuously concave, and without any large processes on the lateral or medial sides. The medial edge of the frontal is longer than the lateral edge. The parietal is approximately triangular in shape, with pit lines and an ossification centre near the medial edge of the bone. The border between the dermosphenotic and the dermopteroic is anterior to the fronto-parietal border. The dermosphenotic and the nasal form the dorsal and anterior rim of the orbit. The parasphenoid has a large processus ascendens posterior and a small processus ascendens anterior. The maxilla has a large trapezoidal shaped maxillary plate that makes up almost half the length of the maxilla. The maxillary plate extends posteroventrally into a strongly pronounced process which is rounded ventrally. The anterior edge of the maxillary plate is largely covered by the jugal, and it makes an angle of about 45° with the denticulated lower margin of the maxilla. The maxillary plate bears sculpture composed of pronounced simple ridges that are not vermicularly curved. The teeth form both medial and outer rows. Those teeth on the medial row are large and slender; they are six times higher than those of the outer row which are more numerous and sharply pointed. The antoperculum is of triangular shape and borders 2/3 of the anterior edge of the operculum. The operculum is dorsoventrally elongated and elliptical in shape, slightly narrowing ventrally. Its height is 2.5 times its length. The operculum bears short ridges diverging radially from the ossification centre of the bone. The suboperculum is square shaped and taperes significantly at the front and then exhibits extended anteroventral and anterodorsal corners. The scales bear conspicuous ridges diagonally traversing across their lateral surface to terminate posteriorly in denticulations.

T y p e l o c a l i t y a n d h o r i z o n. Along the road D1089 at the level of Brive-la-Gaillarde, Corrèze, France. Limestones of Saint-Antoine, lower Permian, Brive Basin, Massif Central, France.

D e s c r i p t i o n. The specimens correspond to adult individuals of approximately 14–18 cm total length. The largest specimen (GMC 126) is a full adult because it exhibits robust short segments in the lepidotrichia in the pectoral and caudal fins. The shape and position of the fin is not preserved.

Rostral part and skull roof. A fragment of elongated nasal represents the rostral region of the skull on the holotype GMC 15 . A fine incision representing the anterior nasal opening is noticeable on the medial edge of the bone. The skull roof is well-preserved in specimen GMC 15 (Textfig. 2a, b) as seen in the frontal, parietal, dermosphenotic and dermopterotic. The frontal is narrow, conspicuously elongated (the length is three times its width). The lateral edges of the right and left frontals are significantly shorter than their medial edges, and without any process. The interfrontal suture is also without process and only slightly wavy. The anterior edge of the frontal is significantly concave in the medial direction. The frontal borders the parietal with a markedly undulating suture. The parietal is triangular in shape, its lateral edge is at least twice the length of the medial edge. The dermosphenotic is also triangular and borders the frontal laterally. Its posterior portion is well preserved on the holotype GMC 15 ( Text-fig. 2a, b ) but displaced slightly backwards. The dermosphenotic widens posteriorly and tapers anteriorly. It contacts the nasal anteriorly and separates the frontal from the orbit. The dermopterotic is well preserved but displaced in GMC 15 : it is oblong, anteroposteriorly extended and with conspicuous lateral processes. The border between the dermosphenotic and the dermopterotic is slightly anterior to the border between the parietal and the frontal. The exposed parts of the skull roof bones and the nasal are ornamented with conspicuous short ridges and tubercles. The ridges on the frontal are uniformly arranged in an anteroposterior direction. The supraorbital sensory canal on the left frontal runs parallel to the lateral edge, and appears as a short groove of about a third the width of the bone. The ossification centre of the parietal is close to its median edge and lies at the beginning of its middle and posterior pit lines. The dermopterotic bears the infraorbital sensory canal on its lateral margin. The specimens GMC 18 , GMC 126 and the holotype GMC 15 have narrow frontals with anterior edge conspicuously concave medially ( Text-fig. 2c ) .

The skull roof bones of Briveichthys chantepieorum gen. et sp. nov. differ from those of Progyrolepis heyleri (see Text-fig. 7a, b here, and Štamberg 2018: text-figs 4, 5) in the shape of the frontal which is anteriorly conspicuously concave, the medial edge of the bone is very long while the lateral edge is significantly shorter; the triangular shaped parietal has pit lines and ossification centre located in the medial part of the bone.

Parasphenoid. This bone is preserved in dorsal view in the holotype GMC 15 ( Text-fig. 4a, b ). It consists of a corpus parasphenoidis, which posteriorly extends into the processus ascendens anterior and the processus ascendens posterior. The corpus parasphenoidis is elongated and narrows anteriorly. The ossification centre lies in the posterior part of the bone, behind the processus ascendens anterior, at a distance of about one fifth of the full length of the bone from its posterior margin. The posterior region of the corpus parasphenoidis is pierced by the foramen for the bucco-hypophysial canal. The posterior edge of this corpus is slightly convex. Its middle part is convex from its anterior edge to the bucco-hypophysial foramen. The processus ascendens anterior is very small and separated from the processus ascendens posterior. The corpus parasphenoidis widens behind the processus acscendens anterior. It extends in a posterolateral direction into a large processus ascendens posterior, which forms an angle of 75° with the axis of the corpus parasphenoidis – this angle is 60°–68° in Progyrolepis heyleri which has no processus ascendens anterior. A further difference being that Progyrolepis heyleri has a shortened posterior part of the corpus parasphenoidis.

Cheek. In the holotype GMC 15 , the orbit is bordered by the dermosphenotic and the nasal dorsally and anteriorly. A short dorsal and anterior parts of a sclerotical ring are also preserved. The preoperculum is well preserved in medial view ( Text-fig. 5c ), together with the antoperculum and operculum on GMC 90 . The preoperculum is long and consists of two branches, a large anterior branch and a narrow posteroventral branch. The two branches meet forming an angle of 135°. The anterior branch is deep anteriorly, with a concave anterior edge and a slightly pronounced anterodorsal corner. The posteroventral branch has the preopercular sensory canal running along its posterior edge. This canal curves and runs anteriorly along its dorsal edge, and then exits on the dorsal margin in front of the anterior edge of the bone. A portion of the preoperculum is exposed in lateral view on the holotype GMC 15 . The deep anterior region of the preoperculum bears a sculpture made of short ridges and tubercles. A large crescent shaped jugal is preserved on GMC 126 ( Text-fig. 3 ), it overlaps the anterior region of the maxillary plate and the dorsal region of the suborbital part of the maxilla. This suborbital part is narrow. The jugal is anteriorly broad and rounded, and dorsally widened. Its anterior concave edge surrounds the orbit. Its partly preserved ornamentation consists of short antero-posterior ridges. A robust hyomandibula is also preserved on GMC 18 and GMC 15 , it is deep and angularly curved. The processus opercularis is formed by the hump of the hyomandibula curve .

Upper jaw. Preserved in specimen GMC 126 is an autopalatinum in the anterior part of the palatoquadratum, and a complete maxilla with a long, low maxillary plate and a narrow suborbital maxillary portion ( Text-fig. 3 ). The length of the maxillary plate is 46 % that of the bone. This maxillary plate is trapezoidal with a ventroposteriorly extended process. This process is rounded ventrally and is larger than that of Progyrolepis heyleri . The anterior edge of the maxillary plate makes an angle of about 45° with the denticulated ventral margin of the maxilla. The ornamentation on the lateral surface of the bone consists of unbranched ridges. The lateral surface of the dorsal area of the maxillary plate bears sparsely arranged, short and anteroposteriorly directed ridges, whereas the ventral area of the maxillary plate bears densely arranged ridges running in a diagonal direction. A narrow suborbital part of the maxilla bears short ridges diagonally arranged across the bone, and tubercles along the ventral edge of the maxilla. The anterior part of the suborbital part of the maxilla is without any sculpture, and it was probably covered laterally by the lacrymal. The maxilla on GMC 15 exhibits a large maxillary plate with a pronounced posteroventral process and the imprint of the horizontal lamina along the ventral edge of the maxilla ( Text-fig. 2a, b ).

Lower jaw. The bone is preserved on GMC 18 , GMC 126 and the holotype GMC 15 . GMC 126 has a robust dentalosplenial with ornamentation on its lateral surface consisting of unbranched diagonally arranged ridges running ventrally to the mandibular canal, and anteroposteriorly arranged ridges running dorsally to the mandibular canal ( Text-figs 3 , 4d ). This ornamentation differs from that of Progyrolepis heyleri which consists exclusively of long ridges regularly arranged in anteroposterior direction ( Štamberg 2018: text-fig. 13). The mandibular sensory canal appears as a series of pores along the ventral edge of the bone, and is curved upward in the anterior part of the dentalosplenial. The coronoids are preserved with numerous small teeth along the dorsal edge of the dentalosplenial .

Dentition. The teeth visible on the maxilla and dentalosplenial form two rows, a lateral and a medial row, the lateral teeth are numerous, sharply pointed and very small, whereas the medial ones are slender, less numerous, irregularly spaced and six times higher ( Text-fig. 4c, e ). The shaft of the tooth bears microsculpture formed by fine protuberances distributed proximo-distally on the surface ( Text-fig. 4f ). This kind of microsculpture is common in Progyrolepis heyleri ( Text-fig. 10h–k ; Štamberg 2018: textfig. 15c–g) and other carnivorous actinopterygians ( Richter 1983, Schindler 2018). In addition to the marginal teeth, there are small teeth on the coronoids of the lower jaw.

Opercular apparatus. In GMC 90 both the operculum and the antoperculum are preserved ( Text-fig. 5a, b ). The antoperculum is triangular, deep and firmly attached to the operculum (it borders 2/3 of the anterior edge of the former). The operculum is oval (its maximum length is in its dorsal region), 2.5 times higher than long, and narrows ventrally. Its ossification centre is in a dorsal position, in the anterior sixth of the bone length. The exposed lateral surface of the operculum exhibits concentric lines in its ventral half and diagonal lines in its dorsal half – the latters are short and arranged radially from the ossification centre towards the posterior margin of the operculum. The shape and ornamentation are fundamentally different from those of Progyrolepis heyleri which has an oblong and vermicularly ornamented operculum.

In Briveichthys chantepieorum gen. et sp. nov., the suboperculum is well preserved and several branchiostegal rays and medial and lateral gulars are distinguished among the dermal bones of the opercular apparatus on the holotype GMC 15 ( Text-fig. 5g ). In lateral view, the suboperculum is square in shape. The bone is deep posteriorly and it enlarges anteriorly, forming two large projections in the anteroventral and anterodorsal corners. The anteroventral projection is pointed, whereas the anterodorsal projection is blunt. On the contrary, the posterodorsal and posteroventral corners of the suboperculum are rounded. This suboperculum is significantly narrowed in its anterior half, which is markedly different in shape from this bone in Progyrolepis heyleri ( Text-fig. 5e, f ). In Briveichthys chantepieorum gen. et sp. nov., seven isolated branchiostegal rays are preserved under the lower jaw in GMC 15 ( Text-fig. 2a, b ), they are elongated, rounded posteriorly and slightly narrowing anteriorly. The total number of branchiostegal rays is estimated to be about 10–12 between the lateral gular and the suboperculum. The lateral gular in front of the branchiostegal rays is conspicuously anteroventrally elongated ( Text-fig. 5d ). The anterior half of the bone is narrow and it borders medially the medial gular with its concave medial edge. The lateral gular is pointed at the front while its posterior half is enlarged. The pit line in the centre of the bone is visible. The medial gular exposed in ventral view on GMC 126 is oval in shape and relatively large, with its length twice its width ( Text-fig. 3 ). It is pointed at the front, widens posteriorly, and has a rounded posterior edge. The pit line is a V-shape and is located in the anterior half of the bone, at the level of its ossification centre. The ventral surface of the medial gular bears short ridges arranged in an anteroposterior direction. According to the shape of the medial and lateral gulars, two thirds of the posterior part of the medial gular may be laterally bordered by the anterior half of the paired lateral gulars.

Dermal bones of the pectoral girdle. The posttemporal and supracleithrum are well preserved. The posttemporal is exposed in dorsal view on GMC 15, it is oblong, elongated in a mediolateral direction and conspicuously narrower medially. Aprominentprocessonitsanterioredgeisdirectedforward.The ornamentation of this bone consists of short ridges arranged in a mediolateral direction. The supracleithrum is dorsoventrally elongated. Its lateral sensory canal runs diagonally across the supracleithrum from the anterodorsal corner of the bone to its mid-postrerior edge. The cleithrum is fragmented and do not provide detailed information about its shape and position.

Dpt soc a ioc Pa

Fr Dsph soc Na ana

pl

Fins. The pectoral fin and the ventral lobe of the caudal fin are partly preserved. In GMC 18 and GMC 126, the pectoral fin consists of at least 15 robust lepidotrichia. The first lepidotrichium is unsegmented. The entire front edge of the pectoral fin is formed by a single lepidotrichium covered along its whole length by fringing fulcra ( Text-figs 3 , 4g ). At least three following lepidotrichia appear segmented beyond the proximal third of their length. The remaining lepidotrichia are segmented along their entire length. These segments are short and wide, especially on GMC 126, this suggests an adult growth stage. The ventral lobe of the caudal fin consists of robust lepidotrichia with short wide segments. The anterior edge of the ventral lobe is protected by fringing fulcra together with pointed terminal segments of lepidotrichia.

Scales. The scales are poorly preserved. They have prominent diagonal ridges, which start anteriorly and protrude into the serrations posteriorly. The posterior border of the scales exhibits up to ten serrations. In the ventral part of the body, the long low scales bear only two or three ridges. The peg and socket articulation is well developed. The scales forming the midline on the back are wide. The ridges protruding into the serrations are parallel with the lateral edges of the scale ( Text-fig. 5h–j ). In the ventral part of the body, behind the skull, the scales are small in size but robust in shape with short dorsoventral ridges. A large part of the anterior and dorsal regions of the scale are not ornamented and overlap the surrounding scales. Three large ridge scales precede the base of the dorsal fin ( Text-fig. 5j ).

C o m p a r i s o n. Briveichthys chantepieorum gen. et

sp. nov. presents the following unusual characters:

• its parietal is anteriorly separated by a very long frontal medially although the frontal is shorter laterally. The parietal shows medial and posterior pit lines. Its ossification centre is located at the beginning of the medial pit lines, whereas it is located on the lateral side of the bone in other Permo-Carboniferous actinopterygians;

• its dermosphenotic is slender, triangular and contacts anteriorly with the nasal, separating the frontal from the orbit, this is not the case in the Elonichthyidae (sensu Poschman and Schindler 2004);

• its operculum is ellipsoid and shows a different sculpture pattern than that in Progyrolepis heyleri ( Text-fig. 9d ), Progyrolepis speciosus ( FRIČ, 1875) ( Štamberg 1991) and Elonichthys germari GIEBEL, 1848 ( Schindler 2018) ;

• its suboperculum is very shallow anteriorly, with the anterodorsal corner being extremely elongated and prongshaped, and the large anteroventral corner forming a process: this is not the case in other actinopterygians such as Progyrolepis or Letovichthys where both corners are prominent ( Stensiö 1921, Aldinger 1937, Štamberg 2007). However, a prong-shaped anterodorsal corner of the suboperculum is also seen in Devonian actinopterygians such as Gogosardinia or Mimipiscis (e.g., Choo 2011) where it is interpreted as a functional replacement for the accessory operculum ( Choo et al. 2009). The fact that the suboperculum of Briveichthys is also very shallow anteriorly is a unique character, yet a slight anterior reduction of this bone is also seen in Coccocephalus wildi WATSON, 1925 ( Poplin and Véran 1996).

Briveichthys chantepieorum gen. et sp. nov. also shares the following characters with other Permo-Carboniferous actinopterygians: • it shares the same morphology of the parasphenoid and antoperculum with that of the Pygopteridae ; • its frontal has a concave anterior border as in Coccocephalus wildi ( Poplin and Véran 1996) , Letovichthys tuberculatus ŠTAMBERG, 2007 and relatives;

• its parietal is subtriangular as in Commentrya traquairi SAUVAGE, 1888 ( Blot 1966) , Coccocephalus wildi ( Poplin and Véran 1996) , Mansfieldiscus sweeti WOODWARD, 1906 ( Long 1988) and Letovichthys tuberculatus ( Štamberg 2007);

• its processus ascendens anterior of the parasphenoid is separated at its base from the processus ascendens posterior as in Pygopterus nielseni ALDINGER, 1937 ( Aldinger 1937: text-fig. 38), Progyrolepis heyleri , Meisenheimichthys palatinus ( SCHINDLER, 1993) , Elonichthys germari GIEBEL, 1848 ( Schindler 2018) and other Elonichthyidae (sensu Poschmann and Schindler 2004);

• its triangular antoperculum borders two third of the anterior operculum edge as in many species such as Progyrolepis heyleri , Pygopterus nielseni and Howqualepis rostridens LONG, 1988 ( Long 1988) . This bone is named the dermohyal in KazancevaSelezneva (1977) who considered its triangular shape as a characteristic of the Pygopteridae . However, it does not fit on the hyomandibular and should be named antoperculum as originally described by Aldinger (1937).

Figures

Text-fig. 2. Briveichthys chantepieorum gen. et sp. nov. a, b: whitened photograph and drawing of the skull, GMC 15, scale bars 5 mm; c: anterior part of the left and right frontal in dorsal view, a conspicuous concave anterior edge of the bones is demonstrated, GMC 18, whitened, scale bar 5 mm. Abbreviations: Clei – cleithrum, De – dentalosplenial, Dpt – dermopterotic, Dsph – dermosphenotic, Ext? – extrascapular?, Fr – frontal, Gul – lateral gular, Hy – hyomandibula, Mx – maxilla, Na – nasal, Pa – parietal, Pop – preoperculum, ppl – parietal pit lines, Pt – posttemporal, Rbr – branchiostegal rays, Scl – supracleithrum, soc – supraorbital canal, Sop – suboperculum, sr – sclerotic ring.

Text-fig. 3. Briveichthys chantepieorum gen. et sp. nov. a, b: drawing and photograph of the jaws, jugal, medial gular and first lepidotrichia of the pectoral fin, GMC 126, whitened, scale bar 5 mm. Abbreviations: Aup – autopalatinum, De – dentalosplenial, ff – fringing fulcra, Gm – medial gular, gpl – gular pit line, ioc – infraorbital sensory canal, Ju – jugal, lep – lepidotrichia, mc – pores of the mandibular sensory canal, Mx – maxilla.

Text-fig. 4. Briveichthys chantepieorum gen. et sp. nov. a, b: photograph and drawing of the parasphenoid in dorsal view, GMC 15, whitened, scale bars 5 mm; c: maxillary plate in medial view with horizontal lamina along the ventral edge of the bone, segment of the lower jaw with assembly of large slender teeth of the inner row and coronoids with small teeth, GMC 15, whitened, scale bar 5 mm; d: detail of the sculpture on the maxilla and dentalosplenial, GMC 126, whitened, scale bar 5 mm; e: detail of the teeth of the inner and outer row and coronoids on the lower jaw, the frame delineates the area illustrated in (f) at higher magnification, GMC 15, scale bar 2 mm; f: microsculpture formed by elliptical proximo-distally elongated protuberances on the large conical teeth, GMC 15, scale bar 100 µm; g: small fringing fulcra tightly attached to the anterior edge of a lepidotrichium, individual fulcral scales are indicated by arrows, GMC 18, scale bar 2 mm. Abbreviation: bhf – bucco-hypophysial foramen, Cor – coronoids, cp – corpus parasphenoidis, De – dentalosplenial, hl – horizontal lamina, mc – pores of the mandibular sensory canal, Mx – maxilla, paa – processus ascendens anterior, pap – processus ascendens posterior.

Text-fig. 5. Briveichthys chantepieorum gen. et sp. nov. a, b: photograph and drawing of the left antoperculum and operculum in lateral view, GMC 90, whitened, scale bars 5 mm; c: left preoperculum in medial view, GMC 90, whitened, scale bar 5 mm; d: left lateral gular in dorsal view, GMC 15, scale bar 5 mm; e: drawing of the right suboperculm of Progyrolepis heyleri in lateral view (after Štamberg 2018: text-fig. 19f), scale bar 5 mm; f: left suboperculum of Progyrolepis heyleri in medial view, GMC 116, scale bar 5 mm; g: left suboperculum of Briveichthys chantepieorum gen. et sp. nov. in medial view, GMC 15, scale bar 5 mm; h, j: photograph and drawing of the scale from the midline on the back, GMC 126, whitened, scale bars 2 mm; k: scales on the back of the body and ridge scale in front of the dorsal fin, GMC 126, whitened, scale bar 2 mm. Abbreviations: Aop – antoperculum, Op – operculum.

Text-fig. 6. Briveichthys chantepieorum gen. et sp. nov. a: reconstruction of the skull roof and the nasal in dorsal view, scale bar 5 mm; b: reconstruction of the skull in lateral view (parts of the skull which are not clearly retained are in grey colour), scale bar 5 mm. Abbreviations: ana – anterior nasal opening, Aop – antoperculum, De – dentalosplenial, Dpt – dermopterotic, Dsph – dermosphenotic, Fr – frontal, Gul – lateral gular, ioc – infraorbital canal, Ju – jugal, Mx – maxilla, Na – nasal, Op – operculum, Pa – parietal, pl – pit line, Pop – preoperculum, Pt – posttemporal, Rbr – branchiostegal ray, Scl – supracleithrum, soc – supraorbital canal, Sop – suboperculum, sr – sclerotical ring.

Text-fig. 7. Progyrolepis heyleri POPLIN, 1999. a, b: whitened photograph and drawing of the skull roof in dorsal view, GMC 56, scale bars 5 mm; c, d: drawing and whitened photograph of the parasphenoid in dorsal view, GMC 84, scale bars 5 mm. Abbreviations: Dsph – dermosphenotic, Dpt – dermopterotic, Fr – frontal, infs – interfrontal suture, ioc – infraorbital sensory canal, Pa – parietal, pl – parietal pit line, soc – supraorbital sensory canal.

Text-fig. 10. Progyrolepis heyleri POPLIN, 1999. a: dorsal lobe of the caudal fin with the fulcral scales along the dorsal edge of the lobe, GMC 55, whitened, scale bar 5 mm; b: basal fulcral scales from the dorsal edge of the caudal peduncle, G 123, whitened, scale bar 5 mm; c: fragment of the body of juvenile specimen with dorsal and anal fins, GMC 11, whitened, scale bar 5 mm; d: isolated scales from lateral side of the body, G 123, whitened, scale bar 5 mm; e: ridges on the scale surface, the frame delineates the area illustrated in (f) at higher magnification, G 123, scale bar 500 µm; f: details of the surface with microtubercles, scale bar 50 µm; g: isolated lepidotrichium of an adult specimen with very short and wide segments and with unsegmented basal part, GMC 101, whitened, scale bar 5 mm; h: large conical teeth from the internal row of the maxilla, G 123, scale bar 2 mm; i: microsculpture formed by elliptical proximo-distally elongated protuberances on the large conical tooth, G 123, scale bar 100 µm; j: large conical tooth from the internal row of the maxilla, G 123, scale bar 2 mm; k: microsculpture formed by elliptical proximo-distally elongated protuberances on the large conical tooth, G 123, scale bar 100 µm.

Text-fig. 9. Progyrolepis heyleri POPLIN, 1999. a: right dentalosplenial of adult specimen in lateral view, GMC 1, scale bar 5 mm; b: left dentalospelenial and suboperculum in medial view, GMC 1, scale bar 5 mm; c: set of bones of the right side of the cheek displaying maxilla, preoperculum, hyomandibula, left and right ceratohyal, epibranchial and neural spine from the axial skeleton, G 123, scale bar 5 mm; d: drawing of the right frontal in dorsal view, GMC 81, scale bar 5 mm; e: right operculum in lateral view, GMC 10, scale bar 5 mm. Abbreviations: Cbr – ceratobranchial, Cer – ceratohyal, Ds – dorsal spine, Hy – hyomandibula, Md – mandible, Mx – maxilla, Op – operculum, Pop – preoperculum.