Ammonoids of the Oxynotum Zone and Raricostatum Zone (Densinodulum Subzone) of Sinemurian, Lower Jurassic, in the Asturian and Lusitanian basins

ABSTRACT This research is focused on the ammonoids of Oxynotum Zone and Raricostatum Zone (Densinodulum Subzone) from Asturian (Spain) and Lusitanian (Portugal) basins. They come from expanded sections from where 216 specimens have been collected. These have been classified in eight genera and 15 species that belong to families Echioceratidae, Oxynoticeratidae and Eoderoceratidae. Most of the ammonoids are typical of the Northwest European Province. Echioceratidae is the most frequent in the Asturian Basin, whereas Oxynoticeratidae is the predominant in the Lusitanian Basin. Eoderoceratidae specimens are scarcer, and they are registered only between the middle part of Oxynotum Subzone and the lower part of Densinodulum Subzone. A new species is proposed for the upper part of Oxynotum Subzone of the Lusitanian Basin, Plesechioceras rochai sp. nov., which is relatively frequent. As well, also in Portugal, Cheltonia sp. is registered in Densinodulum Subzone, which is a form different from Cheltonia accipitris. This provides new evidence to support the hypothesis of the relation micro-macroconch between the genera Oxynoticeras (M) and Cheltonia (m). Remarkable differences in the studied area are the presence of Paroxynoticeras salisburgense and Plesechioceras cf. delicatum in the Asturias Basin, and Crucilobiceras densinodulum, associated to Oxynoticeras lymense, in the Lusitanian Basin. https://zoobank.org/NomenclaturalActs/FEF11B83-03CA-4BD6-BB15-F3E9926B7C4F


Introduction
Echioceratids, Oxynoticeratids and Eoderoceratids species registered in NW Iberian are studied.They are recorded from the interval between the uppermost part of Obtusum Zone (Denotatus Subzone), and the lowermost part of Raricostatum Zone (Densinodulum Subzone).In the NW European Province, these zones, namely Oxynotum and lowermost part of Raricostatum, are recorded so far mainly in two areas of reference: on the one hand, Robin Hood's Bay and Dorset Coast in Britain (Dean et al. 1961;Page 1992;Howarth 2002), and on the other hand, Burgundy in France (Mouterde 1971;Corna et al. 1997a).Dommergues et al. (1994) proposed a correlation between these two areas, and Page (2003) synthesised the horizons for the NW European Province.The base of Oxynotum Zone, Simpsoni Subzone, is correlated with the first occurrence of Gagaticeras, and that of the Oxynotum Subzone with the first occurrence of Oxynoticeras grp.oxynotum (Dommergues et al. 1994).However, it remains uncertainty in the correlations between some horizons within the subzones.
This study is focused in the Asturian and Lusitanian basins, due to their expanded sections and their high stratigraphical control in the collection of ammonoids.Recently, Comas-Rengifo et al. (2021) have proposed a correlation between the Asturian Basin (AB) and Western Tethys basins, showing a high affinity with Robin Hood's Bay and Burgundy, although with the presence of some characteristically taxa from Mediterranean Province.It is also noted the difference with the Lusitanian Basin (LB).Therefore, the main goal of this work is to describe and compare the ammonoid between the AB and LB, reviewing the material and studying new one.A correlation with the standard zonation for the NW European Province is made.

Geological settings and studied sections
Asturias shows Jurassic outcrops almost in continuous way along the coast between Torres Cape (Gijón) and Arra Beach near of Ribadesella (García- Ramos 2010).The Oxynotum Zone of the Asturian Basin is mainly represented in the most expanded sections of Peñarrubia (PÑ) and East Rodiles (ER) (Comas-Rengifo et al. 2021), which are located in the central-eastern Asturian coast (Figure 1A, B).In the LB, the best and continuous succession is observed near São Pedro de Moel, at W of Marinha Grande, in the Polvoeira (PO) -Pedra do Ouro (OU) beaches (Figure 1A, C).In the studied sections, the relevant discontinuities identified with the available biochronostratigraphy, always encompass less than one subchronozone.
The sediments of the upper Sinemurian in the AB belong to Rodiles Formation (Fm), Buerres Member (Mb) (Valenzuela et al. 1986) (Figure 1D).The units are expanded, show few significant discontinuities, and are constituted by alternating limestones and grey marls that are arranged in asymmetrical transgressive-regressive parasequences with variable thickness with a nodular appearance (Comas-Rengifo et al. 2010).The thickness increases from East to West, as a result of a deeping of the basin, therefore the maximum is established in PÑ (Figure 2B) where all the chronostratigraphic limits could be characterised with different degrees of precision.In this section, Comas-Rengifo et al. (2021) have attributed at least 25 m thickness for the Obtusum Zone and more than 24 m for the Oxynotum Zone, although the upper part of this zone has not been possible to study completely.The ER section (Figure 2A) has been studied in detail by Comas-Rengifo et al. (2010,2021), who attribute approximately 20 m thickness for the Oxynotum Zone.Several ammonoid assemblages allow to characterise the Simpsoni and Oxynotum subzones.However, the lower limit of the Raricostatum Zone was stablished with doubts, as above Gleviceras doris (Reynès) there is an interval of several meters of thickness with no register of typical basal Raricostatum Zone ammonoids.Ammonoids from the Oxynotum-Raricostatum boundary are scarce in both sections.The record is discontinuous, the preservation is not good, and they are often incomplete or fragmented, almost always corresponding to inner moulds with a similar lithology than the matrix where they are preserved.There is no evidence of reelaboration.The fossil record is completed by frequent and varied ichnofossils, mainly in PÑ section.It is possible to find a particular level of Rhizocorallium in both sections, ER and PÑ, which can be used as a marker for the correlation between the different sections of the AB.
The S. Pedro de Moel area, located in the westernmost part of the Lusitanian Basin (Figure 1A), shows an important record of the Lower Jurassic, that includes the most continuous succession of the Portuguese Sinemurian (Mouterde 1967a;Duarte 2004;Azerêdo et al. 2010;Duarte et al. 2014aDuarte et al. , 2022)).In this part of the basin, the Sinemurian is recorded of a thick carbonate succession of around 200 m in thickness, showing a transition between shallow-water carbonates to hemipelagic deposits, characterised by organic-rich marl-limestone alternations, a kind of sedimentation particularly well developed in the basin during the Pliensbachian (Dommergues et al. 2004(Dommergues et al. , 2010;;Azerêdo et al. 2010;Duarte et al. 2010Duarte et al. , 2014aDuarte et al. , 2022;;Sêco et al. 2018).
The PO-OU section, located in the southernmost São Pedro de Moel area (Figure 1C; coordinates: 39°43'8.74"N;9°3'0.10"W),involves the uppermost part of the Coimbra Fm and the great part of the Polvoeira Mb of the Água de Madeiros Fm (Duarte and Soares 2002) (Figure 1D).The first ~13.5 m (Figure 2C) belong to the unit H of the Coimbra Fm (Duarte et al. 2014b(Duarte et al. , 2022)), that is composed of bioturbated decimetric micritic limestones/centimetric marl alternations, particularly enriched in benthic macrofauna, mainly brachiopods, and bivalve and gastropod mollusks (Paredes et al. 2013a(Paredes et al. , 2013b(Paredes et al. , 2016;;Vitón et al. 2020a).The remaining part of the studied section (~16.5 m) corresponds to the base of the Polvoeira Mb of the Água de Madeiros Fm (Figure 2D).In relation to the top of the Coimbra Fm, the base of the Polvoeira Mb is characterised by an increase in the thickness of the marly levels, as well as a considerable increase in its organic content, through the occurrence of black shale facies (Duarte et al. 2012).The paleontological content of benthic nature remains basically the same as at the top of the Coimbra Fm (Duarte et al. 2014a).

Materials and methods
A total of 216 ammonites specimens have been studied and identified at genus or species level.Namely, 51 are from AB, and 165 from LB.Most of these come from the field collection made with a detailed stratigraphic work, bed-by-bed, by Complutense University of Madrid, Coimbra University and Jurassic Museum of Asturias (MUJA) researchers in different field works from 2010 to 2022.Particularly in the LB, this longterm work has allowed the stratigraphic information to be completed as it has been possible to access different intervals of the section.For this reason, although PO-OU is a continuous section, there are some apparent discontinuities in the numbering.Therefore, PO134 and OU4 are correlative layers, as are OU116 and OU151.
Each specimen is identified with a catalogue number that is composed of the initials of the outcrop (PO: Polvoeira; OU: Pedra do Ouro; PÑ: Peñarrubia; ER: Rodiles East) accompanied by the number of the level (Figures 3 and 4), follow by the specimen/ sample number.The specimens from LB are kept at the Department of Earth Sciences of the Coimbra University; the ones from the AB are housed at Jurassic Museum of Asturias (MUJA).Some of the specimens herein studied are identified with those letters, followed by the collection number, as they were kept there at the moment of the research work.
At the base of PO-OU section (Coimbra Fm), three specimens were collected from the Oxynotum Zone levels.For this interval, the material of Coimbra Fm is extremely calcareous (shallow-water facies) and the exposure of the sediments is reduced, therefore the occurrence of ammonites is rare.However, there are other fossil groups more frequent such as brachiopods and bivalves.In the Polvoeira Mb a bigger number of specimens was collected, which moreover are generally well preserved.Although due to diagenetic compaction, the ventral view is rarely visible and the suture lines are hardly observable.This taphorecord is characteristic of sediments from deep subtidal environment (Fernández-López 1997).
In Asturias, in the PÑ and ER sections, the presence of fossils is scarcer than in LB.Some of them are pyritised and others used to be inner moulds of incomplete phragmocones.This material was collected and figured by Comas-Rengifo et al. (2021).
In the present paper, we use the palaeontological terminology adopted by Arkell et al. (1957), and follow the measurements proposed by Lukeneder and Lukeneder (2018, Figure 5).The table with the measurements can be found in the Supplemental Data.All of them are given in millimetres, taken to 0.1 mm accuracy with the use of a calliper (Vogel Germany-Electronic Digital Calliper) when possible, and when not, on digital images using the software Adobe Photoshop CS6 version 13.0.1.Biplot analysis and figures have been made with the software PAST 4.09 version (Hammer et al. 2001).

Systematic palaeontology
The ranges of ammonoid species registered in the central area of AB, where are located both outcrops, Rodiles East (ER) and Peñarrubia (PÑ), is shown in Figure 3.The species registered in the section of S. Pedro de Moel (PO-OU) are shown in Figure 4.
In both cases, in the Oxynotum Zone, it has been able to characterise the Simpsoni Subzone by the presence of Gagaticeras gagateum (Young & Bird) and Oxynoticeras simpsoni (Simpson), and the Oxynotum Subzone by the presence of the index species, Bifericeras bifer (Quenstedt) and Gleviceras doris (Reynès).
In the Raricostatum Zone (Densinodulum Subzone), the differences are significantly emphasised.In the AB, where the sedimentary rate is relatively high (Comas-Rengifo et al. 2010), the register is very scarce and, above G. doris, only it has been possible to identify Plesechioceras cf.delicatum, followed of Plesechioceras gr.edmundi-viticola at less than 4 m below the first level that contains Echioceras, namely E. rhodanicum Buckman.In the LB, above the last O. oxynotum (Quenstedt) associated to Plesechioceras, at 2.8 m Oxynoticeras (M)-Cheltonia (m), Gleviceras subguibalianum (Pia), Crucilobiceras densinodulum Buckman and Oxynoticeras lymense (Wright) are found, followed by E. rhodanicum and E. gr.raricostatoides (Vadasz) at little more than 1 m.
The list of specimens considered in the sub-chapters of Material, with biometric information, is given in the Supplemental Data.

Description
Generally serpenticones, low densely ribbed, with rectiradiate, blunt and stout ribs, which pass over the venter neither weakening nor forming chevrons.Absence of keel.Subcircular section.PO62t is a pyritised fragmented specimen, which preserves partial last whorl.Specimens from ER and PÑ sections are moulds, and ER60.6 preserves a fragment of two whorls where it is possible to define the umbilical wall as quite rounded, but with a deep umbilicus.

Remarks
Despite of the incompleteness of the specimens, the ribbing and coiling pattern allow to assign them certainly to G. gagateum.
The specimens described and figured by Dommergues et al. (2005) as Gagaticeras grp.gagateum show chevrons in the ventral area.This feature put them closer to G. finitimum or to G. neglectum than to G. gagateum.For this same reason, Wright (1882) included, with doubts, their specimens in this species.Bernard and Martínez (1996) cite and figure a specimen of G. gagateum, that belongs to Museo Geominero (Spain).However, according to the label, it is located in Velez Rubio (Almeria), in the Betic Range, which is unlikely because that locality is in Mediterranean Province, where there are no other references or citations of Gagaticeras genus, which is thought to be exclusive of northwestern Europe (Howarth 2013).It is more likely that there has been a confusion with the label.

Genus Plesechioceras Trueman & Williams 1925
Remarks Plesechioceras has a similar morphology to 'Paltechioceras' form but from lower horizons (Getty 1973;Howarth 2013).Getty (1973) separates Orthechioceras from Paltechioceras, in opposition to Arkell et al. (1957), because, among other reasons, Orthechioceras species are found 'below Echioceras at much lower horizon than Paltechioceras' (Getty 1973, p.23).However, he considers Plesechioceras as synonym of Paltechioceras, noting that the differences in not attain a sulcate condition, the unusually dense ribbing and slender whorls, are not enough to consider the separation at generic level.According to Dommergues (1993) and Meister et al. (2021), and with the evidence of the specimens belonging to this genus that we find in both studied basins, herein we support the appearance of Plesechioceras at the top of Oxynotum Zone, below the first Echioceras and at a stratigraphic position much lower than Paltechioceras occurrence.Therefore, adding to the morphological differences, we consider that the stratigraphic position reinforces the differentiation of Plesechioceras as an independent genus, as Howarth (2013)

Diagnosis
Serpenticone quite evolute, with a proportional growth of the umbilicus in relation to the diameter (Figure 5A); quite densely ribbed, with 33 ribs in the half last whorl at diameter of 35.3 mm; rectiradiate ribs, or sometimes strongly prorsiradiates; first whorls smooth, but soon their flanks start to be ribbed, and it preserves the rib pattern along the whole growth.

Description
The holotype is 35.3 mm in diameter, and the paratype 42.6 mm.Among the specimens collected there are diversity in size, with diameters between 12.0ca mm (OU83b.12.1) and 58.4 mm (OU77.12)(Figure 5).The paratype OU83b.10 shows the rib pattern of rectiradiate ribs with some prorsiradiates mainly in the outer whorl.In half last whorl, there are 34/35 ribs for the biggest diameters.The high rib density quite homogenous along the growth differentiates it from Plesechioceras platypleura, the other Plesechioceras described in the Oxynotum Zone.Most of the specimens are inner moulds crushed, but relatively well preserved.OU77.12 and OU83.13 (Figure 8U) specimens present a fragment of the external whorl with volume, as inner mould.Although ventral area is not clearly visible neither here, it allows to deduce a subcircular-oval section with keel, with an involution of around 10% over the previous whorl.Howarth (2013), following Dommergues (1982), establishes Plesechioceras as a genus from Densinodulum Subzone.However, Dommergues (1993) describes Plesechioceras platypleura as new species for the Oxynotum Subzone of Burgundy (France), namely in the outcrops of Dezize-les-Maranges (type locality), Jalogny, Nolay and Arnay-sous-Vitteaux.This description is based on 10 specimens, of which the two figured ones, including the holotype, have ferruginous oolites.The species is characterised by the last whorl smooth, but in the same publication, Dommergues (1993: pl. 4, figure 4) figured a specimen of Plesechioceras aff.platypleura completely ribbed.Following this figuration, Comas-Rengifo et al. (2013) and Duarte et al. (2014a) cited some specimens in similar stratigraphic position (Oxynotum Subzone, level OU77) classified with doubts as P. cf. platypleura.New data collected at that OU level and at OU83 have allowed to confirm that these specimens should represent a new species: Plesechioceras rochai.P. rochai ribbing pattern is similar to the inner whorls of P. platypleura and P. delicatum (Dommergues, 1993), but P. rochai is more densely ribbed and it is definitively different in the outer whorls, which are smooth in P. platypleura but completely ribbed in P. rochai.The U/D and Wh/D rate is similar in both species (Figure 5) as it can be expected due to their closely relationship.In the rib density and evolute rate, P. rochai is closer to P. delicatum, which justifies its assignation to Plesechioceras genus.P. rochai differs from P. delicatum in the geometry of the ribs, that are straight, or sometimes strongly prorsiradiates, whereas P. delicatum has concave ones.Moreover, P. delicatum has been described in the base of Densinodulum Subzone, whereas these specimens are in lower stratigraphic level, in Oxynotum Zone, accompanied in the same levels by Oxynoticeras oxynotum.Although Dommergues (1993) consider that P. delicatum can be present at the top of Oxynotum Zone, Blau and Meister (2000) and Page (2003) consider Delicatum as the first horizon of Densinodulum Subzone.

Remarks
Plesechioceras delicatum is only described in France (Dumortier 1867; Dommergues 1982Dommergues , 1993)).Also, Buckman (1927, pl. 694) renamed as Plesechioceras typus the specimen figured by Trueman and Williams (1925) as P. delicatum, although Dommergues (1993) considers this P. typus as synonym of P. delicatum.Schlatter (1987) describes and figures Plesechioceras sp.nov., but being only one specimen and without the stratigraphic information, it is difficult to tell even if it really is a Plesechioceras.
Without doubt, these forms, namely P. delicatum, P. typus and P. rochai, are difficult to differentiate from each other, because they are certainly very closed in the evolutionary line from Palaeoechioceras to Paltechioceras (Guex et al. 2008).

Description
Incomplete phragmocones, where specimens PO100.1 and PO100.2 are clearly visible as wholly septate.Oxycone with an oval section, where the widest part is below the half of the height of the whorl.Ventral area fastigiate, finishing quite sharp.Straight ribs slightly marked in dorsal area that fade towards ventral area and along growth, therefore they are visible mainly in the first half of the last whorl.All the specimens are small size, as those figures by Howarth (2002, pl. 4, figure 5) or by Corna et al. (1997a, pl. 4, figure 12).The ornamentation fits with the figuration of Howarth (2002, pl. 4, figure 5).

Description
Oxycones with U/D between 11.09% (OU83b.17)and 16.92% (OU83b.11.2).Around 24% of the last whorl height embraces the previous one.The three specimens from level OU50 are preserved as inner moulds.The specimens from levels OU77 and OU83, much more numerous, are smaller and with a morphology that resembles to Cheltonia, but with bigger diameter and lower U/D rate.The specimens are crushed.However, the specimen OU77.3 preserves some volume.In this mould, very feeble falcoid ribs can be observed at certain angle.The flank turns in high angle as umbilical wall, although it is not vertical, the maximum width of the whorl is in the dorsal area, from where it becomes narrower towards ventral area, resulting in a fastigiate cross section finishing in a sharp ventral zone, which in these crushed specimens could be confused with a keel due to the relief of the siphuncle.MUJA2046 was figured by Suárez Vega (1974).Pompeckj (1906) described the species Oxynoticeras choffati referring to 'Oxynoticeras cf. and aff.guibalianus' by Choffat (1903).These specimens come from Monte de Vera, Telhadella and Palheira, that are localities on the outskirts of Coimbra which currently are hardly accessible (Vitón et al. 2020a(Vitón et al. , 2020b)).Moreover, Choffat (1903) noted that the level with 'Oxynoticeras cf.guibalianus' is not registered in Polvoeira, where the only ammonite he found is O. oxynotum.Because of the difficulty of confirming the stratigraphic position of the levels with O. choffati, it is not possible to stablish if it is a synonym for O. oxynotum or not.Even more when Pompeckj (1906) recognises that 'O.insigillatum' is the closest form to O. choffati, and Pia (1914) that it is also closed to 'O. sphenonotum'.All of them are specimens from Jamesoni Zone.

Description
Oxycone with an estimated diameter up to 65 mm.It has fine ribs that are stronger in dorsal area and fade towards ventral-lateral zone, but not completely disappear, as they are visible in the specimen OU129m.1.They are slightly falcoid, having the flexion at the middle of the height of the whorl, just above where they start to fade out.The umbilicus is small, around 10% of the maximum diameter.Inner whorls and ventral area are not visible in these specimens.
The rib pattern differs a bit from the lectotype (NHM C2225), in the specimens from OU it is possible to differentiate a bundle of striae at the middle of the height of the whorl, where the flexion happens, giving the appearance of composed ribs.

Remarks
Howarth (2013) establishes 20-30% of U/D ratio as characteristic for Oxynoticeras genus.According to this, the U/D ratio for O. lymense fits better with Radstockiceras genus.As Meister and Schlögl (2019) point out, this species could be an intermediate between those two genera.It can be observed in the chronostratigraphic sequence, the oxycones morphologies tend to reduce their umbilicus in the evolutionary line.As the external morphology resembles to Oxynoticeras oxynotum, we consider keeping the species in this genus.
There are 13 specimens (OU158.1,OU159.1,OU160.3,OU164.2.1,OU164.7,OU165b.1,OU165b.2.2,OU165b.3,OU167b.6,OU167b.10,OU167m.6) that are completely smooth, although with similar U/D rates than the other specimens herein indicated that preserve the ornamentation (Figure 7).Moreover, most of them were found in the same stratigraphic levels.Because of that we grouped them together in the species O. lymense.It is possible that this differentiation is given because of taphonomical reasons.The specimen OU164.9 preserves part of the conch with the typical falcoid ribs, but where it is broken, the surface is smooth in a similar way that those specimens.Furthermore, Meister and Schlögl (2019) points out that in the adult morphology, O. lymense becomes smooth.However, this description does not fit with our specimens, as the bigger ones are those that precisely better preserve the ornamentation.

Description
Oxycones from 17.5 mm (ER68.3) to 27.9 mm (OU83b.21).The apertural border is projected forward.Ribs can be observed in ER specimens, they are straight and strong in the dorsal area and fade out towards ventral area.However, specimens from OU are smooth.ER.68.3 is the specimen most completed.It has a compressed subrectangular whorl section.Last whorl opens up, getting an increasing U/D rate along the growth.The peristome is projected forward.In the last half whorl there are eight ribs slightly prorsiradiate and stout that fade out through the ventrolateral area, but they get stronger again in the outer margin, crossing the ventral area and leaving a crenulated form.Inner whorls are smooth.
Ventral area is flattened with a low keel.

Remarks
All the specimens differ from the lectotype of Cheltonia accipitris (Howarth, 2013, figure 23.2a-b).On the one hand, these specimens do not preserve neither the ventral rostrum nor the corrugations on venter.On the other hand, the lectotype has a bigger U/D rate.However, Simms & Edmunds (2021, figure 5h) figure a specimen of Cheltonia accipitris that resembles to the lectotype and where the suture lines are visible.Taking the measures from the end of the phragmocone, they resemble to ER specimens, specially ER68.3 has a very similar U/D rate.Therefore, ER specimens can be considered as incomplete phragmocones of Cheltonia accipitris.However, OU specimens appear to be completely smooth and have smaller umbilicus, in fact the U/D rate fall below the 30% that Howarth (2013) marks as the lower limit for Cheltonia genus, being closer to those values of Oxynoticeras.This can be considered as intraspecific variability links to regional differences, as they are in the same stratigraphic position and there are no other oxynoticeratids of small size described in previous works.Moreover, Dommergues (1993) points out that these intermediate forms precisely suggest the close relationship between Cheltonia and Oxynoticeras.
The morphology resembles to O. oxynotum, but with smaller diameters.Some authors (Dommergues 1993;Howarth 2013;Simms and Edmunds 2021) consider Cheltonia accipitris as microconch of O. oxynotum.As shown in Figure 6, certainly these specimens assigned to Cheltonia could be the microconchs or juveniles of Oxynoticeras.Specimens from Oxynotum subzone have not an overlay in the diameter measurements, the biggest Cheltonia accipitris is OU83.26(27.1 mm) and the smallest Oxynoticeras oxynotum is OU77s.2(29.3 mm).However, there is an overlay in the specimens from Densinodulum subzone, but those clearly differ in the U/D rate (Figure 7).

Description
Fragments of phragmocones, with numerous falcoid ribs.In OU67.14 some weaker ones are observed between the stronger ribs.Neither the ventral area, nor the inner whorls are preserved.OU77.5 (Figure 8O) is an external mould, that after doing a latex positive mould, seems to show inner whorls smooth.

Remarks
In Reynès (1879), this species is figured with secondary ribs intercalated that fade out through the dorsal area, which result in the counting of double number of ribs in the ventral area than in the dorsal one.However, Dommergues (1993) pointed out that some of their specimens have few or no intercalations or bifurcations at all.

Description
Oxycone with very strong falcoid ribs.Halfway up the whorl secondary ribs develop towards the venter following the same falcoid pattern than the primary ribs.As a result, it can be counted approximately the double ribs in the ventral area than in the dorsal one.Z value included in the table corresponds to the count in half whorl in the dorsal area.Presence of keel.No specimen preserved the inner whorls.

Description
Evolute specimens, preserved as inner moulds where the two rows of tubercles, characteristic of this species, are observed, one central and another in the venter-lateral area.Specimens from PO-OU section are eroded, being preserved the most prominent parts, which are indeed the tubercles.Therefore, there are no information about the cross section or possible ribs over which the tubercles are developed.However, PÑ290 is a pyritised whorl (probably a phragmocone) fragment, less than half whorl, where all the features are clearly visible.In the lateral view, prorsiradiate ribs have on their both ends, umbilical and ventrolateral, pointed tubercles.Ventral area is slightly curved and crossed by thin striae that connect parallel tubercles from both ventrolateral sides.Cross section is quite subquadrate, as the flanks tend to be parallels.ER141 is an external mould of a small whorl fragment, being only visible the ventrolateral area and six ribs.The latex mould reveals very prorsiradiate ribs with sharp end, that could be taken as tubercles.Although the umbilical tubercle row is not preserved, it can be tentatively assigned to Bifericeras cf.bifer.

Additional information
PÑ282 and PÑ290 were collected and figured by Comas-Rengifo et al. (2021).

Description
Bituberculated serpenticones, although it is sometimes apparently unituberculated because the umbilical row is poorly developed and sometimes missing.Both rows of tubercles are better preserved in OU128.3.The main tubercles are placed on the ventro-lateral area over ribs sometimes hard to distinguish because of the state of preservation.

Oxynotum Zone
The lower limit of this zone can be marked with the first appearance of Gagaticeras in both studied basins.In the AB, G. cf.exortum is registered and followed immediately by G. gagateum (Comas-Rengifo et al. 2010, 2021).In the first levels of this zone, species of Eparietites remain, such as E. impendens and E. glaber.In the LB, G. gagateum is registered (Mouterde and Rocha 1972).In the first levels of this zone, Eparietites specimens are scarce.
The ammonoid record obtained (Figures 3, 4) allows us to subdivide this area into the Oxynotum and Simpsoni subzones and to recognise the chronostratigraphic succession detailed herein.
The Simpsoni Subzone is characterised by the presence of the index species and by G. gagateum in the N and NW of Iberia.In the AB, Palaeoechioceras sp. is also recorded.
In Oxynotum Subzone, the lower boundary is marked by the first appearance of O. oxynotum.The succession obtained is very similar in all the research area: O. oxynotum -B.bifer -G.doris.As the most significant peculiarities, on the one hand, in the AB, between O. oxynotum and G. doris, P. salisburgense has been found (Comas-Rengifo et al. 2010), which is a typical species of the 'Lienz Dolomiten Alps' (Blau and Meister 2000;Meister et al. 2017).On the other hand, specimens of Plesechioceras that have morphological similitudes with P. delicatum are found in the LB, where they have been considered as a new species.This species is registered in the levels with O. oxynotum, C. accipitris, G. doris and a specimen badly preserved of Eoderoceras sp.

Raricostatum Zone
The lower limit is uncertain in both basins.In the AB, it is located between the layer ER150 with Gleviceras doris and the layer ER198 with P. cf. delicatum (Figure 3A) that characterises the Densinodulum Subzone (Delicatum Horizon); afterwards, P. cf. edmundi is registered, before the first Echioceras of the group of E. raricostatum, that could be associated to E. rhodanicum.In the LB, this limit is located between the layer OU83 with the presence of P. rochai and the layer OU162 with G. subguibalianum (Figure 4).Immediately above, the latter species are associated to C. densinodulum and O. lymense, that characterise the Densinodulum Subzone (Delicatum Horizon); these species are followed by E. rhodanicum and E. gr.raricostatoides, that are associated to G. subguibalianum, which characterise the lower part of the Raricostatum Subzone (Comas-Rengifo et al. 2013;Duarte et al. 2014a).

Discussion
The Asturian and Lusitanian basins, as being located on the Atlantic edge of the Iberian Peninsula, include the successions of ammonoid genera and species that are essentially very similar to the assemblages of the Northwest European Province (Figure 9).Nevertheless, although they are located relatively close to each other (less than 600 km; Figure 1), several significative differences have been noted.In the AB, the Echioceratidae specimens are more frequent than Oxynoticeratidae ones, and they are mainly registered in the lower part of the Simpsoni Subzone and, occasionally, in Densinodulum Subzone (Suárez Vega 1974;Comas-Rengifo et al. 2010, 2021;Comas-Rengifo and Goy 2010).However, oxynoticeratids are the most frequent in the LB.They are registered in the middle part of the Oxynotum Subzone and the lower part of the Densinodulum Subzone (Mouterde 1967a;Mouterde and Rocha 1981;Dommergues and Mouterde 1987;Dommergues et al. 2004Dommergues et al. , 2010;;Comas-Rengifo et al. 2013;Duarte et al. 2014aDuarte et al. , 2022)).Eoderoceratidae specimens are scarcer, and they are registered between the middle part of the Oxynotum Subzone and, in the LB, in the lower part of the Densinodulum Subzone (Comas-Rengifo et al. 2013, 2021).
In the PO-OU section the access is easier than in ER and PÑ, moreover the layers in the section of the LB have bigger surface of exposition, therefore it can exist a bias in the sampling.But leaving it aside, the main differences that can be deduced from the Results chapter (Biochronostratigraphy section) are: a) among the species of Echioceratidae, Palaeoechioceras is registered in the Simpsoni Subzone of the AB, and Plesechioceras rochai in the upper part of the Oxynotum Subzone in LB; b) among the Oxynoticeratidae, Paroxynoticeras salisburgense is registered in the AB, and this species is common in the Australpine area of the Mediterranean Province (Blau and Meister 2000;Meister et al. 2017), where it defines the middle part of the Oxynotum Subzone; c) very few Plesechioceras are known in the Raricostatum Zone (Densinodulum Subzone) of the AB, and Oxynoticeras lymense, together with Crucilobiceras densinodulum, in the LB.
Among the O. lymense specimens, there are 13 that do not present the characteristic ornamentation.On the contrary, they are smooth.However, the similar measurements, and the presence of specimens with part of the conch also smooth due to breakage, support that all these specimens belong to O. lymense, and that this difference in the ornamentation could be a taphonomical matter.Although it must be confirmed with new material in the future.On the other hand, Cheltonia sp.specimens are registered in these same levels.They are slightly smaller in average than typical C. accipitris, and they also show slight morphological differences as lower U/D rate.In the same way that C. accipitris can be considered the microconch of O. oxynotum (Dommergues 1993;Howarth 2013;  The new species, Plesechioceras rochai, occurs in the upper part of the Oxynotum Subzone and differs from P. platypleura in the continuous ornamentation throughout growth.On the other hand, it differs from P. delicatum in the geometry of the ribs and in the stratigraphic position, while P. rochai is associated with O. oxynotum, P. delicatum is registered at the lower part of Densinodulum Subzone.

Conclusions
The detailed study of the sections of ER and PÑ in the AB, and the PO-OU section in the LB has allowed to improve the knowledge of the ammonoid assemblages in NW Iberia between the Oxynotum (Simpsoni Subzone) and Raricostatum (Densinodulum Subzone) zones.The horizons recognised in both basins are well correlated with each other and with those defined for the Northwest European Province and only occasionally with those of the Mediterranean Province.
Gagaticeras gagateum, Oxynoticeras simpsoni, O. oxynotum, Cheltonia accipitris, Gleviceras doris and Bifericeras bifer are common species in AB and LB, and moreover, they are common with other basins in both Great Britain and France.The main difference in the Oxynotum Zone is the presence of Paroxynoticeras salisburgense in the middle part of the zone in AB, and that of Plesechioceras rochai in the middle/upper part of the zone in the LB.In Raricostatum Zone, the difference is the presence of Plesechioceras cf.delicatum in the AB and that of Crucilobiceras densinodulum and Oxynoticeras lymense in the LB.
The new species described, Plesechioceras rochai, occurs in the upper part of the Oxynotum Subzone, likely as an endemism of the LB.Also in the LB, but in the Densinodulum Subzone, a species of Cheltonia, similar to C. accipitris from the Oxynotum Subzone, has been recognised.It must be reviewed when new material is available.
) are the presence of G. gagateum and O. simpsoni in Simpsoni Subzone, O. oxynotum (associated to Cheltonia spp.and B. bifer) in the lower and middle part of Oxynotum Subzone, and Gleviceras spp. in the transition between Oxynotum and Raricostatum zones.The registered assemblages are similar to the rest of the cited subzone, with the exception of the lowermost levels of Densinodulum Subzone.In other Northern Iberian Peninsula basins, such as Basque-Cantabrian and Iberian ones, the ammonoids are in general very scarce or almost non-existent in the studied interval.The presence of Palaeoechioceas, Gagaticeras and C. accipitris in the Basque-Cantabrian Basin (Braga et al. 1988) allow to correlate the lower part of the Simpsoni Subzone relatively well.In the Iberian Range, only in the transgressive episode of the cycle LJ2-1 (Gómez and Goy 2005, figure
Simms and Edmunds 2021), Cheltonia sp. could be the microconch of O. lymense.It would reinforce the hypothesis of the relation micro/macroconch between Cheltonia and Oxynoticeras genera.
(Young & Bird 1828)cones quite evolute.Rounded to depressed whorl section.Ribs are rursiradiate and cross the ventral area, where a feeble keel is observed.It is well preserved in ER58.3, although it is a small fragment of phragmocone.ER58.4 is a fragment of three ribs.The most complete are ER58.1 and ER58.2.ER58.1 is a slightly deformed specimen, whereas ER58.2 is an external mould.MUJA4711 is from La Llastra, a deformed pyritised inner whorls broken through a suture line.Finally, PÑ specimens are quite complete where most of lateral-view characteristics are observed.Complementary informationER58.1 and ER58.2 are figured by Comas-Rengifo et al.(2021,  figure9f, g).Gagaticeras gagateum(Young & Bird 1828)(Figure8A-E)