Anacheirurus Reed, 1896

Pérez-Peris, Francesc, Laibl, Lukáš, Vidal, Muriel & Daley, Allison C., 2021, Systematics, morphology, and appendages of an Early Ordovician pilekiine trilobite Anacheirurus from Fezouata Shale and the early diversification of Cheiruridae, Acta Palaeontologica Polonica 66 (4), pp. 857-877 : 860-869

publication ID	https://doi.org/ 10.4202/app.00902.2021
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scientific name	Anacheirurus Reed, 1896
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Genus Anacheirurus Reed, 1896 View in CoL

Type species: Cheirurus (Eccoptochile) frederici Salter, 1864 ; Tremadocian Slates ; Wales, upper Tremadocian, Upper Ordovician .

Species included: Anacheirurus frederici ( Salter, 1864) ; Anacheirurus adserai ( Vela and Corbacho, 2007) ; Anacheirurus nanus ( Mergl, 1984) ; Anacheirurus bohemicus ( Růžička, 1926) ; Anacheirurus ? plutonis Bulman and Rushton, 1973; Anacheirurus ? atecae ( Hammann, 1971); Anacheirurus? sougyi (Destombes in Destombes et al., 1969).

Diagnosis.—A genus of Pilekiinae with glabella sub-quadratic to sub-oval in shape, with smooth surface; pleura deeply furrowed, with posterior band extended distally in a spine of variable length; posterior tergites of the trunk bear elongated spines directed backward; pygidium with two to three axial rings, and with one to three pair of pygidial spines; pygidial pleurae furrowed with a deep furrow as in the thoracic pleurae, pygidial spines oriented mainly backward.

Remarks.— Anacheirurus has been recognized as bearing three pairs of pygidial spines ( Bulman and Rushton 1973; Fortey 1980; Peng 1990; Mergl 2006). However, the type species of the genus, Anacheirurus frederici , probably bears only two pairs of pygidial pleural spines ( Lane 1971). Anacheirurus adserai , a species otherwise morphologically nearly identical to A. frederici , bears only one pair of pygidial pleural spines. The number of pleural spines allocated within pygidium seems to be a highly variable character in early members of Cheiruridae , and consequently of little use when defining genera or even higher taxa. On the other hand, both A. frederici and A. adserai share the same overall morphology of the trunk, with the last seven pairs of spines elongated. Considering the uniqueness of this character within Pilekiinae and morphological similarity between these two species, it seems likely they are both form a monophyletic group. For that reason, we suggest using these elongated posterior spines as a diagnostic character of Anacheirurus , being aware this might restrict the original concept of the genus (and consequently the number of species assigned to it). Except for A. frederici and A. adserai , the remaining species assigned to Anacheirurus lack the thoracic information. We decided to keep them within the genus until the thoracic information is available.

Anacheirurus adserai ( Vela and Corbacho, 2007) Figs. 1 View Fig , 2A, B View Fig , 3–7 View Fig View Fig View Fig View Fig View Fig .

?1969 Parapilekia sougyi n. sp.; Destombes in Destombes et al. 1969: 192, text-fig. 6, pl. 4: 7–11.

1985 Parapilekia sp. ; Destombes in Destombes et al. 1985: 189.

2006 Parapilekia sp. ; Destombes 2006: pl. 2: 5.

2007 Anacheirurus frederici ( Salter, 1864) View in CoL ; Vela 2007: 27.

2007 Lehua adserai n. sp.; Vela and Corbacho 2007: text-figs. 3–6.

2007 Lehua corbachoi n. sp.; Vela 2007: 28, text-figs. 2, 3.

2007 Lehua colli n. sp.; Vela 2007: 29, text-fig. 4.

2007 Lehua ponti n. sp.; Vela 2007: 30, text-figs. 5–7.

2008 Anacheirurus frederici ( Salter, 1864) View in CoL ; Corbacho 2008: 4.

2008 Lehua adserai Vela and Corbacho, 2007 ; Corbacho 2008: 4.

2008 Lehua corbachoi Vela, 2007 ; Corbacho 2008: 4.

2008 Lehua colli Vela, 2007 ; Corbacho 2008: 4.

2008 Lehua ponti Vela, 2007 ; Corbacho 2008: 4.

2008 Lehua velai n. sp.; Corbacho 2008: 5, text-figs. 1–3.

2008 Lehua iannacconnei n. sp.; Corbacho 2008: 6, text-fig. 4.

2008 Lehua tahirii n. sp.; Corbacho 2008: 7, text-fig. 7.

2010 Lehua vinculum ( Barrande, 1852) ; Bonino and Kier 2010: 290, pl. 84: a.

2010 Lehua sp. ; Bonino and Kier 2010: 290, pl. 84: b.

2010 Lehua corbachoi Vela, 2007 ; Bonino and Kier 2010: 290, pl. 84: c. 2010 Lehua velai Corbacho, 2008 ; Bonino and Kier 2010: 290, pl. 84: d. 2010 Lehua ponti Vela, 2007 ; Bonino and Kier 2010: 290, pl. 84: e.

2011 Lehua adserai Corbacho and Vela, 2007 ; Corbacho and Vela 2011: 48, pl. 1: 1; 49, pl. 2: 6.

2011 Lehua corbachoi Vela, 2007 ; Corbacho and Vela 2011: 48, 49, pl. 1: 2; pl. 2: 2, 4, 7.

2011 Lehua tahirii Corbacho, 2008 ; Corbacho and Vela 2011: 48, 49, pl. 1: 3; pl. 2: 8, 9.

2011 Lehua velai Corbacho 2008 ; Corbacho and Vela 2011: 48, 49, pl. 1: 4; pl. 2: 1, 3, 5.

2015 Lehua adserai Corbacho and Vela, 2007 ; Van Roy et al. 2015a: 546.

2015 Lehua corbachoi Vela, 2007 ; Van Roy et al. 2015a: 546.

2015 Lehua tahirii Corbacho, 2008 ; Van Roy et al. 2015a: 546.

2015 Lehua velai Corbacho, 2008 ; Van Roy et al. 2015a: 546.

?2015 Parapilekia sp. ; Van Roy et al. 2015a: 546.

2015 Lehua adserai Corbacho and Vela, 2007 ; Valent and Corbacho 2015: 51.

2015 Lehua corbachoi Vela, 2007 ; Valent and Corbacho 2015: 51.

2015 Lehua tahirii Corbacho, 2008 ; Valent and Corbacho 2015: 51.

2015 Lehua velai Corbacho, 2008 ; Valent and Corbacho 2015: 51.

2016 Anacheirurus adserai ( Vela and Corbacho, 2007) ; Martin et al. 2016b: 149, figs. 2, 3, 5, table 1.

2018 Lehua corbachoi Vela, 2007 ; Lebrun 2018: 107, fig. D.

Type material: Holotype: GMSB 73845 , complete specimen (selected by Vela and Corbacho 2007) . Paratype: MGB 46755 View Materials , complete specimen (selected by Vela and Corbacho 2007) .

Type locality: Fezouata Shale in the central Anti-Atlas of Morocco, presumably located north of the city of Zagora. The original material doubtfully assigned to the locality “Tanssikhte” (see section Remarks for details) .

Type horizon: Fezouata Shale, Upper Tremadocian (Tr3) ( Araneograptus murrayi Zone ), Lower Ordovician. The type locality was incorrectly assigned by Vela and Corbacho (2007) stratigraphically to the Upper Fezouata Shale , “lower–middle Arenig” (= Floian age). See section Remarks for details .

Material.—Assigned specimens MGL 102153, MGL 102170, MGL 102172, MGL 102179, MGL 102225, MGL 103863, MGL 104146, MGL 104533, YPM 226573, YPM 517074, YPM

522182, YPM 525125, YPM 530933, ML 20-269198. All the material come from the Fezouata Shale, Upper Tremadocian (Tr3), Araneograptus murrayi Zone , Lower Ordovician.

Description.— Exoskeleton: Exoskeleton micropygous ( Fig. 1A, B, C View Fig ), ovoid to sub-ovoid in outline (excluding spines) ( Fig. 1A 1 View Fig , C). The sagittal length of the observed holaspid individuals ranges from 5.6 mm ( MGL 101262) to 35 mm ( YPM 522182). Cephalon semi-elliptical in outline, relatively short (sag.), length/width ratio ranges from 1.04 to 0.88. Glabella sub-quadratic in shape, parallel-sided, slightly tapering anteriorly. Anterior glabellar margin bowed anteriorly. Posterior glabellar margin slightly bowed anteriorly medially. Three pairs of well-defined lateral glabellar furrows ( Fig. 1C View Fig ). S1 deeper, longer (tr.), and wider (sag.) than S2 and S3, directed inwards and slightly backwards, adaxially strongly curved backwards, not connected with the occipital furrow. S2 and S3 sub-parallel, directed inwards and slightly backwards. L1 sub-oval in shape, L2 and L3 rectangular in shape. Frontal lobe wider (tr.) than longer (sag.), with an inverted triangular shape. Occipital ring sub-rectangular in shape, slightly narrower (tr.) than the glabella, longer (sag.) in medial part, tapering distally, the medial part of anterior margin slightly convex anteriorly. Anterior border short sag.), slightly bowed anteriorly almost transversal. Anterior border furrow narrow (sag.), relatively shallow, of the same depth as the axial furrow. Posterior border furrow long (sag.), deeply incised. Posterior border short (sag.) proximally, expanding (sag.) gradually distally, expanding abruptly beyond the medial region of the fixigenal field, wider at the genal angle, distally curving forward and slightly inward. Lateral border widest (tr.) next to the genal angle, tapering forward. Genal spines developed at a genal angle, relatively long, directed backwards, thick at the base, and tapering backward. Fixigenal field sub-triangular in shape, with narrow (tr.) anterior part triangular in shape and wide (tr.) rectangular posterior part. The anterior part of the fixigenal field crossed by the eye ridge. Librigenal field small with an elongated triangular shape, outer margin convex outwardly. Eye lobe semi-circular in shape, outwardly convex, narrow ( Fig. 1A 1 View Fig ).

Fixigenal and librigenal fields densely sculpted by small pits equally distributed around the surface. Small palpebral lobe, elongated, oval in shape, not differentiated from the eye ridge, posterior tip opposite (exsag.) to anterior part of L2. Palpebral furrow deeply incised, narrow, going from the posterior tip of the palpebral lobe through eye ridge to the axial furrow. Eye ridge prominent, directed slightly obliquely forward, reaching the axial furrow on the S3. Cephalic doublure is not preserved.

Proparian facial suture. Anterior branch shorter than posterior, running posteriorly slightly outwardly, lightly curved. Posterior branch mostly transverse, almost perpendicular to the sagittal axis, turning posteriorly abruptly beyond the border furrow.

Conterminant hypostome ( Figs. 1A View Fig 2, 2A View Fig ). Anterior part not well preserved. Anterior margin almost transversal ( Fig. 2A View Fig ). Middle body oval in outline with an anterior lobe oval in shape, ventrally vaulted. Posterior lobe shorter (sag.) than the anterior lobe, U-shaped. Posterior border expanded, gently curved posteriorly, smooth without spines.

Thorax composed of eleven tergites (e.g., Fig. 2B View Fig ). Thoracic axial ring sub-rectangular in outline, distal tips lightly rounded, slightly shorter (sag.) medially, width (tr.) less than one-third of the pleural width, gradually narrowing (tr.) in posterior tergites. Articulation half-ring with sagittal length half of the axial ring length, longer medially, tapering distally, anterior margin bowed anteriorly. Thoracic pleurae unconstrained, narrow flange in the anterior and posterior margin, carrying a deep, oblique pleural furrow that divides the pleura into an anterior and posterior band. Fulcrum subtle, situated in the distal second third of the pleura. Anterior band with the same length (sag.) as the posterior, but narrower (tr.). The posterior band extended beyond the fulcrum in the pleural spine, needle-like and sub-circular in cross-section. Five anterior tergites with short pleural spines that are directed posterolaterally, and increasing subtly in length from anterior toward posterior tergites. Six posterior tergites with long, thick, rather posteriorly directed pleural spines ( Figs. 1C, D View Fig , 2A View Fig ). Pleural spine of the sixth tergite longer and thicker than the rest, curved outward and backward, and extending (exsag.) beyond the pygidial terminal piece. Pleural spines in posterior trunk decreasing in size gradually from the sixth to the eleventh tergite.

Pygidium reduced with two axial rings plus terminal piece and one pair of pleural spines ( Fig. 1B, D View Fig ). Articulating half-ring short (sag.) with half of the length of the first axial ring, longer (sag.) medially, tapering distally, anterior margin anteriorly bowed. First pygidial axial ring rectangular in outline, longer (sag.) and wider (tr.) than the second pygidial axial ring. Terminal piece narrower (tr.) than pygidial axial rings, similar in length (sag.) to second axial ring; anterior margin transversal, posterior margin slightly bowed posteriorly. One pleural tergite strongly curved backwards, with a distinct pleural furrow dividing the pleura in anterior and posterior bands, the posterior band extended in a pair of long pleural spines directed backwards.

Ontogeny: The smallest recorded specimen ( MGL 104146) is a late meraspid stage that is 2.6 mm long ( Fig. 3A View Fig ). The cephalon of this juvenile stage is generally identical to the cephala of the adult individuals, only the genal spines are proportionally longer (such longer genal spines are evident also in some small holaspid specimens, Fig. 3B, C View Fig ). The trunk bears at least ten or eleven axial rings, posteriors of which are difficult to recognize, and twelve pairs of pleural spines. In contrast to the holaspids, the pleural spines of this small individual show a slightly different pattern in respect to their mutual sizes. The spines of tergites one to seven are quite long with equal or nearly equal length. The spines on tergites one to five are not significantly smaller than the rest, as is the case for holaspids. From the seventh tergite, the spine sizes decrease in length rapidly, such that the tenth to twelve pairs are minute. The equal number of pleural spines between this juvenile specimen and the adults suggests that this juvenile might have already reached the total number of trunk segments characteristic for A. adserai , despite the posterior-most axial rings not being recognizable. It is not clear whether the last pre-terminal tergite (no. 13) was present, for this one does not bear any spines in the holaspid individuals. Seven anterior trunk tergites are separated by articulating structures, forming the thorax. The posterior tergites, bearing five pairs of pleural spines, are still fused, forming a meraspid pygidium. This tergite-rich meraspid pygidium suggests that the A. adserai holaspid pygidium developed into its final form by depletion of the conjoined tergites of the thorax, as is known in many other trilobites (e.g., Hughes et al. 2006, Laibl et al. 2014). MGL 101262 (5.6 mm long) and MGL 104533 (5.9 mm long) are the smallest holaspids found ( Fig. 3B, C View Fig ). The morphology of the pleural spines is similar to the morphology of the adult specimens. The only differences to fully-grown individuals are proportionally longer genal spines and the slightly larger size of the first five thoracic pleural spines.

Appendages: Antennae: A uniramous antenna is partially visible in the counterpart of MGL 102179 ( Fig. 1A View Fig 2 View Fig , white arrow). The proximal part of the right antenna is preserved in the front of the cephalon. The individual podomeres are poorly recognizable but seem to be rather short, and quadratic in outline. The total number of podomeres and the total length of the antenna are not known because the distal tip is missing.

Protopodites: The protopodite is not preserved in any of the studied specimens. In general, the appendages in the Fezouata Shale trilobites are visible in the parts where the exoskeleton has been removed. The protopodite should be located below the axial region, which usually is not removed. As a consequence, the proximal parts of the appendages are not visible. In MGL 10225 and YPM 226573 some structures can be interpreted as a part of the protopodite Figs. 4B View Fig , 5A View Fig 2 View Fig , A 4 View Fig ), however, the nature of the structures is not clear and not enough information is available to interpret the morphology or the presence of a gnathobase.

Endopodites: The endopodites are preserved in the MGL 103863 ( Fig. 4A View Fig ), MGL 10225 ( Fig. 4B View Fig ), and YPM 226573 ( Fig. 5 View Fig ). The proximal parts of the endopodites are poorly preserved, and the attachment point to the protopodite is not visible in any of the specimens. The three anterior-most endopodites belong to the cephalon ( Fig. 4A View Fig 2, B 2 View Fig ), corresponding with attachment sites on S2, S1, and SO ( Fig. 4A, B View Fig ). In the thorax, each tergite bears a pair of endopodites ( Fig. 4B View Fig ). The posterior thoracic (behind the tenth tergite) and the pygidial endopodites are not preserved.

The endopodites are divided into seven podomeres (numbered in Fig. 5A View Fig 2 View Fig , A 4 View Fig ). The most proximal podomeres 1–3 have a rectangular shape and are more robust than the rest of the podomeres. The podomere 3 is just slightly shorter and thinner than the previous ones (1 and 2). The following podomeres (4–6) are much narrower and more elongated in shape. They gradually decrease in width and increase in length distally, with the podomere 6 being the narrowest and most elongated ( Fig. 5A View Fig 2 View Fig , A 4 View Fig ). The podomeres 5 and 6 are both nearly twice as long as the most proximal podomeres. The podomere 7 is reduced, forming a tripartite claw, with a fork shape ( Fig. 5A View Fig 3 View Fig , A 5 View Fig ).

The morphology of the endopodites is consistent along the body, at least up to the tenth thoracic endopodite. The cephalic endopodites are shorter than the thoracic ones, with thicker proximal podomeres ( Fig. 4A, B View Fig ). Large endites are visible in the posterior endopodites, but no endites are visible in the anterior endopodites. In YPM 226573, one of the posterior endopodites bears two large endites on podomeres 2 and 3 ( Fig. 5B View Fig 4 View Fig , B 6 View Fig , B 5 View Fig ). The endopodites are usually flexed at a joint between the third and the fourth endopodite. In most of the specimens, the proximal part of each endopodite is directed anterolaterally while the distal region beyond the joint is directed posterolaterally.

Exopodites: The exopodites are preserved in MGL 102172 ( Fig. 6 View Fig ) and YMP 517074 ( Fig. 7 View Fig ), with the latter preserving the only complete exopodite (ninth thoracic exopodite; Fig. 7A View Fig 4 View Fig , A 5 View Fig ). The rest of the exopodites are partially covered in the anterior region, obscuring the complete shape. The exopodite consists of a flat, elongated, wider (tr.) than longer (sag.) flap. Around the flap, there is a reinforced marginal rim ( Fig. 7A View Fig 5 View Fig ) showing a smooth surface. The anterior margin of the flap is slightly concave, while the posterior is almost straight, with the distal part curving forward. The flap is divided into two separate lobes by an articulation that extends from the anterior to the posterior region in an almost straight line, perpendicular to the main axis of the flap ( Fig. 7A View Fig 4 View Fig ). The proximal lobe is sub-rectangular in outline, longer (tr.) and wider (exsag.) than the distal lobe. Attached to the posterior rim, there are long and flat imbricated lamellae ( Figs. 6A View Fig 2 View Fig , A 3 View Fig , 7A View Fig 2 –A View Fig 5 View Fig ). The distal lobe is sub-oval in outline, also with bristles attached to the posterior margin. The lamellae on the proximal lobe are directed posteriorly, reaching the posterior tergite border, and are wider in the transversal section and longer than the bristles on the distal lobe, which are shorter and thinner. The bristles on the distal lobe are directed slightly outwards. The joint between the protopodite and the exopodite is not visible; it is covered by the thoracic axial rings. The three anterior exopodites in YPM 517074 display a similar morphology, bearing long and robust lamellae attached to the posterior rim of the flap, however, the anterior region of the flaps is overlapped by the lamella of the next most anterior exopodite ( Fig. 7A View Fig 2 View Fig , A 3 View Fig ). The exopodites show different spatial distributions between specimens. In YMP 517074 the posterior exopodite is oriented transversally ( Fig. 7 View Fig ), perpendicular to the sagittal axis, with the lamellae pointing backward. In MGL 102172 the exopodite is forwardly and slightly outwardly oriented with the lamellae pointing transversally to the sagittal axis ( Fig. 6 View Fig ).

Remarks.—The species discussed herein was originally described by Vela and Corbacho (2007) as Lehua adserai . These authors assigned this species to the genus Lehua Barton, 1915 , but unfortunately did not provide any justification or discussion why they did so. The Lehua was erected by Barton (1915). The characters defining Lehua , as presented by Barton (1915), are: glabella not tumid; eyes absent or extremely rudimentary; genal spines present; frontal lobe much less than one-third of the glabella; pleura with transverse constriction or node; the inner portion cut by a diagonal furrow and pygidium not differentiated from the thorax. Later, Prantl and Přibyl (1947: 18), amended the description of Lehua as having: “… 11 thoracic segments and six-lobate pygidium. Free cheeks small, shifted forward. Facial suture forming a small arc; at first, it runs close along the margin of the glabella and at the level of the first lateral glabellar lobe it turns back in an arc to the lateral margin of the cephalon. Neither visual organs nor palpebral lobes are developed. Pygidium with raised subtriangular axis composed of four rings, the fourth being completely stunted. Three pairs of sword-like curved, free, flat pleurae of unequal length. The lower margin of the pygidium is sharply cut off in a straight line perpendicular to the axis.” These diagnostic features of Lehua are not present in the species described by Vela and Corbacho (2007) as Lehua adserai , making this generic assignment questionable. On the contrary, L. adserai shows numerous similarities with the type species of the Anacheirurus ( A. frederici ) and should be therefore placed within this genus. Both A. adserai and A. frederici are morphologically nearly identical. Anacheirurus adserai differs from A. frederici in having a shorter (sag.) anterior border, an eye ridge not adjacent to facial suture, by the absence of S4, and by the number of pygidial spines with probably two pairs in A. frederici and one pair in A. adserai .

Whittard (1967: 285) pointed out that the morphological differences between Lehua and Anacheirurus are not enough to warrant two different genera, suggesting Lehua should be considered as a junior synonym of Anacheirurus , an argument later followed by Martin et al. (2016b) in their review of Fezouata Shale trilobites. Based on the new material studied herein, we advocate for the idea formulated by Lane (1971) that Lehua is a valid genus different from Anacheirurus . Lehua vinculum ( Fig. 2C–E View Fig ), shows several major morphological differences when compared to A. frederici . First, the glabella of L. vinculum is slightly expanded in the front, while in A. frederici it is sub-parallel with the median part slightly wider. Second, the posterior margin of the cephalon in L. vinculum is not transverse; the more distal parts are curved slightly forward. In A. frederici , on contrary, the distal part of the posterior margin is curved backward, creating a slightly expanded posterior border close to the genal spines. Third, the palpebral lobes and eye ridges are not present in L. vinculum , whereas in A. frederici they are prominent and well developed (however the lack of eyes in Lehua could be taphonomic, as the anterior part of the fixigena is not well preserved in the type species). Finally, the main difference between L. vinculum and A. frederici is related to the morphology and arrangement of the thoracic pleurae. In L. vinculum , all the thoracic and pygidial pleural spines are of the same size (only slightly decreasing in length posteriorly on the pygidium), contrary to A. frederici , which has posterior thoracic and pygidial spines that are longer than the anterior thoracic pleural spines. The thoracic pleurae of L. vinculum are divided into an inner and outer parts, with the inner part consisting of one-quarter of the length of the pleura and the outer part three quarters. Only the inner part of the pleura in L. vinculum bears an oblique pleural furrow. The pleurae in A. frederici are not divided into an inner and an outer part, instead, the pleural furrow is deep, oblique, and traversing the whole width (tr.) of the pleura dividing it into anterior and posterior parts of similar size.

Since the morphology of the pleural furrow is relevant for the classification of different groups inside the family Cheiruridae (e.g., Barrande 1852; Salter 1864; Schmidt 1881; Reed 1896; Barton 1915; Öpik 1937; Lane 1971), this character has a broader implication for the systematic position of A. adserai . Anacheirurus adserai displays a pleural morphology diagnostic of the subfamily Pilekiinae ( Lee and Chatterton 1997) , whereas L. vinculum displays a pleural morphology typical of the subfamily Cheirurinae ( Lane 1971) . Lehua is a more derived taxon belonging to a late diverging group that contains highly derived taxa such as Cheirurus and Crotalocephalus .

Some authors ( Bulman and Rushton 1973; Přibyl et al. 1985; Sdzuy et al. 2001) suggested that Parapilekia sougyi (Destombes in Destombes et al., 1969), described from the Zemmour locality( Mauritania)by Destombes in Destombes et al. (1969) should also belong to the Anacheirurus . The morphology of the cephalon of P. sougyi and A. adserai is generally identical, with the only notable difference in glabellar furrows, which are not connected to the axial furrow in Parapilekia sougyi . This character might be, however, caused by differences in the preservation. Destombes et al. (1969) also described a fragmentary pygidium with three axial rings (two rings and a terminal piece?) and one pair of spines directed backward. Such a morphology corresponds with what is seen in Anacheirurus adserai . This suggests that Parapilekia sougyi is a species of Anacheirurus , possibly even synonymous with A. adserai (in which case the name A. adserai would be a junior subjective synonym of A. sougyi ). However, the type material of A. sougyi does not allow more detailed comparison, due to its fragmentary nature. Until more complete specimens of A. sougyi from the type and/or nearby localities are available, we recommend keeping A. sougyi and A. adserai as two separate species.

Seven species of Anacheirurus , assigned previously to Lehua , were originally described in the area of Zagora, Morocco ( Vela and Corbacho 2007; Vela 2007; Corbacho 2008). In the following years, Vela and Corbacho (2011) considered three of them ( A. colli , A. ponti , and A. iannacconnei ) as junior synonym of A. corbachoi , keeping four different species of Anacheirurus from the Fezouata Shale. Their characters separating individual species are in general related to the shape of the cephalon and the glabella, the presence and morphology of the anterior notch in the cephalon, and the number and shape of posterior pygidial spines. These species are based on specimens that were heavily prepared, a process that may modify or exaggerate the original morphology, and can lead to misinterpretations of anatomical characters ( Fortey 2009; Gutiérrez-Marco et al. 2017; Gutiérrez-Marco and García-Bellido 2018). Martin et al. (2016b) in a study about the trilobite community in the Fezouata Shale concluded that there is only one morphotype of pilekiid, and it was assigned to Anacheirurus adserai which is the oldest species of Anacheirurus described. Detailed observations of numerous specimens from Fezouata Shale, which had not undergone the same degree of preparation suggest, in concordance with Martin et al. (2016b), that all the species may be junior subjective synonyms of A. adserai for the following reasons.

Firstly, the anterior cephalic notch that is variously developed in some specimens (Vela and Corbacho 2011: pl. 1: 2) represents the dorsal arching of the cephalon. Such arching is expressed to a higher or lower degree on the compressed specimens, ranging from an indistinct or distinct notch to a protrusion, depending on the original orientation of the cephalon to the bedding plane. Hughes and Rushton (1990) explained how similar variability in the pygidial shape of trilobite Cermatops discoidalis ( Salter, 1866) is caused by different depositional orientations of the pygidium. Similarly, Hughes (1995) listed morphological characters in trilobite Bailiella lantenoisi Mansuy, 1916 , that can be a subject of taphonomic variation when preserved in shales. Four of these characters (frontal area length, border length, preglabellar field depression, and anterior border furrow) are generally related to the shape of the anterior cephalic margin (see Hughes 1995: table 1), corroborating this part of the trilobite exoskeleton is prone to taphonomic variation when preserved in shales. These examples demonstrate that certain types of variation in trilobite body parts, when preserved in shales, should not be considered as real biological features, but taphonomic artifacts.

Secondly, the exact number of pygidial spines is generally hard to determine in complete individuals of Anancheirurus, as the boundary between the thorax and the pygidium is difficult to identify owing to the similarity of thoracic and pygidial pleurae. Especially in species where both thoracic and pygidial pleurae bear spines but otherwise do not show any major morphological differences between thorax and pygidium, the last articulation structure is not easy to discern (see Esteve et al. 2017 for a similar issue). The variation in the number of pygidial spines in individual species described by Corbacho and Vela (2011) may be subject to such a bias, especially in heavily prepared specimens.

There is also an uncertainty regarding the exact stratigraphic position of several Anacheirurus species from the Anti-Atlas region. The type locality of A. adserai , A. corbachoi (and its subjective synonyms A. colli , A. ponti , A. iannacconnei ), A. tahirii , and A. velai was named as “Tanssikhte”, west of Zagora ( Vela and Corbacho 2007: fig. 1; Vela 2007: fig. 1). The type locality was assigned stratigraphically to the Upper Fezouata Shales, “lower–middle Arenig” (= Floian age). Thirty additional trilobite species were identified, with some of them typical for Floian strata (belonging to genera Foulonia , Pateraspis , or Ormathops ), but others typical for Tremadocian (such as Bavarilla zemmourensis ) ( Vela and Corbacho 2007; Vela 2007). However, the latest review of trilobite stratigraphic distribution in the Fezouata Shale made by Martin et al. (2016b) shows that Anacheirurus adserai is restricted to Araneograptus murrayi Zone of the upper Tremadocian. We agree with Martin et al. (2016b) statement and we question the original locality and stratigraphic assignement Anacheirurus adserai .

Stratigraphic and geographic range.—Anti-Atlas of Morocco and possibly also northern Mauritania (= A. sougyi , see section Remarks for details). Fezouata Shale, Araneograptus murrayi Zone, Tremadocian , Lower Ordovician.