Correlation of Assemame sections with Bou Tiouit, Tarhoucht.
Introduction.
Over the last several years, five articles have been published by two teams working on the new Moroccan species of Acadoparadoxides from the middle part of the Brèche à Micmacca Member of the Jbel Wawrmast Formation, which belongs to the uppermost "Lower Cambrian" around 510 million years in age. The two teams have strongly differing views on the validity of the new species. One team, led by Gerd Geyer of Würzburg, Germany (referred to here as the "Geyer team"), regards the new species as valid taxa which are largely separated stratigraphically and differ morphologically from each other. The other team, led by J. Javier Álvaro of the Institute of Geosciences at Madrid, Spain (referred to here as the "Álvaro team" or sometimes as the "Spanish team"), treats all but one of them as expressions of a single species, Acadoparadoxides mureroensis, from northern Spain, which it claims is "characterised by a broad morphological variation" and whose pygidium has a "variable outline, grading from subtriangular to subhexagonal and subovate" (Álvaro et al , 2018). The five articles are as follows, and have been arranged in order of publication. The article which began the argument was "The Paradoxides puzzle resolved: . . . " by Gerd Geyer and Tony Vincent. It is dated '2015'; however, it was initially published online in 2014, shortly before the article by Zamora et al (2014) which refers to it, so it is placed first in the following list. There are links to all these articles at the end of this page of the website.
GEYER, G. & VINCENT, T. (2015). The Paradoxides puzzle resolved: the appearance of the earliest paradoxidines and its bearing on the Cambrian Series 3 lower boundary. Paläontologische Zeitschrift 89 (3), pp. 335 - 398.
ZAMORA, S., ÁLVARO, J.J., CLAUSEN, S. & ESTEVE, J. (2014). Open quarry of the Brèche à Micmacca Member crossing the 'telesto level' at Assemame, central Anti-Atlas. In Stratigraphic overview of the Ediacaran and Cambrian from the Anti-Atlas, Morocco (ed. J. Devaere, S. Clausen & J.J. Álvaro), pp. 72 - 75, Lille: University of Lille I.
ÁLVARO, J.J., ESTEVE, J. & ZAMORA, S. (2018). Morphological assessment of the earliest paradoxidid trilobites (Cambrian Series 3) from Morocco and Spain. Geological Magazine 115, pp. 1566 - 1595.
[This article was published in the journal in 2018, but was available online in 2017, so some documents may refer to it as "Álvaro et al (2017)".]
GEYER, G. NOWICKI, J., ZYLINSKA, A. & LANDING, E. (2019). Comment on: Álvaro, J.J., Esteve, J. & Zamora, S. 2018, Morphological assessment of the earliest paradoxidid trilobites (Cambrian Series 3) from Morocco and Spain. Geological Magazine 156, pp. 1691 - 1707.
ÁLVARO, J.J. & ESTEVE, J., (2020). Reply to Comment on: Álvaro, J.J., Esteve, J. & Zamora, S., 2019. Morphological assessment of the earliest paradoxidid trilobites (Cambrian Series 3) from Morocco and Spain [Geological Magazine] by Geyer G., Nowicki J., Zylinska A. & Landing E. Geological Magazine 157, pp. 1971 - 1982.
In Geyer & Vincent (2015) several new species of Acadoparadoxides were described from the mountain named Bou Tiouit, near the village of Tarhoucht in the eastern Anti-Atlas. Following this, Zamora et al (2014) published the Álvaro team's results of collecting in the Brèche à Micmacca near the village of Assemame, about 10 km south of Tarhoucht. The trilobite material collected there included several Acadoparadoxides cranidia and pygidia which the authors assigned to five different species sensu Geyer & Vincent (2015). All these remains were collected at a single level which was designated 'Level 4', and were pictured in the following figure from their article ~
In the next article, Álvaro et al (2018), additional Acadoparadoxides cranidia and pygidia from Assemame were figured. Morphometric measurements were made on all the remains from Assemame, together with those from Bou Tiouit which had been figured in Geyer & Vincent (2015), and compared with measurements made on remains of Acadoparadoxides mureroensis from Spain. Detailed statistical analysis of the measurements supposedly indicated that little morphological difference could be detected between the Moroccan species and A. mureroensis, except for A. nobilis, whose pygidium with a slightly indented posterior margin had not been found in Spain. The other early Moroccan species were therefore considered to be invalid taxa. Acadoparadoxides mureroensis itself was redescribed in the article, although no mention was made of the course of the facial sutures in the cranidium, which is an important feature for distinguishing between cranidia of the Moroccan species. Seemingly, the facial sutures could be in any position, apparently making them unsuitable for identifying the various species.
In the two figures of specimens from 'Level 4' at Assemame in Álvaro et al (2018), some of the specimens published in Zamora et al (2014) are duplicated, but overall the two papers figure eight different cranidia and ten pygidia, all of which were reassigned to A. mureroensis in Álvaro et al (2018). Here are the two figures from that article ~
Meanwhile, however, the Geyer team prepared a geometrical analysis of all available Acadoparadoxides cranidia from the Tarhoucht area and Assemame, and the results can be seen in the 'Geometric analysis of cranidia' page on this website. In addition, on two visits to Assemame a section in the Brèche à Micmacca close to the quarries investigated by the Álvaro team was measured by the Geyer team, and compared with the section at Bou Tiouit.
The Assemame quarries themselves were also visited by the Geyer team, and the stratigraphic position of 'Level 4', where the Álvaro team had obtained its collection of Acadoparadoxides remains, was established. The general results were published in a 'Comment' article in Geological Magazine by Geyer et al in 2019, and the full details of the Assemame story can be followed in the two documents at the end of this page of the website.
Lastly, in 2020 the Spanish team published a 'Reply' to the 'Comment' article, in which various allegations are made about the rôle and collecting régime of Tony Vincent in the Tarhoucht area, incorrect attributions of Acadoparadoxides material and supposed size bias in the cranidia and pygidia collected, as well as criticism of the diagnosis and description of the Moroccan species by Gerd Geyer and colleagues. New evidence is presented in the form of a collection of Acadoparadoxides pygidia which the authors claim to have excavated at a single level at Bou Tiouit, near Tarhoucht, and as the pygidia can be assigned to four different species sensu Geyer & Vincent (2015), this appears to support the Álvaro team's contention that they are all remains of the supposedly morphologically variable A. mureroensis. A response to the Spanish team is anticipated for publication in due course, but in the meantime a brief critique of its collection at Bou Tiouit is outlined on the page headed 'A curious collection' on this website.
Correlation of Assemame with Bou Tiouit, Tarhoucht.
Among the various criticisms aimed at the Geyer team by the Álvaro team is the allegation that stratigraphy itself has been used as a diagnostic character to describe the new species from Morocco. This may or may not be the case, but perhaps an instructive example of the desirability of taking into account the stratigraphic setting when attempting the identification of species' remains, while carefully avoiding its use as a diagnostic character itself, concerns the figured material of Acadoparadoxides collected by the Spanish team from its 'Level 4' at Assemame. Morphological re-assessment by the Geyer team suggests that five of the ten pygidia figured in the articles by the Álvaro team can be confidently referred to the distinctive species temporarily designated 'Acadoparadoxides cf. mureroensis' in Geyer & Vincent (2015). These are teardrop-shaped with a rounded posterior margin, and often have a thick, elevated platform supporting the rachis (Álvaro et al, 2018, Fig. 17 f, k-n). The remaining five pygidia are broad and triangular with large, rounded posterolateral corners and a straight posterior margin, and are therefore of A. levisettii morphology (Álvaro et al, 2018, Fig.17 a, b, e, g, h). Further, in Fig. 17 l of the same article, in the thorax attached to the teardrop-shaped pygidium, the pleural spines in the anterior half of the thorax are bent particularly strongly backwards, producing the relatively narrow, straight-sided U-shaped outline to the thoracopygon typical of A. cf. mureroensis, which contrasts with the wider parabolic outline known in both A. levisettii and A. pampalius. Geometrical analysis revealed that in six of the eight figured cranidia the cranidial width across the anterior border is exactly equal to that across the palpebral lobes, while in the remaining two cranidia the width across the anterior border is less than that across the palpebral lobes. The co-occurrence of these two cranidial morphologies, produced by the position of the anterior branch of the facial suture, is also characteristic of A. cf. mureroensis, to which all eight cranidia can be reassigned, whereas in A. levisettii the cranidial width across the anterior border always exceeds that across the palpebral lobes. [The characteristics of the facial sutures of the early species of Acadoparadoxides have been established by the geometrical analysis conducted by the Geyer team, and can be studied on the 'Geometric analysis of cranidia' page of this website.]
These morphological findings suggested that 'Level 4' at Assemame should occupy a stratigraphic position comparable with that at 15 m above the base of the Brèche à Micmacca Member at Bou Tiouit, Tarhoucht, where A. cf. mureroensis occupies a narrow stratal band immediately above the top of the biostratigraphic range of A. levisettii. Lithostratigraphic correlation of Bou Tiouit with a section at Zizaoun Tazemamt, Assemame (Geyer et al, 2019, Fig. 3), about 150 metres west of the excavations incorporating 'Level 4', indicated that this level should occur at Assemame at about 28 m above the base of the Brèche à Micmacca. Figure 3 of Geyer et al (2019) is reproduced here for reference ~
It was surprising, therefore, to discover that, in the stratigraphic chart of the Assemame section in Álvaro et al (2018), Fig. 3, the position of 'Level 4' has been marked at a calculated 18 m above the base of the Brèche à Micmacca, a level where correlation with Bou Tiouit suggests that Acadoparadoxides pampalius should be the only paradoxidid species to be found. Identification of the paradoxidine material from 'Level 4' as that of A. cf. mureroensis and A. levisettii on morphological grounds was secure, but in order to demonstrate the reliability and usefulness of the biostratigraphic framework of early species of Acadoparadoxides provided by detailed collecting at Bou Tiouit, Tarhoucht, it became necessary to visit the Assemame section and its excavations featured in Álvaro et al (2018), to establish the true stratigraphic position of 'Level 4'. This was carried out in October, 2018 and, as outlined in Geyer et al (2019), p. 1694 and Figs 2 and 3 of that article, 'Level 4' was found to be centred upon a position 4.4 m below a prominent limestone layer, 7 cm thick, which also occurs at the nearby Zizaoun Tazemamt section at 32.6 m, and which marks the top of the Brèche à Micmacca Member sensu Álvaro et al (2018), Fig. 3. 'Level 4' was therefore found to lie at about 28.2 m above the base of the Brèche à Micmacca, almost precisely as had been predicted by the morphologically based paradoxidine biostratigraphy at Bou Tiouit and its lithostratigraphic context, and confirming the error in the chart of Álvaro et al (2018), Fig. 3. This chart and Fig. 2 of Geyer et al (2019) are reproduced here for reference, and they are followed by an enlargement of the chart with annotations showing the correction to the position of 'Level 4', about ten metres above its indicated level ~
Consequently, it can be said that lithostratigraphic observations at Assemame, and morphological re-assessment of Acadoparadoxides remains collected there from 'Level 4' by the Spanish team, illustrate the reliability of the biostratigraphic succession of discrete early species of Acadoparadoxides at Bou Tiouit, Tarhoucht, proposed in Geyer & Vincent (2015) and Geyer et al (2019), with specific combinations of morphological characteristics being indicative of particular stratigraphic levels, leading to relatively precise correlation between sections in the Brèche à Micmacca Member in the eastern Anti-Atlas of Morocco.
The fine details of the correlation of the Assemame sections and 'Level 4' with Bou Tiouit, Tarhoucht, are set out in the two documents below, the first of which also includes geometrical analysis of the eight Acadoparadoxides cranidia collected by the Spanish team from 'Level 4' ~
10 - Comparison of Sections, Assemame.pdf11 - Update to Correlation of Zizaoun Tazemamt (1).pdfHere are links to all the articles mentioned above ~