The trace fossil Cardioichnus planus from the lower Miocene of Algeria : the first record from Africa and a probable endemic tracemaker

Abundant Cardioichnus planus SMITH & CRIMES, 1983 have been reported for the first time from Africa. They occur in lower Miocene deep-marine deposits near Tiaret in northwestern Al-geria. Morphological features of the trace, coupled with the spatial interrelationships between Cardioichnus and the echinoids documented in this area, provide evidence that the endemic spatangoid Echinocardium nummuliticum PÉRON & GAUTHIER, 1885 represents the trace-maker of the studied C. planus . Cardioichnus is known from the upper Jurassic to the Pleistocene and it has a wide geographic and environmental distribution.

Here, the ichnospecies C. planus is recorded for the first time from Algeria and in the African continent.The interpretation of abundant C. planus from the lower Miocene Tiaret Marl Formation in western Algeria is presented and discussed.Furthermore, the spatangoid echinoid Echinocardium nummuliticum PÉRON & GAUTHIER, 1885 has been proposed as the probable producer of the studied C. planus specimens.Finally, a stratigraphic, palaeogeographic and palaeoenvironmental review of Cardioichnus and its ichnospecies is presented.The studied material comes from the Tiaret Marl Formation (CHERIF et al., 2021) of Sidi Ali Mellal section in the Tiaret basin (POLVÊCHE, 1960).The section is located some kilometeres to the northwest of the town of Tiaret (Fig. 1A-C).The lower Miocene Tiaret Marl Formation crops out widely in the Tiaret area.It overlies Visean rhyolites (POLVÊCHE, 1960), or Jurassic marine deposits (CHERIF et al., 2015(CHERIF et al., , 2018;;HALAMSKI & CHERIF, 2017;BELAID et al., 2020), and is overlain by the upper Miocene Tiaret Sandstone Formation (CHERIF et al., 2021).
In the Sidi Ali Mellal section, the ichnoassemblage of the first unit contains the trace fossil Cardioichnus (Fig. 1D).

MATERIAL AND METHODS
More than seventy specimens of C. planus have been observed and collected by the authors during several field trips in to the Tiaret basin between 2019 and 2020.The studied collection (N = 65) is deposited at the Laboratory of Geology of Sahara at Kasdi Merbah Ouargla University (CAR-SAM-001 to CAR-SAM-065).These sixty-five specimens were selected because of their preservation.They were photographed both in the field and the laboratory.The measurements were taken in the laboratory (Table 1).Also, the ichnotaxonomic identification of C. planus is in accordance with SMITH & CRIMES (1983).

Description
Cardioichnus planus is very abundant in the Sidi Ali Mellal section.It is a hypichnion in the sandstone beds, and co-occurs with Scolicia vertebralis (Fig. 2).
The studied traces (Fig. 3A-F) are heart-shaped mounds.They are ovoid or subquadratic in outline, bilobate and bilaterally symmetrical.Their two lateral lobes are generally rounded, curved, and merged at the central axial depressed V-shaped depression.Some specimens show a well developed posterior impression with a sharp edge at the terminus, which extends from the V-shaped depression, and separates the two distinct lobes.The mounds are 20-37 mm long and 16-26 mm wide, with a length/width ratio of about 1.05-1.42.The mean length (N = 65) is 25.58 mm, whereas the mean width is 20.87 mm (Table 1).

Remarks
The selected samples have been attributed to Cardioichnus planus due to their dimensions (l= 20-37 mm; w= 16-26 mm), and the length/width ratio which are similar to that described by SMITH & CRIMES (1983) and BRUSTUR (2005).The ovoid to subquadratic form of the studied specimens, their preservation as a convex body and the presence of two curved lateral lobes merged at the median longitudinal V-shaped depression and posterior impression characterize C. planus (SMITH & CRIMES, 1983;UCHMAN, 2007a), and confirm their determination as this ichnospecies.

Probable local tracemakers of the Algerian C. planus
Spatangoid echinoderms or heart urchins are known from the early Jurassic (BUATOIS & MÁNGANO, 2018).They are the producers of Bichordites, Cardioichnus and Scolicia.Bichordites and Scolicia are attributed to the burrowing of spatangoids.Scoli cia is the best known among these ichnogenera.Its oldest occurrence is from the Tithonian of Bulgaria (TCHOUMATCHENCO & UCHMAN, 2001).
All Cardioichnus ichnospecies are considered to be resting traces produced by spatangoid echinoids (SMITH & CRIMES, 1983;PLAZIAT & MAHMOUDI, 1988;MAYORAL & MU-ÑIZ, 2001;KAPPUS & LUCAS, 2019).The determination of tracemaker is problematic.In the Oligocene of Trentino (northeastern Italy), Cardioichnus isp.and the associated Bichordites have been attributed to Eupatagus ornatus (BERNARDI et al., 2010).Furthermore, Bichordites recorded from the middle Miocene of Spain has been attributed to the grazing activity of spatangoid echinoids assigned to the genus Maretia (GIBERT & GOLDRING, 2008).Recently, the spatangoid Heteraster has been considered as the tracemaker of the ichnospecies C. biloba, described from the mid-Cretaceous of New Mexico in the United States of America (KAPPUS & LUCAS, 2019).
The studied outcrop belongs to the southern Tellian border of the Ouarsenis range which constitutes the northern edge of the Tiaret Miocene basin.The most important palaeontological in-  vestigations which yielded a rich echinoid fauna in the southern border of the Ouarsenis range have been carried out at the Kef Ighoud section, located 50 km to the ENE of our study area.Kef Ighoud echinoid-rich deposits have been attributed to the Eocene (POMEL, 1885), the Oligocene (DALLONI, 1936;MATTAUER, 1958) and finally to the lower to middle Miocene on the basis of their planktonic foraminifera (DERKAOUI, 2017).
Several spatangoid echinoids have been described from the lower-middle Miocene of the Kef Ighoud section (COTTEAU et al., 1885;POMEL, 1885) , 2007).This echinoid fauna is important due to its endemism, and has never been reported out of this area.
Probable producers of Cardioichnus are proposed on the basis of the similarities in size and morphology between the spatangoids and Cardioichnus (KAPPUS & LUCAS, 2019).However, among the spatangoid assemblage of Kef Ighoud, three species have similar sizes to the studied Cardioichnus specimens.They include Echinocardium dubium (l= 26 mm; w= 25 mm), E. nummuliticum (l= 26 mm; w= 23 mm) and Euspatangus hangen mulleri (l= 25; w= 25).According to the l/w ratio, the two Echi nocardium species resemble Cardioichnus planus.However, the morphology of Echinocardium nummuliticum allows proposing it as the probable tracemaker (Fig. 4).The size decreases near the posterior part of the test where a prominent rostrum is present.Accordingly, the posterior impression at the terminus of the trace could be interpreted as the imprint of this rostrum.Alternatively, the ambitus of this species is heart-shaped, whereas it is rounded in E. dubium.This confirms that C. planus most closely resembles E. nummuliticum in size and morphology.
Echinocardium species occur generally in shallower environments.POMEL (1887) indicated that the attribution of E. nummuliticum to this genus by PÉRON & GAUTHIER is incorrect, and these specimens should be revised.However, it co-occurs with other sea urchins such as Schizaster (POMEL, 1885)  which could be found in deep basins (MCKINNEY, 1986).Also, echinoids of the Echinocardium group are responsible for traces with one drain such as Bichordites, which could be continuous with Cardioichnus and produced by the same organism.Bichordi tes occurs from shallow-marine settings (UCHMAN & KREN-MAYR, 1995;BERNARDI et al., 2010;CARUSO & MONACO, 2015) to slope deposits in soft substrates (VILLEGAS-MARTÍN et al., 2014).

Stratigraphy, palaeoenvironments and palaeogeography
The oldest Cardioichnus were observed in the Kimmeridgian of northwestern Algeria, near this study area (BOUCHEMLA et al., 2020) , 1995, 1999).Finally, the newest records of this trace fossil are from the Pleistocene of the Tropical Eastern Atlantic (Cape Verde), in a moderate to high-energy shallow marine environment, where it constitutes part of the proximal Cruziana ichnosubfacies (MAY-ORAL et al., 2013), and from the shallow deposits of southern Italy (CARUSO & MONACO, 2015).

CONCLUSIONS
The studied samples allow the following conclusions to be drawn: Abundant Cardioichnus planus has been found for the first time in Africa; The studied specimens are dated as being of the lower Miocene age, and co-occur with Scolicia vertebralis; The endemic spatangoid echinoderm Echinocardium num muliticum PÉRON & GAUTHIER, 1885 represents the probable tracemaker of the studied C. planus.

Figure 1 .
Figure 1.A) Location of Orania (Northern Algeria) in the Western Mediterranean.B) The main structural domains of Western Algeria.C) Geological map of the study area.

Figure 2 .
Figure 2. The stratigraphic succession and vertical distribution of trace fossils of the lower Miocene Tiaret Marls Formation in the Sidi Ali Mellal section.

Table 1 .
Length, width and l/w ratio of the studied 65 Cardioichnus planus (in mm).

Table 2 .
Stratigraphic and geographic record of the ichnogenus Cardioichnus.
(CHERIF et al., 2021;1;GIBERT & ROBLES, 2005)nal offshore environment characterizing the archetypal Cruziana ichnofacies.This area belongs to the south Tethyan Ocean.This constitutes the only record of this ichnogenus in the Jurassic.In the lower Cretaceous, Cardioichnus occurred in similar environments to those of the upper Jurassic of Algeria.It has been reported in seve ral domains from Spain (Tethyan Realm)(MONACO et al., 2005;GIANNETTI et al., 2014)to North America(KAPPUS & LUCAS, 2019, 2020).Cardioichnus is common in flysch deposits from as early as the upper Cretaceous.It has been found in Iran, Zoophycos and Nereites ichnofacies.Cardioich nus also has been reported from the Oligocene and the Oligo-Miocene of the Mediterranean Tethys in shallow(BERNARDI et al., 2010)to deep-sea deposits(BRUSTUR, 2005;RIAHI et al., 2014).Miocene records of Cardioichnus are widespread, e.g., it occurs in New Zealand(MANLEY & LEWIS, 1998),Argentina (LÓPEZ-CABRERA et al., 2008), and the western Mediterranean basin, especially in Spain(MAYORAL & MUÑIZ, 2001;GIBERT & ROBLES, 2005)and Algeria(CHERIF et al., 2021;  this paper).Miocene Cardioichnus have been reported from infralittoral shelf sands to deep-sea deposits.In the Pliocene, Car dioichnus is known only in Spain from sandy storm beds, typical of the Cruziana ichnofacies (GIBERT & MARTINELL