Thalassinoides horizontalis Myrow, 1995

Carvalho, Carlos Neto De & Bayet-Goll, Aram, 2023, Thalassinoides horizontalis Myrow, 1995 from the Middle-Upper Ordovician shallow marine siliciclastics of Iran (Lashkerak Formation), Comptes Rendus Palevol 22 (27), pp. 569-583 : 578-579

Thalassinoides horizontalis Myrow, 1995

( Fig. 6)

Thalassinoides horizontalis Myrow, 1995: 62-63 , figs 6, 7a, b. — Blissett & Pickerill 2004: 360, pl. 10, fig. A. — Tiwari, Majkonwar, Malsawma, Malte & Patel2011: 1139, pl. 4d. — El-Hedeny, Hewady & Al-Kalitany 2012: 728, figs 6A, B. — El-Sabbagh, El-Hedeny & Ferraj 2017: 11, 12, figs 4c, 5, 6d, e, 7e, f. — Darngawn, Patel, Joseph & Shitole 2018: 176, pl. 3, figs 3, 4. — Bendella, Benyoucef, Mikulăś, Bouchemla, Martinell & Feri 2021: 539, fig. 5E.

Protopaleodictyon atkeni Morgan, Henderson & Pratt, 2019: 217 , figs 3, 5, 6.

Thalassinoides isp. Bayet-Goll, Buatois, Mángano & Daraei, 2022a: 17, fig. 6f.

MATERIAL. — Numerous field observations; 20 specimens were measured.

DIAGNOSIS. — Horizontal, branching network of smooth-walled, unlined burrows, lacking vertically oriented shafts. Burrow diameter identical within individual specimens; constrictions or swellings at both junctions and inter-junction segments are absent (emended by Blissett & Pickerill 2004, after Myrow 1995).

DESCRIPTION

Thalassinoides from Lashkerak Formation consists of mainly horizontal,sole bed preserved,branching networks and pseudo-polygonal networks,with mainly horizontal Y-shaped branching burrows ( Fig. 6). Burrows have a rounded cross section in and show passive filling. The persistent diameter of the burrows ranges between 5 and 10 mm. They have variable length and are straight to winding ( Fig.6D, E). The margin is smooth with no lining.Branching occurs regularly every 2-3 cm ( Fig.6B, F). Angle of branching varies between 100-120°.

REMARKS

In the description of the ichnogenus Thalassinoides, Ehrenberg (1944) stated that it is composed by cylindrical-to-elliptical burrows that form a three-dimensional to horizontal branching polygonal network with vertical shafts connected to the surface, where branching is regular and swells are found at the branches and elsewhere.According to the original diagnosis of Thalassinoides horizontalis by Myrow (1995), these networks are mostly horizontal, regularly branching of unlined burrows lacking vertical shafts, with tunnels straight to curved showing almost constant burrow diameter (no swellings). So, the main ichnotaxobases for T. horizontalis are the lack of vertical offshoots, lack of swellings, regular branching and almost constant burrow diameter, also in accordance to the emended diagnosis of Blissett & Pickerill (2004).In his controversial revision of the ichnogenus Thalassinoides, Schlirf (2000) included T. horizontalis in his Spongeliomorpha suevica (Reith) , which diagnosed as mainly horizontal, but sometimes partly vertical to oblique burrow systems with unlined, smooth lined or ornamented walls with Y- and T-shaped branches, typically enlarged at junctions or elsewhere, and variable diameters within a given system. This diagnosis is too broad as includes different burrow systems with different expressions of behavior and preservation in substrates with different consistencies.We do not intend in this paper to revise the ichnogenus Thalassinoides, or raise again the discussion about the usefulness of keeping Spongeliomorpha, Thalassinoides and Ophiomorpha as separate ichnogenera. However, the ichnotaxobases of T. horizontalis as defined originally by Myrow (1995) were included in this broad diagnosis or were not taken into account by Schlirf (2000), such as the constant diameter along the tunnels, and therefore the characteristic lack of swellings, or turning chambers, in the branching areas and elsewhere, in a burrow network recognized by the lack of vertical shafts by all the subsequent authors that described this as a valid ichnospecies ( Blissett & Pickerill 2004; Tiwari et al. 2011; El-Hedeny et al. 2012; El-Sabbagh et al. 2017; Darngawn et al. 2018; Bendella et al. 2021; this paper), allows to maintain T. horizontalis as valid distinctive ichnotaxon.

The small diameter burrows in Lashkerak Fm. do not swell at branching areas and do not show constrictions ( Fig. 6), and they are organized in regularly branching, mostly horizontal burrow systems, matching with the diagnosis by Myrow (1995) for Thalassinoides horizontalis . Some oblique burrows could have been the connection of the burrow system in a lower level with the water-substrate interface ( Fig. 6D), making them closer in morphology to the Zhushadong specimens from Cambrian Age 4 ( Zhang et al. 2017). According to Myrow (1995), this pattern may have had the function of conduits through which water would be pumped during filter-feeding, or as a feeding structure, in case of an agrichnial burrow. In the examples of Thalassinoides described by Myrow (1995), it is frequent the presence of an “outer wall” resulting from a diagenetic halo. This kind of preservation in carbonates is particularly evident for Thalassinoides from different ages (e.g. Fürsich 1981; Ekdale & Bromley 2003), and the eodiagenetical processes and dolomitization ( Jin et al. 2012) inside and in the vicinities of the disturbed sediment ultimately develop a nodular fabric. The diagenetical processes are usually different in siliciclastic settings and for this reason the halo typical of Thalassinoides horizontalis in carbonates cannot be found in the unlined burrows from the Lashkerak Formation, as it is not found in the examples described by, e.g. El-Sabbagh et al. (2017) and Bendella et al. (2021).