Spongeliomorpha in continental settings.

RICARDO NESTOR MELCHOR; EMILIO BEDATOU,; RICHARD BROMLEY

Praga, República Checa

Workshop; Third Workshop on Ichnotaxonomy; 2006

Institute of Geology, AS CR, Prague

The main characteristics of the ichnogenus Spongeliomorpha de Saporta are a dominantly horizontal, unlined burrow system showing Y- and T-branching, and ornamented with scratches of different orientation. Spongeliomorpha is herein considered as an ichnogenus distinct from Ophiomorpha Lundgren and Thalassinoides Ehrenberg, because of its scratch-ornamented walls. Another character of ichnotaxonomic value for Spongeliomorpha is the angle of bifurcation of the burrow system. The synonymy of Steinichnus Bromley & Asgaard, 1979 under Spongeliomorpha was advanced more than two decades ago, although no formal proposal has been published to date. Subsequently, different authors have used both ichnogenera to refer to burrow systems with striae transversal or oblique to the burrow axis.             The specimens of Spongeliomorpha commonly found in continental settings display a pattern of striations that is consistently oblique to transverse to the burrow axis. There are two named ichnospecies of Spongeliomorpha with this feature: S. carlsbergi Bromley & Asgaard, 1979 and S. milfordensis Metz, 1993. Metz (1993) recognized the morphological similarity between S. carlsbergi and S. milfordensis and indicated that the main differences are thicker striae and striae that form an average oblique angle with the axis of the burrow in the latter, while S. carlsbergi displays transversal striations. For this study, we compared the angle of striations with the axis of the burrow for the holotype and paratype material of Spongelomorpha (Steinichnus) carlsbergi (Bromley & Asgaard, 1979) and the paratypes and photographs of the holotype of Spongeliomorpha milfordensis (Metz, 1993). The histogram of the orientation of the scratch ornament (Figure 1) suggests that both ichnospecies show a marked overlap in the range 30-90°, although S. milfordensis has more readings than S. carlsbergi for acute angles (30-60°), whereas the latter shows more readings at higher angles (60-90°). In addition, the average orientation of scratches is fairly similar (Figure 2). A hypothesis test performed to compare the average orientation of scratch ornament between the type series of S. carlsbergi and S. milfordensis suggest that both values are different with a confidence of 99.99%, although the difference in average orientation is only 10 degrees (Figure 2). The apparent variability in the distribution of scratch ornament is easily accommodated if the type series of Steinichnus carlsbergi (specimen GGU 146315, housed at the Geological Museum, Copenhagen; paratype of Steinichnus carlsbergi, illustrated in Bromley & Asgaard 1979, fig. 11B) is considered (Figure 2). This comparison indicates that the angles of striations in a part of the same burrow are dominantly transversal (“carlsbergi type”) and dominantly oblique in another adjacent portion (“milfordensis type”). However, to compare both ichnospecies the holotype material must be contrasted. A visual comparison suggests that both ichnospecies are similar. This subjective impression is further supported by a hypothesis test for the mean of the two populations, performed to compare the average orientation of scratch ornament between the holotype material of S. carlsbergi and S. milfordensis. This test suggests (p = 0.22) that both values are statistically similar (Figure 2). In consequence, it is considered that the morphological differences between both type specimens are minor and they should be included under a single ichnospecies. In this case, S. milfordensis is proposed as junior synonym of S. carlsbergi. Further micromorphological characterization of Spongeliomorpha (Steinichnus) carlsbergi is obtained by study of a longitudinal thin section of a specimen of the type locality and unit. The burrow displays a massive filling composed of calcareous siltstone and microsparite, similar to the host rock, with micritic, darker rip-up clasts, up to 5 mm long. Similarly, paratype material of S. milfordensis shows a massive filling in thin section, although in this case it is composed of siltstone devoid of rip-up clasts, which is identical to the host rock. It is proposed that Spongeliomorpha carlsbergi should be restricted to burrow systems with deeply striated walls, striations ranging from transverse to oblique to the burrow axis, with occasional T- or Y-bifurcation; showing no lining, and having a massive filling. The published records of S. carlsbergi are restricted to Late Triassic – Early Jurassic continental deposits, although recent accounts of the ichnospecies from Miocene and modern deposits of Argentina considerably extend its age range.   Figure 1. Histogram of the orientation of striations in the type series of Steinichnus carlsbergi and Spongeliomorpha milfordensis.   Average angle (degrees) Standard Deviation Steichnichnus carlsbergi holotype material 70,9 1,9 Spongeliomorpha  milfordensis holotype material 68,2 3,7 Steichnichnus carlsbergi type series 70,6 12,9 Spongeliomorpha  milfordensis type series 60,1 13,6 Steichnichnus carlsbergi type series  “carlsbergi type” portion 74,8 9,6 Steichnichnus carlsbergi type series “milfordensis type” portion 65,6 15,5   Figure 2. Summary of the average orientation and standard deviation for the type material of Steinichnus carlsbergi and Spongeliomorpha milfordenbsis. As a corollary of this ichnotaxonomic analysis, it is considered that the distinction of ichnospecies should be based on clear morphologic differences that can be easily observed and measured and not on subtle morphological contrasts that require statistical studies to define the assignment of new material.