3-D burrow morphology of Chondrites targionii (Brongniart, 1828) from an Upper Jurassic firmground of Southern Germany

Chondrites is a common ichnogenus and typified by Chondrites targionii, with peak appearance in Upper Cretaceous deep-marine deposits. These burrows were commonly developed in soft substrate, which suffered from subsequent compaction that obliterated their three-dimensional (3-D) morphology and characteristics aiding the producers’ interpretation, although deep-tier firmground burrows occur too. Micro-CT-scanning of a sample from an Upper Jurassic (Kimmeridgian) carbonate firmground (southern Germany) allowed the reconstruction of the 3-D morphology of C. targionii for the first time. For comparison with the sample, all six C. targionii syntypes were morphometrically analysed. Within the sample, two-dimensional (2-D) and 3-D morphometric analyses identified 48 burrow parts containing 221 branches (up to the third branch order), another 29 tunnels with obviously very low branch width, one crossing tunnel and another tunnel with scale-leaf-shaped structures on its outer surface. Eleven identified burrow portions are likely parts of three larger burrow systems. With intermediate effect size, statistically compared branch widths revealed significantly decreasing widths from unbranched tunnels over root branches to their side branches, which, along with the scale-leaf-shaped structures, may indicate a vermiform tracemaker. The results of the 3-D analyses revealed the ratio of tunnel width to burrow system width as useful discriminator of C. targionii covering a larger portion of its morphological variation