Chondrites Sternberg, 1833

Emended diagnosis: regularly branching tunnel system consisting of a small number of sub-vertical master-shafts, connected to the ancient sediment-water interface, that branches at depth to form a dendritic network. Fill can be active or passive (diagnosis emended after Osgood, 1970; Fürsich, 1974; Fu, 1991; Uchman, 1995; Blissett and Pickerill, 2004; Uchman et al., 2012).

Remarks: Non-radial forms (hypidiomorphic) may be included within the ichnogenus.The tunnels of Chondrites neither touch nor cross each other (Fu, 1991), although some exceptions are reported (Blissett and Pickerill, 2004; Uchman et al., 2012). Branching may occur in a single plane or as three-dimensional forms, and may consist of up to six orders of branching (Fu, 1991). According to Fu (1991), the branching angle may range from 15° to 90° with an average value between 30°- 60°. The burrow fill is typically uniform/structure-less, though there are some rare accounts of meniscate fill (Fu, 1991). Burrow diameter is narrow and varies between burrow systems (1−5 mm according to Fu (1991); 0.1–10 mm according to Savrda and Bottjer, 1991), but it is generally constant within a single system (Fu, 1991; Savrda and Bottjer, 1991). According to Uchman (1999), tunnels are narrower than 0.95 mm.

Remarks. Chondrites is ethologically classified as a feeding trace (fodinichnia). It is generally assigned to depositivorous and/or suspensivorous annelids or sipunculids.

Morphology, Fill and Size: Chondrites is one of the most common and widely distributed trace fossils; due to its rootlike appearance it was originally interpreted as a plant fossil.  It consists of tunnel systems possessing a single or a small number of master shafts, presumably open to the surface, which ramifies with depth under acute angle to form a dendritic or root-like system (Osgood 1970; Fu 1991). Most of the burrows show an active fill, sometimes with portions preserving a meniscate structure. The burrows are unlined. The tunnel diameter remains constant in different parts of the burrow and typically is in the range of less than 1 mm to a few millimeters.

Diagnosis: Dendritic, smooth walled, regularly but asymmetrically branched small burrow systems that ordinarily do not interpenetrate or interconnect. Diameter of components within a given system remains more or less constant.

Recent work suggests that Chondrites may represent specialized feeding behavior that involves chemosymbiosis, being interpreted as a sulfide pump (Fu, 1990, Seilacher, 1990; Bromley, 1996). It has been regarded that the Chondrites animal developed adaptations to cope with oxygen-depleted conditions (Bromley & Ekdale, 1984; Savdra, 1992).