Rochester Shale crinoids also had other parasites. In some assemblages a very high proportion of Icthyocrinus specimens had their columns and calyx plates riddled by circular, parabolic pits (Brett, 1978b), a trace fossil referred to by Brett (1985) as Tremichnus. These pits, which were probably formed by some type of worm, were most certainly harmful to their hosts and like many parasites were quite specific to particular host crinoids. Icthyocrinus was the most frequently infested but Tremichnus occurs less commonly in a few other crinoids, including calceocrinids and Eucalyptocrinites.
Carnivores in the Rochester seafloor communities were relatively uncommon and surely not as significant as in modern communities. Trilobites may have been partly detritivores or scavengers, feeding on decaying organisms; they lacked biting mouthparts and so were constrained to feeding on relatively soft food. Certain trilobites, however, were probably predators on soft-bodied organisms. By analogy with modern horseshoe crabs, trilobites may have utilized the bristly inner joints of their legs (gnathobases) to grasp soft bodied organisms, such as worms, from within the sediment and pass them forward to the mouth, which was underlain by a wedge-shaped plate, the hypostome. In some cases, trilobites may have worked the food between anterior legs and the hypostome to break it down into small pieces to be ingested by a suctorial mouth. In rare cases the coffee bean shaped trace fossils of trilobites (Rusophycus) (Figure 429) from the Rochester Shale and elsewhere have been observed to intercept a worm burrow in the sediment and it is likely that these represent hunting traces of trilobites (Tetreault 1990). Forms like Trimerus that had smooth, spade-like cephala may have been adapted to furrowing in sediment after prey; they match in size and shape some associated Rusophycus traces (Tetreault 1990). Large Arctinurus were evidently rather slow moving and molted infrequently, as evidenced by the presence of encrusting worm tubes, bryozoans and small attached brachiopods (Figures 483 - 485) on their dorsal exoskeletons (Tetreault 1990, 1992) as do modern large horseshoe crabs, and may have scuttled on the surface in search of detritus.
Ophiuroids or brittle stars were also probably actively crawling scavengers and predators, using their entire slender sinuous rays to propel themselves along the seafloor in search of organic matter and microorganisms, which they processed with the small "jaws" at the corners of their star shaped mouths.
Starfish or sea stars are known to be important predators in modern communities. These animals actively crawl using tube feet and some forms are capable of wrapping their arms around bivalved prey such as clams and exerting a pull with tube feet. Eventually, these animals weaken their prey sufficiently to allow a portion of the starfish stomach lining to be inserted and start digesting their victim's tissues. This same mode of life may have been present in Palaeaster (Figures 218 and 219) from the Rochester Shale.
The master predators of the Silurian seafloor in the absence of any fishes, were probably nautiloid cephalopods. Based on modern analogs, notably the chambered Nautilus, these were streamlined, actively swimming predators. Dawsonoceras could be up to a meter long and up to 10 cm in diameter. By analogy to Nautilus it probably swam using a form of jet propulsion by taking water into its mantle cavity and forcing it out through a narrow nozzle referred to as a hyponome. Relatively sophisticated eyes enabled nautiloids to track prey. Nautiloids had a cluster of tentacles for seizing other organisms such as trilobites. These surrounded a mouth equipped with a sharp, parrot-like beak that could tear into their prey. A number of trilobites illustrated in this book show scallop-shaped divots in their exoskeletons that are interpreted as bite marks. These may very well record predatory strikes by large nautiloids; however, some of these trilobites clearly survived the attacks and lived to partially heal their wounds. Jawed fishes would become a more important element in the Middle Devonian, and they may have had a strong influence on prey taxa, but they were only just coming on the scene during the later Silurian and none lived in the Appalachian basin seas.
Carnivores in the Rochester seafloor communities were relatively uncommon and surely not as significant as in modern communities. Trilobites may have been partly detritivores or scavengers, feeding on decaying organisms; they lacked biting mouthparts and so were constrained to feeding on relatively soft food. Certain trilobites, however, were probably predators on soft-bodied organisms. By analogy with modern horseshoe crabs, trilobites may have utilized the bristly inner joints of their legs (gnathobases) to grasp soft bodied organisms, such as worms, from within the sediment and pass them forward to the mouth, which was underlain by a wedge-shaped plate, the hypostome. In some cases, trilobites may have worked the food between anterior legs and the hypostome to break it down into small pieces to be ingested by a suctorial mouth. In rare cases the coffee bean shaped trace fossils of trilobites (Rusophycus) (Figure 429) from the Rochester Shale and elsewhere have been observed to intercept a worm burrow in the sediment and it is likely that these represent hunting traces of trilobites (Tetreault 1990). Forms like Trimerus that had smooth, spade-like cephala may have been adapted to furrowing in sediment after prey; they match in size and shape some associated Rusophycus traces (Tetreault 1990). Large Arctinurus were evidently rather slow moving and molted infrequently, as evidenced by the presence of encrusting worm tubes, bryozoans and small attached brachiopods (Figures 483 - 485) on their dorsal exoskeletons (Tetreault 1990, 1992) as do modern large horseshoe crabs, and may have scuttled on the surface in search of detritus.
Ophiuroids or brittle stars were also probably actively crawling scavengers and predators, using their entire slender sinuous rays to propel themselves along the seafloor in search of organic matter and microorganisms, which they processed with the small "jaws" at the corners of their star shaped mouths.
Starfish or sea stars are known to be important predators in modern communities. These animals actively crawl using tube feet and some forms are capable of wrapping their arms around bivalved prey such as clams and exerting a pull with tube feet. Eventually, these animals weaken their prey sufficiently to allow a portion of the starfish stomach lining to be inserted and start digesting their victim's tissues. This same mode of life may have been present in Palaeaster (Figures 218 and 219) from the Rochester Shale.
The master predators of the Silurian seafloor in the absence of any fishes, were probably nautiloid cephalopods. Based on modern analogs, notably the chambered Nautilus, these were streamlined, actively swimming predators. Dawsonoceras could be up to a meter long and up to 10 cm in diameter. By analogy to Nautilus it probably swam using a form of jet propulsion by taking water into its mantle cavity and forcing it out through a narrow nozzle referred to as a hyponome. Relatively sophisticated eyes enabled nautiloids to track prey. Nautiloids had a cluster of tentacles for seizing other organisms such as trilobites. These surrounded a mouth equipped with a sharp, parrot-like beak that could tear into their prey. A number of trilobites illustrated in this book show scallop-shaped divots in their exoskeletons that are interpreted as bite marks. These may very well record predatory strikes by large nautiloids; however, some of these trilobites clearly survived the attacks and lived to partially heal their wounds. Jawed fishes would become a more important element in the Middle Devonian, and they may have had a strong influence on prey taxa, but they were only just coming on the scene during the later Silurian and none lived in the Appalachian basin seas.
page 18