Early Mesozoic evolution of alivincular bivalve ligaments and its implications for the timing of the ''Mesozoic marine revolution''

The early Mesozoic radiation of the Pteriomorphia was accompanied and furthered by the development of several new types of alivincular ligaments. These new types evolved as modifications of the primitive alivincular-areate (new term) ligament, which is characterized by an ontogenetic shift of both the central resilium and the straight lateral ligament in the direction of main shell growth. Arching of the attachment surface of the ligament led to the alivincular-arcuate (new term) ligament type, which has been realized by the Ostreidae only. By contrast, a replacement of the lateral ligament by hinge teeth, limiting the (primary) ligament to a central groove (alivincular-fossate, new term), has evolved independently in three families (Dimyidae, Plicatulidae and Spondylidae). Functionally, both kinds of modification effectively impede shearing of the valves and are interpreted as an antipredatory adaptation advantageous in the cemented habit of these families. The alivincular-alate (new term) ligament of the Entoliidae and Pectinidae differs from the other types of alivincular ligaments by different growth directions of resilium and lateral ligament, which result in an internal position of the resilium suitable for fast and powerful opening of the valves. This arrangement is an important prerequisite for effective swimming, which, in its turn, is a behaviour chiefly used to escape from predator attacks. The simultaneous early Mesozoic appearance of different antipredatory adaptations within independent clades hints at increased predator pressure as a stimulant and may therefore point to a contemporaneous proliferation of durophagous predators. Hence, an important aspect of the 'Mesozoic marine revolution' might have started earlier than previously thought.     

Polarizing analysis of the crimped collagen ligament of the maxillary barbel in Parauchenipterus galeatus, and the functional implications

A polarizing analysis of the crimped collagen ligament in the maxillary barbel of the catfish Parauchenipterus galeatus showed clearly that the maxillo-mandibular ligament of this species is formed by two types of collagen fibres. The first is composed of uncrimped, and the second of crimped collagen fibres. An anatomical examination of the muscular and osteological components which act in the movement of the barbel revealed that abduction of the barbel is performed by the extensor tentaculi muscle and the mechanism of adduction by muscular relaxation of the extensor tentaculi combined with release of the stored elastic energy of the maxillo-mandibular ligament. This ligament has rubber properties and because of this can store energy during abduction (first stage of the cycle of barbel movement). The stored energy is released when the action of the extensor tentaculi muscle ceases. The second half of the locomotory cycle is to return the maxilla to its original position by the antagonistic action of the ligament. The crimping of the ligament permits the take up of slack, allowing greater extension, with less chance of snapping. The connective tissue between the hyomandibular and the maxilla may assist the adduction of the barbel.