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The stem crustacean Oelandocaris oelandica re-visited

100%

Stein M., Waloszek D., Maas A., Haug J. T., Muller K. J.

Acta Palaeontologica Polonica

2008

tom 53

nr 3

The arthropod Oelandocaris oelandica from the upper Middle Cambrian “Orsten” of Sweden was recently recognized as a member of the early phase of crustacean evolution based on additional morphological detail from new specimens. Here we present a detailed investigation of all available material. It includes the description of a 400 μm long specimen probably representing an early developmental stage. Variation in size correlated with variation of trunk−segment numbers allowed recognition of different instars. The largest specimens do not exceed an estimated length of about 1 mm, indicating that our material may consist only of immature specimens. The characteristic, extremely long antennula of O. oelandica branches into three long rods. It may have served as the major structure to sweep in food, aided by the two subsequent appendages. These and the more posterior limbs were also responsible for locomotion. Minute pores on the outer edges of the posterior limbs and on the trunk tergites possibly contained sensilla originally, which may have served as water−current detectors. The presence of a minute proximal endite only on the third head appendage suggests a rather basal position of this species within Crustacea, because comparable developmental stages of other known stem crustaceans have such an endite on more of their appendages. Reconstruction of O. oelandica and its life attitudes (referred to the largest instar known) benefited from the application of 3D modelling. These helped, e.g., in identifying the combination of the plesiomorphic feeding function of the antennulae and the specialisation of the exopods of the next two appendages as a step toward the development of a sweep−net mode of feeding, one of the key novelties in the evolution of Crustacea. Such a mode of feeding coupled with locomotion of the three anterior appendages is still practiced in the naupliar and metanaupliar phases of many extant eucrustaceans, and even some adults.

Phyletic evolution and iterative speciation in the persistent Pristiograptus dubius lineage

88%

Urbanek A., Radzevicius S., Kozlowska A., Teller L.

Acta Palaeontologica Polonica

2012

tom 57

nr 3

The paper focuses on patterns of the evolution of the simplest and longest−ranging (approximately 18 Ma) Silurian graptolite Pristiograptus dubius. The Pristiograptus dubius species group consists of the P. dubius stem lineage represented by a sequence of a number of subspecies displaying only small morphological changes as well as derivative species produced from the stem lineage by means of iterative speciation. This long raging graptolite lineage is the only one, apart of one retiolitid, which survived the most severe environmental event for graptolites, the Cyrtograptus lundgreni Event. Based on three−dimensional, isolated material two P. dubius groups taxa are distinguished. One group has an obtuse angle between the thecal lip and the succeeding thecal wall, the second group has a right or acute angle. Other characters differentiating P. dubius forms are: the shape of the apertural lips, differences in rhabdosome shape and size, and a different number of sicular rings. Sixteen species and subspecies of Pristiograptus from the East European Platform, Poland and Lithuania are discussed. Five new subspecies P. dubius magnus, P. dubius paezerensis, P. dubius praelodenicensis, P. dubius postfrequens, and P. dubius postmagnus are proposed.

Ograniczanie wyników

2 Acta Palaeontologica Polonica

1 Haug J. T.

1 Kozlowska A.

1 Maas A.

1 Muller K. J.

1 Radzevicius S.

1 Stein M.

1 Teller L.

1 Urbanek A.

1 Waloszek D.

1 2012

1 2008

Wyniki wyszukiwania

The stem crustacean Oelandocaris oelandica re-visited

Phyletic evolution and iterative speciation in the persistent Pristiograptus dubius lineage