The age of the primitive ichthyopterygian Thaisaurus is more accurately defined due to the discovery of a new species of the ammonoid genus Marcouxia from beds about 2.4 m below the horizon from which the holotype of Thaisaurus chonglakmanii was collected in the Phukhaothong Dolomite Member of the Chaiburi Formation in the Phatthalung area, southern Thailand. The shell of Marcouxia chaiburiensis sp. nov. is characterized by a quadrate whorl section, an arched venter with a sub-acute keel-like elevation, and spiny tubercles on the ventrolateral shoulders as well as numerous regularly spaced, radial or slightly prorsiradiate ribs. Because the range of Marcouxia is limited to the Columbites parisianus Subzone of the lower Spathian (upper Olenekian, Lower Triassic) in the western USA, the age of Thaisaurus is likely constrained to the early Spathian, thus suggesting it is one of the oldest known ichthyopterygians.
Abstract Relationships between growth increments of internal shell and age was studied in three neritic decapod cephalopods cultured in laboratory through their entire life cycles. The studied cephalopods were the nektic Sepioteuthis lessoniana d’Orbigny, 1826, Sepia pharaonis Ehrenberg, 1831 and Sepiella inermis Van Hasselt, 1835. Most of the relationship models are in cubic parabolic, except when numbers of increments were estimated from age in S. pharaonis . Differences of numbers of increments from the real age were higher in the pelagic S. lessoniana when compared to the benthic sepiids . The differences were higher in juvenile stages (< 60 days after hatching) than adult stages (> 60 days) in the three species. The increment rate is close to the “one day one increment” assumption. The differences of numbers of increments from the ages and the rate of increment apposition revealed the transition point of the life cycle from 60 days of age, corresponding to the sexual maturity or adult stages. Numbers of increments with higher accuracy are reevaluated to be reliable for age determination at least for the neritic species in the tropical zone, where environmental conditions are more stable, regarding the life styles and stages in life cycles of each species.
Comprising more than 800 extant species, the class Cephalopoda (octopuses, squid, cuttlefish, and nautiluses) is a fascinating group of marine conchiferan mollusks. Recently, the first cephalopod genome (of
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