A STUDY OF THE THECAL MORPHOLOGY OF EARLY ORDOVICIAN SINOGRAPTUS TYPICALIS:AN ADDENDUM

Thecal morphology of the Sinograptus typicalis Mu is crucial to understand its ecological environment. This study re-presented thecal morphological features of Sinograptus and reinterpreted its ecological environment on the basis of previous studies.Two stipes of the Sinograptus were reconstructed and each has a complete thecal,which comprises three dorsal thecal folds and three ventral thecal folds.The first dorsal thecal fold is prothecal fold of the thecal(th_n),which extends beneath the second dorsal thecal fold(prothecal fold of thecal th_n+1),forming an L-shape curve.Subsequently,the thecal develops a ventral thecal fold that is perpendicular to the stipe beneath the prothecal fold of thecal th_n+2.Thecal apertures are present at tip of the thecal.Crevice-like thecal apertures with transverse expansion occur at the upper part of the thecal in the left-hand side stipe,whereas only the dorsal part of thecal apertures is observed in the right-hand side stipe,showing that the thecal apertures on the left-and right-hand side stipes of the Sinograptus point to opposite directions.This indicates that the Sinograptus suspended and rotated slowly within a low-energy hydrodynamic environment.Spines of dorsal and ventral thecal folds are well developed in the Sinograptus, suggesting that they are spine stenothermal species living in warm water.Therefore,Sinograptus suspended in warm marine water with weak hydrodynamics.

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