Tres maderas de gimnospermas cretácicas del Norte de México

Abstract

EIGHT WOOD SAMPLES, ONE FROM LOWER CRETACEOUS SEDIMENTS OF LAMPAZOS, SONORA, AND SEVEN FROM UPPER CRETACEOUS (MAESTRICHTIAN) SEDIMENTS OF THE OLMOS FORMATION, COAHUILA. HAVE ANATOMICAL CHARACTERISTICS SIMILAR TO THE WOOD OF CONIFERS. WOOD OF THE PLANT FROM SONORA RESEMBLES THE WOOD OFEXTANT ARAUCARIACEAE. HOWEVER, PRESENCE OF AXIAL SECRETORY CANALS AND SEPTATE FIBERS IN THE FOSSIL MATERIAL MAKES IT UNLIKE ANY EXTANT TAXON OF THIS FAMILY. THESE SAME CHARACTERS TYPIFY BRACHYOXYLON JEFFREY WITH WHICH THE WOOD OF SONORA COMPARES POSITIVELY. WOODS FROM COAHUILA ARE SIMILAR TO THE WOOD OF PODORCARPACEAE AND TAXODIACEAE PODOCARPOXYLON GOTHAN AND TAXODIOXYLON HARTING EM. GOTHAN ARE DISTINGUISHED BASED ON THE SHAPE OF TRACHEIDS AS SEEN IN TRANSVERSE SECTION, NUMBER AND TYPE OF PITS IN THE CROSSFIELDS, AND TO SOME EXTENT THE AMOUNT OF AXIAL AND RADIAL PARENCHYMA. LITTLE KNOWLEDGE ON THE VARIABILITY OF THE WOOD OF THESE PLANTS RESTRAINS THEIR SPECIFIC IDENTIFICATION. PRESENCE OF THESE PLANTS IN CRETACEOUS SEDIMENTS CONTINUES TO DOCUMENT THE DIVERSITY OF MESOZOIC CONIFERS IN NORTHERN MEXICO, REINFORCES THE PRESENCE OF A FAVORABLE CLIMATE FOR PLANT GROWTH DURING DEPOSITION OF THESE GEOLOGIC UNITS, AND SUGGESTS THAT THE VASCULAR CAMBIUM CEASED ITS ACTIVITY FOR SHORT PERIODS OF TIME ONLY.