562 



NATURE 



[October 8, 1903 



is shown by the existence of Glossopteris in Upper Permian 

 rocks of Russia, bv the occurrence of several southern types 

 in plant-bearing beds of the Altai mountains, and by the 

 existence in Western Europe during the early stages of the 

 Triassic era of such southern genera as Neuroptendium and 

 Schizoneura. 



Triassic, Jurassic, and Wealden Floras. 

 It is unfortunate that the records of plant-life towards the 

 close of the Palaeozoic and during the succeeding Triassic 

 period are very fragmentary ; the documents are few in 

 number and instead of the fairly continuous chapters in 

 which the records of the Coal age have been preserved, 

 wo have to be content with a few blurred pages. During 

 the Triassic period the vegetation of the world gradually 

 changed its character; the balance of power was shifted 

 from the Vascular Cryptogams, the dominant group of the 

 Pala;ozoic era, to the Gymnosperms. It is not until we pass 

 up the geologic series as far as the Rhaetic formation, that 

 wi come to palaeobotanical records at all comparable in 

 their completeness with those of the Permo-Carboniferous 

 era ; but before considering the Rhaetic vegetation we must 

 glance at such scattered relics as remain of the vegetation 

 belonging to the period of transition between the Palaeozoic 

 and Mesozoic facies. It is regrettable that this transitional 

 period is unusually poor in documentary evidence that 

 might throw light on the gradual change in the facies of 

 Palceozoic vegetation. The new order, when once estab- 

 lished, persisted for many succeeding ages without under- 

 going any essential alteration. 



One of the few floras of early Triassic age of which 

 satisfactory relics have been preserved is that described in 

 1844 by Schimper and Mougeot from the Bunter Sandstones 

 of the Vosges. The genus Neuropteridium, a plant which 

 may be a true fern, or possibly a surviving member of the 

 Cycadofilices, is represented by a species which can hardly 

 be distinguished from that which flourished in South 

 America, South Africa, and India in the Permo-Carbon- 

 iferous period. This genus and another southern type, 

 Schizoneura, both of which are met with in the Triassic 

 rocks of the Vosges, would seem to point to a northern 

 migration of certain members of the Glossopteris flora, 

 which took place at the close of the Palaeozoic era. In the 

 Lower Triassic flora Conifers are relatively more abundant 

 than in the earlier periods ; such genera as Albertia (re- 

 sembling in its vegetative features some recent species of 

 Araucaria), Voltzia (with cones that cannot be closely 

 matched with those of any existing members of the Coni- 

 ferae), and other representatives of this class are common 

 fossils. Lepidodendra have apparently ceased to exist ; 

 Sigillaria may be said to survive in one somewhat doubtful 

 form, Sigillaria oculina. The genus Pleuromeia, which 

 makes its appearance in Triassic rocks, is known only in 

 the form of casts exhibiting a strong likeness to some 

 Palaeozoic Lycopods, and is perhaps more akin to Isoetes 

 than to any other existing plant. The Calamites are now 

 replaced by large Equisetaceous plants, which are best de- 

 scribed as Horsetails with much thicker stems than those 

 of their modern descendants. 



From Recoaro in Northern Italy some of the Vosges 

 genera have been recorded, and a few other European 

 localities have furnished similar relics of a Triassic vegeta- 

 tion. Passing to the peninsula of India, we find the genus 

 Glossopteris abundantly represented in strata which there 

 is good reason for regarding as homotaxial with the 

 European Trias, and the occurrence in the same beds of 

 some other genera of Permo-Carboniferous age shows that 

 the change in the character of the southern vegetation at 

 the close of the Palaeozoic era was much more gradual than 

 in the north. 



The comparative abundance of plant remains in the 

 northern hemisphere in rocks belonging to the Rhaetic form- 

 ation, a series of sediments so named from their development 

 in the Rhastian Alps, is in welcome contrast to the paucity 

 of the records from the underlying Triassic strata. From 

 Virginia and adjacent districts in the United States a rich 

 flora has been described, which by some authors is assigned 

 to the Keuper or Upper Triassic series, while others class 

 it as Rhaetic. A similar assemblage of plants is known also 

 from the Lettenkohle beds of Austria which, as Stur has 

 shown, clearly belong to the same period of vegetation as 

 the American flora. We need not, however, concern our- 



NO. 177 1, VOL. 68] 



selves with discussions as to the precise stratigraphical 

 position of these American and European plant-beds, but 

 may conveniently group together floras of Upper Triassic 

 and Rhjetic age since they exhibit but minor differences 

 from one another. Plants of Upper Triassic or Rhajtic age 

 are known from Scania and Franconia in Europe, Virginia 

 and elsewhere in North America, Honduras, Tonkin, 

 Australia, South Africa, Chili, and other parts of the world. 

 The geographical distribution of plants of approximately 

 Rhaetic age is shown in the following table, No. II., on p. 

 563, which demonstrates an almost world-wide range of a 

 vegetation of uniform character. The character of the plant- 

 world is entirely different from that which we have described 

 in speaking of the Palaeozoic floras. Gymnosperms have 

 ousted Vascular Cryptogams from their position of 

 superiority ; ferns, indeed, are still very abundant, but they 

 have undergone many and striking changes, notably in the 

 much smaller representation of the Marattiaceae. The 

 Paleeozoic Lycopods and Calamites have gone, and in their 

 place we have a wealth of Cycadean and Coniferous types. 

 As we ascend to the Jurassic plant-beds the change in the 

 vegetation is comparatively slight, and the same persistence 

 of a well-marked type of vegetation extends into the 

 Wealden period. It is a remarkable fact that after the 

 Paleeozoic floras had been replaced by those of the Mesozoic 

 era, the vegetation maintained a striking uniformity of 

 character, from the close of the Triassic up to the dawn of 

 the Cretaceous era. This statement is open to misconcep- 

 tion ; I do not wish to convey the idea that a palaeobotanist 

 would be unable to discriminate between floras from Rhaetic 

 and Wealden rocks ; but I wish to emphasise the fact that 

 in spite of specific, and to a less extent of generic, peculi- 

 arities, which enable us to determine, within narrow limits, 

 the age of a Mesozoic flora, the main features of the vegeta- 

 tion remained the same through a long succession of ages. 

 The accompanying tables (Nos. III. and IV.) illustrate the 

 geographical distribution of some of the leading types of 

 Mesozoic plants during the Jurassic and Wealden periods, 

 and demonstrate not only the striking differences between 

 the Mesozoic and Palaeozoic floras, but also the much greater 

 uniformity in the vegetation of the world during the 

 Secondary era than in the preceding Permo-Carboniferous 

 epoch. 



Mesozoic Floras. 

 It may be of interest to glance at some of the leading 

 types of' Mesozoic floras with the view of comparing them 

 with their modern representatives. We are so familiar with 

 the present position of the flowering plants in the vegeta- 

 tion of the world, that it is difficult for us to form a con- 

 ception of a state of things in the history of the plant- 

 kingdom in which Angiosperms had no part. 



a. Conifers. 



How may we describe the characteristic features of Rhaetic 

 and Jurassic floras? Gymnosperms, so far as we know, 

 marked the highest level of plant-evolution. Conifers were 

 abundant, but the majority were not members of that group 

 to which the best known and most widely distributed 

 modern forms belong. 



A comparison of fossil and recent conifers is rendered 

 difficult by the lack of satisfactory evidence as to the system- 

 atic position of many of the commoner types met with in 

 Mesozoic rocks. There are, however, certain broad 

 generalisations which we are justified in making ; such 

 genera as the Pines, Firs, Larches, and other members of 

 the Abietineae appear to have occupied a subordinate posi- 

 tion during the Triassic and Jurassic eras ; it is among the 

 relics of Wealden and Lower Cretaceous floras that cones 

 and vegetative shoots like those of recent Pines occur for 

 the first time in a position of importance. There are several 

 Mesozoic Conifers to which such artificial designations as 

 Pagiophyllum, Brachyphyllum, and others have been 

 assigned, which cannot be referred with certainty to a 

 particular section of the Coniferae ; these forms, however, 

 exhibit distinct indications of a close relationship with the 

 Araucarieee, represented in modern floras by Araucaria and 

 Agathis. The abundance of cones in Jurassic strata show- 

 ing the characteristic features of those of recent species of 

 Araucaria affords trustworthy evidence as to the antiquity 

 of the Araucarieaj and demonstrates their wide geographical 

 distribution during the Mesozoic era. At the present day 

 the Araucarieae comprise the two genera Araucaria and 



