Structure , habitat and seed of Mariopteris ZEILLER

The genus Mariopteris ZEILLER is distributed from the Namurian A to the early Stephanian in Europe. In the ­Dobrudzha­Basin,­numerous­specimens­are­found­mainly­in­flood­plain­claystones­and­siltstones.­The­habitat­and­ reconstruction­of­the­plants­suggests­a­creeping­manner­of­growth.­Climatic­changes­are­reflected­in­the­architecture­ of­species.­One­specimen­has­an­attached­seed­proving­the­classification­of­the­genus­as­a­pteridosperm.


the stem
It is straight or slightly sinuous and covered by longitudinal striations and regular short transverse bars.A group of plants, regardedbyDANZE-CORSIN(1953)asalinae, do not have transversebarsandBOERSMA(1972)excludesthemfrom Mariopteris.
There are few illustrations of stems.The stem is slightly elliptical with the leaf stalks, or their marks, situated at equal distances on the stem.One pair of leaves is located on the lowerexternalpartandanotherpairisapproximatelyhalf thedistancetothetopSTUR (1885,pl.22,fig.1).STUR (1885,p.285,pl.22,fig.1)regardsthisarrangementasaspiraloffourbasesinonecycle.DANZE-CORSIN(1953, figs.7-9)supposedtheretobeahelixbuiltoffiveleaves.ZEILLER (1888) regards the arrangement of the leaves as two generatrice situated at an angle less than 180°.The longitudinalstriationofthestem,illustratedinmanyfigures,is straight and does not indicate any spiral growth.The position of each of the four consecutive leaves is repeated along the stem, and can be regarded as helical that is not the result ofaxialrotation.Therefore,thefourfoldalternationcharacterizestheleafarrangement-Fig. 1.
There is neither information nor data for the total length of the stem, although the length of one cycle of leaves in a well-developedstemis280mm(STUR,1885,Pl.22,fig.1).Theknownmaximumwidthforastemis22mmwitha gradual reduction in each cycle of about 1 mm.If this reductioninwidthwaseven,thelengthwouldbeabout21x280 mm = 6.0 m or more.

stalks and leaves
The leaves are pinnate with a massive, longitudinally striate naked stalk (petiole) up to 150 mm long and 7 mm wide at its base.Thestalkaxesaredirectedupwardsata40°angletothe stemaxis(DANZE-CORSIN1953,figs.7-9).Thestalkbifurcatesforthefirsttimeatanopenangleintwoparts(rachises in many authors), which in their turn bifurcate up to 5 times.After the third bifurcation they are covered by pinnae (lamina).The leaf blade is parallel to the stem and to the ground.Itsaxisiseitherparallelto,oratsomeangle,tothestemaxis.The pinnae in the blade are in one plane that is parallel to the stemaxisandtheground.Thepinnuleslength,widthandneuropteroid or pecopteroid base depends on the species.BOERS MA (1972, tables VII to XIV) gives data for the size of the leaf elements for 7 species.Table 1 summarizes some of the data.Pinnules near the top of the plant can have spinelike lobes or are totally formed as spines.
In the mariopterids, two architectural types of leaf blades are known and designated as bipartite and quadripartite.DANZE-CORSIN(1953,p.57-58,p.256)indicatedthatthe fronds of Mariopteris have in its base and along its length quadripartite leaves but are bipartite in its sub-terminal part.The transformation to bipartite in quadripartite leaves is doc-umentedbyDANZE- CORSIN(1953pl.40,fig.1;Pl10,fig. 1,respectivelypl.11,fig.2).Thiscanberegardedasacceleration during the growth in the leaf architecture of a species.She (idem p. 58, plate 56) supposed that simple pinna were situated at the terminal part of the stem, but this is not evidentinthefigures.BOERSMA(1972)doesnotmention anything about simple pinna, although he accepts that the genus Mariopterisconsistsoftwogroupsofspecies-one that has bipartite leaves and another that has quadripartite leaves.Thebipartiteleafpossessesexteriorpinnathatgradually diminish in length.The quadripartite leaf possesses a long pinna at the base of the tertiary rachis, followed by a pinnaofcalceolateform.Forthisreasonhedividesthemariopterids into two genera: Karinopteris BOERSMA for bipartite fronds and Mariopteris (ZEILLER, BOERSMA emend.)for quadripartite fronds.BOERSMA (1972) does not comment on the opinion of DANZE-CORSIN that bipartite leaves are situated at the top of plants with quadripartite leaves.However, he indicated as "aberrant forms" those specimens of Mariopteris (ZEILLER, BOERSMA emend.) that have bipartite leaves.M. nervosa KIDSTON (1925, pl.55)should all be regarded as "minute fronds of Mariopteris".This suggeststhattheproposedtaxonomyshouldbere-examined.
The leaves are large and require strong stalks to support them.The length of the leaves is longer than the length of the distance between the leaf stalks.The plane of the leaf blade most probably is parallel to the ground.In adult plants thelowerpairofpinnulesisabout7cmwhilethenextis about 9 cm above the ground.DANZE-CORSIN(1953,fig.7,p.45)acceptsasAphlebiasomesmallexcrescencesatthebaseoftheleaves.Itisnot supported here.

Pinna, pinnules and venation
The pinnae are lanceolate and slightly or more asymmetric elements formed on stalk branches.The width and the length (whichcanbemorethan300mm),dependonthespecies and on the position on the leaf.The pinnules are the smallest part of the leaf blade and their morphology depends on the species as detailed by BOERSMA (1972).They are attached by pecopteroid or sphenopteroid bases.The basal basiscopic and acroscopic pinnules in a pinna of ultimate order, have strongly developed outgrowths of their basal basiscopic lobes.In some cases, the terminal pinnules of a pinna, and part or all of the pinnules in lower situated pinna, are formed in spines, asfiguredbyHUTH (1912,VIII,143,fig.1).The pinnules have a midvein that arises obliquely from the stalk, curves and reaches at least halfway up the pinnules and secondary vein branches that emerge alternately from the midveinanddichotomiseoneormoretimes,endingattheapex of the pinnules.

the spines
The spine-like form of the pinnules seems to be provoked byheliotaxes,whenayoungspecimenisinfullshadowbellow adult plants.In such cases, the leaves of its top part are directed upwards towards the light.They are covered by spine-like pinnules as documented in HUTH (1912, VIII 143,Fig.1).Thespineshelptheenvelopedleavesbehindthe plant top to intertwine through the leaves of the adult species, (that shade out the light), and then to continue development over the other leaf.This is shown by M. carnosa (COR-SIN,1932,Pl.68).Theolderspecimenhaslargerpinnules andnospines(leftpartofthefigure).Theyoungerspecimen (visible along x-y line), has smaller pinnules and numerous spines that are at y -the top of the specimen.When the plant reaches the sunny surface, the spine-like pinnules start to transformthemselvesfromtheirbasetowardstheapexas normal pinnules.Different phases of this transformation are illustratedinnumerousfiguresofCORSIN (1932-pl.63, fig.2,pl.72,figs.2,4),DANZE-CORSIN(1953)andinthat of HUTH (1912).Therefore, Mariopteris under some conditions can be intertwining with plants at an early phase of growth.

reProductIon
Until now, it has generally been accepted that the mariopterids are pteridosperms, but there is a lack of evidence of their methodofreproduction.GOTHAN(1935,p.8-14)supposed that vegetative reproduction was possible by means of buds.BOERSMA (1969) established that Mariopteris latifolia (BRONGNIART) ZEILLER is really a fern, and referred it to a new genus Fortopteris BOERSMA, as Fortopteris latifolia (BRONGNIART) BOERSMA.It has quadripartite constructed leaves.This species (and genus) differs from the mariopterids by some denticulation of its pinnules, and by its stem having no transverse bars.The missing transverse bars on the stem stimulated DANZE-CORSIN to create a "group alineae" in the mariopterids.
The possibility that Mariopteris is a seed fern is based on an analogy with Dicksonites pluckenetyii which has small seeds situated on the lower surface of its pinna.In Mariopteris the majority of fossils show the upper surface.This suggestedre-examinationofslabswithMariopteris in the Dobrudzha collection.A seed-bearing Mariopteris beneckeii (sampleN16344)wasfoundonasamplefromborehole number 218 at 1406 m depth, corresponding to a level about 218mabovethebaseoftheMogilishteFormationinthe middlepartoftheLangsettian.ThespecimenshownonFig. 3isfromayoungleafinwhichapartisreversed,exposing its lower surface possessing at least one small seed.This suggests that Mariopteris as many other ancient plants e.g.some lepidodendrons, after some time in a vegetative phase entered a reproductive phase that then led to the death of the plant.Two advantages of this strategy are that the seed is dispersed far from the mother plant root (6 m. or more) and may be out of the Mariopteris carpet thereby enlarging its range, and secondly in opening up places in the habitat for a new generation of plants.
In the one metre interval at 1406 m depth in borehole 218, there are seven samples of Mariopteris and another two with Eusphenopteris.Individual pieces of shale are about 1 cmthickandtheunexposedpartsofthemalsocontainplant remains.The plants were preserved with their upper surface upwards,eitherbybeingflattenedbyaninrushofturbid floodwater,ordeathafterareproductionphase.Leavesreversed with their lower surface upwards are rarely found and illustrated.This seems to indicate that mariopterids lived in places with a low water dynamic.

Mariopteris constructIon
Parts of the published accounts of several species are used for the construction of the Mariopteris plant.Those stems possessing the marks of leaf bases show them to be equally spaced with the distances between them being no more than 170mm.Theleavesareflatanddirectedatangletothestem direction.The construction of the fronds is regarded as phases in architecture building with the steps as: appearance -acceleration-retardation-disappearance-Figure4.

HAbItAt
The habitat of the mariopterids has been rather poorly discussedinthepast,althoughtheopinionbyGOTHAN(1913 p.88) that Mariopterislivedontheforestfloorasacreeping, climbing and intertwining plant is generally accepted.Unfortunately, there is no real evidence to support this view.KERP&BARTHEL(1993plate5,figures1-4)acceptthat the documented hooks are of a climbing Karinopteris, but the plant seams to be Eusphenopteris So, a climbing habit of Mariopteris is not supported by the facts.
IntheDobrudzhaCoalfield,Mariopteris specimens occur in all coal bearing lithostratigraphic units.They are rare intheRakovskiFormation(NamurianA),frequentinthe Mogilishte,MakedonkaandKrupenFormations(Namurian Duringflooding,thecreepingplantsweresubmergedin muddywaterandfinesedimentmayhavebeendepositedon them..ThiscouldexplainwhyMariopteris has no stomata ontheuppersurface,astheywouldhavebeenfilledbythe finesediment.Thestalkwasmassiveenoughtosupportthe largeleafandthefinesedimentcoveringitaftertheretreat ofthefloodwater.Sedimentcouldbewashedawaybyrainfall, but if it dried the plant might die.In cases when the floodscoveredtheplaintosignificantdepth,theamountof thefinesedimentarydepositcouldbesothickandheavy, that the Mariopteris plants were crushed below it.This is the most probable reason why the leaves are mainly preserved with their smooth upper surface uppermost.
The reproduction problem could also be bound up with floods.Theareaofafloodplainthatiscoveredbydeepwater looses all its Mariopteris carpet, but the carpet seems to berestoredveryrapidly,postflooding.Ferns,ofcourse,can quicklyre-establishfromdispersedspores.ForMariopteris there are two possibilities that are at present hypothetical.The plants may quickly spread from areas beyond the reach ofthefloodwatersorbere-establishedbyseeds.

strAtIgrAPHIc dIstrIbutIon In euroPe
The genus is known from the Namurian A to the Cantabrian and the stratigraphic distribution in Europe is considered by HUTH (1912).M. acuta, M. beneckeii, M. muricata and M. carnosa are found in the Donets Basin (NOVIK, 1952) and the North Caucasus (ANISIMOVA, 1979: NO-VIK, 1978).The species diversity in the Dobrudzha Coal-fieldiscomparablewiththatofotherbasinsaroundtheVariscan ranges in Europe.

conclusIon
Mariopteris plants formed a cover in the coastal and alluvial plains and around the swamps.This, combined with their creepinggrowthform,explainswhytheyaresowidelydis-  tributed in the Variscan foreland of Europe.Their creeping habit helped them to cover the plains and to migrate along riverbedsandthefloodterraces.Eveniftheyadvanced yearlybyonly1metre,inthe2-3Millionyears(lifespan of M. acute and of M. medicate)theycouldmigrate2-3.000 kilometres along river banks, the coastal margin of the sea, the outer fringes of large lakes and through the swamps from the British isles to the Turkish Black sea coast and the Caucasus.The climatic changes towards drier conditions and longer periods without rainfall would have reduced their habitat until they disappeared around the beginning of the Stephanian.

AcKnowledgement
This paper was presented at the Zagreb meeting 2011 and is a contribution to IGCP Project 575 "Pennsylvanian terrestrial habitats and biotas of southeastern Euramerica".I am grateful to the reviewers for their remarks and help with language editing.

Figure 3 .
Figure 3.The seed-bearing specimen 16344 Dobrudzha Coalfield borehole 214, 1508 m in the Mogilishte Formation in the mid part of the Langsettian.I think this image would benefit from annotation eg an arrow pointing to the seed?

table 2 :
Mariopteris and associated plants in flood plain of the Svoge and Dobrudzha Coalfields.