1mm

5. What are the possible affinities of these mesofossils?

Small is beautiful:

Jennifer L. Morris* , Dianne Edwards , John B. Richardson1 1 2

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2School of Earth and Ocean Sciences, Cardiff University, Wales, UKNatural History Museum, London, UK

*morrisj2@cf.ac.uk

new investigations into Lower Devonian plant mesofossils and their sporesin situ

Funded by a Leverhulme Trust Research GrantWebsite: jennymorris.web.officelive.com

4. Can ontogenic or developmental stages berecognised?

3. Were these mesofossils adapted to lifeon land?

1. How diverse are theseminute plants?

2. Are their sporesrecognised taxa from thedispersed spore record?

in situ

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The Ordovician to Early Devonian was a time of great‘experimentation’ by vegetation to conquer the land, a periodfrom which the major lineages of plants we know todaydiversified.

It is likely that several attempts at phototerrestrialisation weremade: several groups may have adapted novel innovations forsurvival on land, but most became extinct, followed by thediversification of more successful lineages.

Basal vascular plants emerged during the Early Silurian: theearliest megafossil record is from the Wenlock , although triletespores, thought to have derived from tracheophytes, have beenfound from the Late Ordovician . These basal plants continued todiversify into the Late Silurian and Early Devonian (seerhyniophytoid images on right).

Cryptospores are also a major component of the dispersed sporerecord, found back to the Mid Ordovician , with evidence of rarecryptospore-producers from the Late Ordovician . Little is knownregarding these plants due to their poor preservation in the rockrecord.

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Introduction Mesofossils�

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A second component of Early Devonian vegetation hasbeen discovered; a group of minute naked branchingaxes with terminal sporangia of variable morphologies(see figure on right).

An assemblage of these mesofossils from a quarry nearBrecon, Wales, show complex branching of thesubtending axes, and hence can be considered aseparate group from the larger rhyniophytes.

However fossils from this locality are predominatelycoalified compressions, and little anatomical detail hasbeen preserved.

Similar mesofossils from a locality in Shropshire areexceptionally preserved via charcoalification (seebelow), with anatomical detail and spores,including cryptospores.

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in situ

This assemblage is currently being investigated with thefollowing questions.

Typical rhyniophytoids of the Early Devonian

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5mm1mm

mesofossils megafossils

~1mm in sporangium height

1. Simple discoidal to funnel-shaped

3. Oval sporangia

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Cooksonia pertoni Streelispora Aneurospora Ambitisporites10µm

Cooksonia banksii200µm 5µm

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5µm

Cooksonioid sporangia Ambitisporites

Resilitheca microgranulate triletes

Sporathylacium500µm500µm

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cuticle-covered discs with stalksAneurospora Emphanisporites

CymbohilatesStreelispora

2. Sporangia with two equal valves 4. Elongate sporangia

5. Valvate sporangia

6. Bifurcating sporangia

200µm 200µm 5µm

microgranulate triletes

200µm

Tortilicaulis microgranulate triletes

References1. Edwards D., Feehan J., and Smith D.G. 1983. A late Wenlock flora from Co. Tipperary, Ireland. , 86: 19-36.2. Steemans P., Le Hérissé A., Melvin J., Miller M.A., Paris F., Verniers J. and Wellman C.H. 2009. Origin and radiation of the earliest vascular land plants. 324: 353.3. Strother P.K., Al-Hajri S. and Traverse A. 1996. New evidence for land plants from the lower Middle Ordovician of Saudi Arabia. , 24(1): 55-58.4. Wellman C.H., Osterloff P.L. and Mohiuddin U. 2003. Fragments of the earliest land plants. 425: 282-285.5. Edwards D. 1996. New insights into early land ecosystems: A glimpse of a Lilliputian world. 90: 159-174.6. Boyce K.C. 2008. How green was The importance of size in understanding the early evolution of physiology in the vascular plant lineage. 34(2): 179-194

Botanical Journal of the Linnean Society

Science

Geology

Nature

Review of Palaeobotany and Palynology

Cooksonia? Paleobiology .

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Sporangial morphology is highly diverse:

mechanisms are also variable,from:Dehiscence

from discoidal, funnel-shaped, bivalvedreniform, oval, elongate to birfurcating (seeboxes 1 to 6); and therefore representseveral genera.

disintegration of the sporangial wall orcuticle ( and cooksonioidsporangia);splitting marginally into two equal valves( and );longitudinal splitting into two equal valves( );dehiscence associated with twisting;(valvate dehiscence (into 3 or more valves).

Cooksonia

Resilitheca Sporathylacium

Salopella

Tortilicaulis)

200µm 200µm

Salopella Ambitisporites

fused laevigate tetrads

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Both have beenfound within these fossils.

Some taxa have been described from thedispersed spore record (e.g.

) , while others are new(including a new species of ).

The: one plant species may produce

spores from different taxa (e.g.can contain

).

The; this is one

of the main challenges of linking thespores with the dispersed spore record.

Streelispora

newportensis, Cymbohilates horridus,

Ambitisporites

Aneurospora

Cooksonia

pertoni Aneurospora,

Streelispora or Ambitisporites

in situ

trilete and cryptospore taxa

spores also reveal crypticdiversity

same spore taxa can be known fromseveral different plant species

in situ

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Evidence to suggest that these mesofossils were adapted to life onland include:

(a and b);(c and d);

(e and f).

There is some debate as to whether the sub-aerial axial parts ofthese plants were of sufficient diameter to support all the functionsrequired for survival on the land, in particular photosynthesis

gametophyte-dependant , although there is no fossil evidence tosuggest this.

6. Ithas been suggested that these minute plants may have been

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stomatacuticular coveringswater - conducting cells in the axial parts

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The difference stages of sporangial maturity have been observed,from:

immature, sporangia (g);sporangia (h);and , empty sporangia (i).

Spore development has also been recognised, from:spores (j), associated with (k);spores, particularly tetrads (l);

(m), sometimes covered withdisintegrating envelopes / peripheral layers (n);

(o).

pre-dehiscenceduring active dehiscence

post-dehiscence

immature tapetal residueenvelopedloose to parting tetrads/dyads

loose monads/hilates

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Despite having very well preserved sporangial anatomy, this assemblage of mesofossils only have, and therefore .

F ithout the knowledge of the branching habit, it has not previouslybeen possible to distinguish these mesofossils as a separate group to the larger rhyniophytes e.g.

etc. It is

A from the Brecon Beacons, preserved as coalifiedcompressions, include specimens with longer lengths of subtending axes (see p to s). Somespecimens have (at least pseudomonopodialbranching has been observed), and therefore more complex than the simple dichotomousbranching of the rhyniophytes. This suggests that a

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The presence of trilete spores and tracheids in some specimens confirm that they are earlyembryophytes e.g. However the presence of cryptospores is morecharactertistic of bryophytes.

rom this assemblage alone, w

However, complex branching suggests that this group are not , as extant.

These mesofossils may represent stem-group embryophytes with bryophytic characteristics, andmay represent several lineages that later failed to diversify, but from which the larger rhyniophytesevolved. A similar assemblage is known from Ludlow, of Late Silurian age , and therefore thisgroup may have evolved prior to the Late Silurian from gametophyte-dependant unbranchingbryophytes, the presence for which has been inferred from the cryptospore record.

Cooksonia hemispherica, Uskiella

in situ

Cooksonia pertoni. in situ

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short lengths of subtending axes branching habit cannot be determined

possible that these mesofossils represent just the tipsof larger plants.

second assemblage of mesofossils

subtending axes that are complexly branched

separate group of turf-sized vegetation wasliving alongside the larger rhyniophytes

bryophytesbryophytes have unbranching axes

senso stricto

200µm

Cymbohilates horridus200µm

microgranulate tetrads

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laevigate spores

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(no in situ spores as walls homogenised)

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a b

c d

e f

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g h i

j kl

m n o

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p q r s

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Chelinohilates