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Thorezia vezerensisgen. et sp. nov., a new seed plantwith multiovulate cupules from the Late Devonian ofBelgiumCyrille Prestiannia & Philippe Gerrienneb

a Earth and History of Life D.O., Royal Belgian Institute of Natural Sciences, B-1000Brussels, Belgiumb Geology Department, University of Liège, Allée du 6 Août B18/P40, B-4000 Liège, BelgiumPublished online: 23 Apr 2014.

To cite this article: Cyrille Prestianni & Philippe Gerrienne (2014): Thorezia vezerensisgen. et sp. nov., a new seed plant withmultiovulate cupules from the Late Devonian of Belgium, Historical Biology: An International Journal of Paleobiology, DOI:10.1080/08912963.2014.901315

To link to this article: http://dx.doi.org/10.1080/08912963.2014.901315

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Thorezia vezerensis gen. et sp. nov., a new seed plant with multiovulate cupules from the LateDevonian of Belgium

Cyrille Prestiannia* and Philippe Gerrienneb

aEarth and History of Life D.O., Royal Belgian Institute of Natural Sciences, B-1000 Brussels, Belgium; bGeology Department,University of Liege, Allee du 6 Aout B18/P40, B-4000 Liege, Belgium

(Received 2 December 2013; accepted 2 March 2014)

The multiovulate cupule of a new spermatophyte, Thorezia vezerensis gen. et sp. nov., is described from the Late Devonianaged sediments from Trooz Quarry in Belgium. In gross morphology, it conforms to the Moresnetia morphotype and has acupule that is composed of four independent quarters that each dichotomises three times. Each cupule quarter contains onesingle ovoid preovule with a long pedicel and an integument that has small apical teeth surrounding a rudimentarymicropyle. Morphological variability in the materials examined is interpreted as being related to preovule maturity, and fromthis a good understanding of the ontogenetic development from preceding dispersal has been developed. Up to now, 17spermatophyte species have been described, 11 of which come from eastern Laurussia. This diversity in eastern Laurussiacontrasts strongly with the low diversity characterizing contemporaneous floras from other phytogeographical areas. Wesuggest here that the arid climatic conditions prevailing in eastern Laurussia favoured the development of diversespermatophyte communities and contributed to reduced diversity and abundance of contemporaneous free-sporing plantdiversity.

Keywords: Cupule; Evieux flora; fossil plant; integument; preovule; Upper Devonian

1. Introduction

The Evieux Formation in southern Belgium (Upper

Devonian, Upper Famennian, Condroz Group) has yielded

numerous plant remains collectively known as the Evieux

Flora (Stockmans 1948; Fairon-Demaret 1996a). Although

the flora is dominated by remains of Archaeopteris

halliana and Rhacophyton condrusorum, a large number

of reproductive units from spermatophytes (seed plants)

have been discovered during the past 20 years (Fairon-

Demaret and Scheckler 1987; Fairon-Demaret 1996a,

1996b; Prestianni and Gerrienne 2006; Prestianni and

Gerrienne 2010; Prestianni et al. 2013).

The Evieux Flora records some of the most ancient

evidence for spermatophytes in the fossil record (Hilton

1998; Prestianni 2005; Prestianni and Gerrienne 2010); the

Givetian proto-ovule Runcaria heinzelinii Stockmans

(1968) emend. Gerrienne et al. (2004) (Gerrienne and

Meyer-Berthaud 2007) excepted. In these deposits, an

unexpected diversity is observed in what appears to be the

earliest diversification within seed plants. Five morpho-

types of early seed plants have been previously

distinguished on the basis of the organisation of the

cupule and the shape of the integument (Prestianni 2005;

Prestianni and Gerrienne 2010). Of these, the Moresnetia

morphotype – or ‘moresnetian architectural model’ in

Hilton and Edwards (1999) – is characterised by a multi-

units radially symmetrical cupule, formed by successive

cruciate dichotomous divisions. Cupules typically contain

up to four ovules and exceptionally six. Preovules consist

of a hydrasperman nucellus surrounded by several variably

fused integumentary lobes. The Moresnetia-type of

organisation is by far the most diversified morphotype of

earliest spermatophytes: it has been reported from nearly

all the Late Devonian seed plant localities (Fairon-

Demaret and Scheckler 1987; Rothwell et al. 1989;

Krassilov and Zakharova 1995; Hilton and Edwards 1999;

Hilton 2006; Cressler et al. 2010).

In the 1990s, a diversified plant assemblage including

the cupulate structures described here was collected from

the Evieux Formation of the Trooz Quarry near Liege in

Belgium. The cupulate structures were tentatively

identified as Moresnetia sp. by Fairon-Demaret (1996a),

who noted that they did not show any sign of overtopping

and that they hence were readily distinguishable from

Moresnetia zalesskyi Stockmans (1948) emend. Fairon-

Demaret and Scheckler (1987). Here, we describe these

Moresnetia-like plant remains in detail and assign them to

a new genus of early seed plant.

2. Materials and methods

The specimens studied here were collected between 1996

and 1999 by R. Gaipl, a German amateur palaeontologist,

from the Trooz quarry (Province of Liege, Belgium). This

large quarry is situated on northern side of the Vesdre

valley along the road from Trooz to Fraipont (Figure 1). It

q 2014 Taylor & Francis

*Corresponding author. Email: cyrille.prestianni@gmail.com

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is part of the allochthonous Vesdre massif (Prestianni et al.

2010). Plants were collected from the upper part of the

quarry which corresponds to the Evieux Formation.

Recently, another slightly younger plant-bearing bed

attributed to the Dolhain Formation (late Famennian) was

reported from the same quarry. It consists of charcoalified

wood remains attributed to a yet unknown plant

(Prestianni et al. 2010).

The age of the plant samples has been determined on

the basis of the available literature and on new

palynological analyses as the exact collecting horizon

within the quarry is unknown. Unpublished palynological

analyses attributed the layers where plants were very likely

collected to the VCo palynozone (Streel, personal

communication).

Fifty-two specimens of the new genus were collected.

They occur among fragmentary and disarticulated remains

of A. halliana (Goeppert) Lesquereux, R. condrusorum

Crepin, Barsostrobus famennensis Fairon-Demaret, twigs

of Sphenophyllum cf. subtenerrimum Nathorst and

cupulate ovules of Dorinnotheca streelii Fairon-Demaret

(Fairon-Demaret 1996b). They are preserved as slightly

weathered and drifted coalified compressions/impressions.

They were prepared mainly by the ‘degagement’ technique

(Leclercq 1960; Fairon-Demaret et al. 1999). Specimens

were photographed dry, using crossed polarising filters to

enhance the contrast.

All studied specimens are housed within the palaeobo-

tanical collections of the University of Liege (ULg).

3. Description

3.1 General features

The following descriptions are based on 3 branched

specimens and 49 fragmentary remains that allow a

detailed description of cupular and ovular morphologies to

be made. Among these specimens, there is a high degree of

variation in shape, overall size and features. Camera lucida

drawings illustrate this variability (Figure 2).

Each fossil comprises several dichotomising segments

of axes that correspond to the cupule, surrounding a single

central space that contains up to four preovules. The

cupule is borne terminally on a short and thick axis

(Figure 3a). Each cupule divides proximally into four

segments and from here each segment dichotomises 3

times, forming 8 ultimate cupule tips; the entire cupule

therefore comprises 32 ultimate tips. Preovules are stalked

and borne just above the proximal division of the cupule

forming the four quarters. The integument completely

surrounds the megasporangium. Its apical end is

characterised by four small lobes from which a relatively

long and narrow salpinx protrudes.

3.2 Detailed description

The cupule morphology in this species is highly variable

(Figure 2). Some specimens present very narrow cupules

having the aspect of a tight bunch of axes (Figure 3a). The

organisation of the basal part of these cupules is

impossible to assess reliably, giving the aspect of a fused

organ. By contrast, at the other side of the variation range,

some cupules are much wider, having the aspect of a loose

Figure 1. (Colour online) Location of the Trooz Quarry, in theVesdre Valley.

Figure 2. Camera lucida outline of the three major preovule morphotypes encountered among the material of T. vezerensis. Note theimportant morphological variability of the cupules.

2 C. Prestianni and P. Gerrienne

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truss of axes (Figure 3d) in which the cupule organisation

is clear and the cupule structure readily identified.

Only the distalmost part of the cupuliferous branching

system is known. It exhibits an isotomous (equally

dichotomous) branching structure. The most complete

specimen (Figure 3a) consists of two cupules borne at the

tip of an isotomous axis. The axes bearing the cupules are

short and thick and range from 6.2 to 6.8mm long and

from 1.1 to 1.5mm wide. Cupules appear to be extensions

of the ultimate axis, and neither abcission zone nor clear

boundary is visible to distinguish the axis from the cupule

(Figure 3a, c and e). We chose to arbitrarily place the base

of the cupule at the level where the ultimate axis width

starts to increase (Figure 3a at arrow). Cupules range from

11.3 to 12mm long and from 5.5 to 6.7mm wide. In most

specimens, the organisation of the proximal part of the

cupule is unclear (Figure 3a, c and e). The base of each

segment is not discernible; it might be fused with the base

of the adjacent segments. Closer examination of specimen

ULg 15201a nevertheless reveals three division levels (see

dashes labelled 1, 2 and 3 in Figure 3c). Divisions occur at,

respectively, 2.1mm, 3.9mm and 5.6mm from the cupule

base. Ultimate cupule segments are very thin, 0.2–0.3mm

wide. They are up to 2.3mm long and remain more or less

straight giving an impression of rigidity. Their number is

variable; a maximum of 16 segments have been observed.

Further detail of the cupule organisation is visible on

specimen ULg 15215 (Figures 3e and 4a–c). The proximal

part of this specimen consists of a dichotomous single axis

(Figure 3e (1)). One resulting axis is not preserved. The

Figure 3. Cupule and preovules of T. vezerensis from the Late Devonian Trooz quarry (Evieux Formation). (a) Dichotomous branchingsystem presenting two cupules. A preovule (p) is present. The base of the cupule is identified by an arrow. ULg-15201e; scale bar ¼ 1mm.(b) Enlargement of (a) showing details of preovule organisation. ULg-15201e; scale bar ¼ 0.5mm. (c) Cupule showing the three levels ofdichotomy marked by the dashes labelled 1, 2 and 3. ULg-15201a; scale bar ¼ 1mm. (d) Mature cupule showing three distinct quarters(1, 2 and 3) and one that is partly covered (d). A preovule (p) is present; ULg-15200; scale bar ¼ 2mm. (e) Dichotomising axis (1) withonly one attached cupule. Cupule spread out in the sediment showing the organisation in four quarters of the cupule. One quarter (3) istwisted while the three other quarters (2) are only visible by their base (dash); ULg-15215; scale bar ¼ 2mm.

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second one is very short and bears a single cupule (Figure 3e

(2 and 3)). This cupule appears slightly disarticulated. Two

parts are observable (2 and 3 in Figure 3e). The main part

(Figure 3e (2)) is 9.3mm long and 8mm wide. It conforms

morphologically to the specimen described above. The

smaller part (Figure 3e (3)) is 9.2mm long and 4.3mm

Figure 4. Cupule and preovules of T. vezerensis from the Late Devonian Trooz quarry (Evieux Formation). (a) Enlargement from Figure3e showing the position of the preovule tip within the cupule ultimate segments; ULg-15215b; scale bar ¼ 2mm. (b) Preovule showingthe smooth aspect of the integument. ULg-15223; scale bar ¼ 1mm. (c) Enlargement of (b) showing the upper part of the preovule. Threesmall teeth form a micropyle-like structure. They are interpreted as representing the remains of integumentary lobes. ULg-15223; scalebar ¼ 1mm. (d) Enlargement of Figure 3d under low-angle light. It highlights the presence of a transversal crack revealing the presence ofa fourth cupule quarter; ULg-15200; scale bar ¼ 2mm. (e) Enlargement of Figure 3d showing details of preovule organisation. Amicropyle-like structure formed by two integumentary teeth is visible at the arrow; ULg-15200; scale bar ¼ 1mm. (f) Fertile cupulecontaining one preovule. It shows the position of the preovule within the cupule at the level of the third dichotomy; ULg-15228a; scalebar ¼ 2mm. (g) One displaced cupule quarter showing preovule attachment; ULg-15220b; scale bar ¼ 2mm. (h) Badly preserved cupuleshowing preovule attachment within the cupule. Several preovules are present; ULg-15212; scale bar ¼ 1mm. (i) Enlargement of (h)showing the long axis subtending the preovule; ULg-15212; scale bar ¼ 1mm. (j) Enlargement of (i) showing the organisation of thepreovule distal part. ULg-15212; scale bar ¼ 1mm.

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wide. We interpret it as a displaced fragment of the cupule.

This interpretation is based on the observation of a distorted

segment at the base of this fragment (see arrow in

Figure 3e). In addition, the cupule part b appears to bemade

of three units arising at the same level as testified by their

subtending axes (see dash in Figure 3e). The cupule thus

appears to be made of several separate parts.

Specimen ULg 15200 (Figures 3d and 4d,e) is 11.9mm

long and 10.9mm wide. In this specimen, the cupule

appears to be separated basally into three independent units

(1, 2 and 3, Figure 3d). Using low-angle polarised

illumination (Figure 4d), we observe a crack that

presumably corresponds to the limit of the cupular unit (1

in Figure 3d). It highlights the presence of a fourth unit (4 in

Figure 3d)which is partly covered by unit 1 (1 in Figure 3d).

Each independent unit is flat. They appear to have a basal

webbed part. They are ended by eight terete ultimate

segments resulting from three successive dichotomies in

the same plane. Considering the presence of four units

resulting three very close dichotomies, a total number of 32

ultimate appendages is expected for the whole cupule.

Only five specimens contain preovules, with individual

cupules containing one (Figure 3b and c) or rarely two

(Figures 3e and 4a) preovules. However, the bad

preservation, the drifted state of the fossil material and

the lack of preovule in most cupules suggest that the

number of preovules per cupule could be larger. Preovules

are oval in outline (Figures 3b and 4a,b; pl. 4d, e and g).

Their overall shape is, however, variable. Small thin

representatives are observed as well as (more commonly)

long and wide specimens. They range from 2.1 to 3.2mm

long and from 0.90 to 2mm wide. The external surface of

the preovule is smooth (Figure 4b, e and h). There are no

traces of ribbing or of integumentary lobes at the surface of

the preovule body. The traces of the integumentary lobes

are only shown by the presence of two to three small

‘teeth’ corresponding to the distalmost part of the

integumentary lobes; the teeth surround a micropyle-like

structure (see arrows in Figure 4c and e). Considering that

only half of the preovule is visible, a total number of four

to five lobes are expected. An elongated, presumably

tubular salpinx protrudes from the integument (Figure 4c

and j); it is usually 1.3mm long.

The position of preovules within the cupule is difficult

to establish. In most specimens, the preovule base is hardly

visible or covered by the cupule. Only two specimens

allow the identification of the attachment site of the

preovule within the cupule (Figure 4h–j). As shown by

those specimens, preovules are attached to the cupule by a

slender, 1mm long, pedicel. They are attached singly

nearly at the base of the cupular unit so that a maximum of

four preovules per cupule is expected, one per cupule

quarter. A cutaway version of a cupule showing the

attachment level of the ovule is given in Figure 7. The

widest part of the preovule is situated approximately at the

middle of the cupule height that corresponds to the third

dichotomy of each cupular unit and to the proximal part of

the ultimate segments (Figures 3d and 4f).

Simple allometric analyses were performed (Figure 5).

First, we compared the length up to the first division with

the total length of the cupule. We were able to show the

presence of a single homogenous assemblage set along the

same variability line (Figure 5a). Second, we compared the

maximum length of cupules versus their maximum width.

Figure 5b shows the results of this comparison. The

regression line and the R 2-associated values highlight that

these two variables are not linked to each other. Third, we

compared preovulewidth and lengthwith cupulemaximum

width (Figure 6). This shows that both preovule length and

width evolve proportionally to cupule width. In addition,

each diagram show that the data-set represents a single

species that ranges in size and shape as part of a continuum

(Figures 5 and 6).

Figure 5. Relationship between (a) cupule maximum length and cupule length at first division and (b) cupule maximum length andcupule maximum width. N ¼ 36.

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The morphology of Thorezia vezerensis is recon-

structed in Figures 7 and 8.

4. Systematic palaeobotany

Division SPERMATOPHYTA

Genus Thorezia Prestianni & Gerrienne, gen. nov.

Type: T. vezerensis Prestianni et Gerrienne, sp. nov.

Diagnosis: Vegetative parts unknown. Cupule consisting of

up to 32 ultimate segments resulting from a first proximal

division that forms fourmore or less radially disposed equal

quarters. Each quarter dichotomises a further three times to

produce eight terminal segments in the same plane.

Ultimate cupule segments terete and elongated. Individual

cupule with at least one preovule borne on inner surface of

cupule near first proximal division. Stalked preovules

symmetric in outline. Integument glabrous, basally entire,

forming ca. 4 short integumentary tips at salpinx base.

Salpinx tubular and short.

Etymology: In honor of Pr. Thorez.

T. vezerensis Prestianni and Gerrienne, sp. nov.

Holotype: Specimen ULg 15200 (Figure 4a). Adpression

of a fertile cupule.

Repository: Palaeobotanical collections of the University

of Liege, Liege, Belgium.

Type Locality: Trooz Quarry, at Trooz, near Liege, Vesdre

Valley, Province of Liege, Belgium.

Type stratum: Evieux Formation of the Condroz sandstone

group.

Biostratigraphic horizon: VCo palynozone (this work and

Streel, personal communication)

Age: late Famennian, Late Devonian.

Etymology: ‘Vezere’ is the original Celtic and later Latin

name of the river Vesdre in the valley of which the fossils

have been discovered.

Diagnosis: Characters as for the genus. Cupule stalk 6–

7mm long, 1–1.5mm wide. Cupules ranging from 5 to

15mm in length and from 4.0 to 11.0mm in width,

depending on maturity. Preovules 1.2–2.1mm wide and

2.5–3.5mm long.

Figure 6. Relationship between cupule maximum width andpreovule length and width. N ¼ 10.

Figure 7. Schematic cut away reconstruction of the cupuleshowing the organisation of one single cupule quarter as well asthe position and insertion level of one ovule.

Figure 8. Reconstruction of a cupule of T. vezerensis.

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5. Discussion

5.1 Comparison

T. vezerensis has a cupule including four quarters

surrounding one to several partly lobed hydrasperman

preovules. Such organisation is shared by many other Late

Devonian and Mississippian species. It was first

recognised in Elkinsia polymorpha [VH palynozone,

Hampshire Formation, West Virginia] (Gillespie et al.

1981; Rothwell et al. 1989). This species has basically the

same morphology as M. zalesskyi [VCo palynozone,

Evieux Formation, Belgium] and Xenotheca devonica [LL

palynozone, Baggy Beds, North Devon] (Fairon-Demaret

and Scheckler 1987; Hilton and Edwards 1999) to which

E. polymorpha has repeatedly been suggested to be

synonymous at the generic level (Hilton and Edwards

1999; Prestianni and Gerrienne 2010). In these plants, the

cupule comprises four quarters with four ultimate

segments each. In the plant investigated here, four quarters

are indeed present showing an additional dichotomy

leading to eight ultimate segments per quarter rather than

four. In addition, in contrast with other plants, the cupule

in T. vezerensis appears to be flattened with all cupule

segments in the same plane.

Preovules in Elkinsia, Moresnetia and Xenotheca

present from 4 to 11 integumentary lobes that are fused up

to 50% of their length (see Table 1). Their surface is

characteristically ribbed when integumentary lobes are

fused. By contrast, preovules in T. vezerensis are smooth

and present a complete integument on almost its entire

length. We consider these features sufficient to state that

Elkinsia, Moresnetia and Xenotheca are taxonomically

different from the specimens described here. Archae-

osperma arnoldii (Pettitt and Beck 1968) is another Late

Devonian cupulate preovule. It shares with the plant

described here a similar cupule organisation. However, as

for Elkinsia, Moresnetia and Xenotheca, A. arnoldii

presents only four ultimate segments per cupule. Preovule

in A. arnoldii, like those of the plant described here,

comprises a basally entire integument on almost all their

length, and four to five small integumentary lobes are

present distally. However, unlike our plant, the integument

of A. arnoldii is characteristically covered by small spines.

The differences in cupule and preovule organisation lead

us to consider A. arnoldii and the plant described here as

separate taxonomic entities.

Among all Devonian preovules, Glamorgania gayeri

(Hilton 2006) [LL/LE/LN palynozones, Taffs Well, Wales]

appears to be morphologically the closest to T. vezerensis.

The cupule of G. gayeri starts with a first dichotomy and is

constituted by two more or less equal halves that divide

each three times to produce eight long ultimate segments.

This is different from our plant where the cupule starts

with three very close dichotomies leading to the

establishment of four more or less equal quarters. Like

those of T. vezerensis, the preovules of Glamorgania

present four distal integumentary lobes; however, they

differ markedly from those of Thorezia in being

asymmetrical. Despite some shared characters, the plant

from Trooz appears thus to be different from Glamorga-

nia. We consider those differences large enough to justify

the erection of a new taxon at the generic level for our

specimens.

From the comparison presented above, the specimens

described here can be distinguished from all previously

described taxa. Their cupule, however, presents many

characters in common with that of Glamorgania and the

structure of their integument is very close to that of

Archaeosperma. Despite these similarities, the specimens

of Thorezia show a precise combination of characters that

does not match those of other previously described

preovules. We thus conclude that it is necessary to

establish new fossil genus and species.

5.2 Ontogeny

In a first approach, the apparent variability of the cupule

led us to consider the existence of different morphotypes

(Figure 2). Using simple allometric analyses, we, however,

Table 1. Comparison of main characters of Late Devonian preovulate cupules presenting a lobed integument (Fairon-Demaret andScheckler 1987; Rothwell and Scheckler 1989; Hilton and Edwards 1999; Hilton 2006).

Species X. devonica M. zalesskyi E. polymorpha G. gayeri T. vezerensis

OvuleLength 2.7–5.5 1.0–5.0 3.5–6.5 1.8–3.1 2.5–3.5Width 0.8–2.5 1.0 þ 1.0–2.6 1.1–1.8 1.2–2.1Numberof int. lobes 4–7 8–10 4–5 ca. 4 4Fusion (%) ,50 Chalazal (b10) ca. 33 ca. 90 ca. 90Shape symm. symm. symm. asymm. symm.

CupuleLength 8–16 8–10 8–18 9–11 1–5Width 4–12 7–12 6–12 3–5 4–11Number of units 4 4 4 2 4Number of lobes 16 8–16 16 16 32

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showed the presence of a single homogenous assemblage

set along the same variability line (Figure 5a). Considering

this and the morphological similarity of the studied

specimens, a taxonomic variability among the material can

be excluded. Nevertheless, comparison of cupule length

and cupule width showed more contrasted results

(Figure 5b). With an R 2 of 0.3, these two variables are

very weakly correlated. Whether this is the consequence of

taphonomic or ontogenetic processes can be decided

thanks to Figure 6. This figure shows that both preovule

length and width evolve proportionally to cupule width.

The ovule size increase is very likely the consequence of

ontogenetic processes linked to the gametophyte matu-

ration. The cupule width thus increases as the ovule

matures. More ‘open’ cupules would thus correspond to

more mature ovules.

5.3 Diversity

The recognition of T. vezerensis increases the known early

seed plant diversity in Laurussia. Currently, 6 of the 17

described Famennian preovule species have been collected

from Belgian localities (Hilton 1998; Prestianni and

Gerrienne 2010). If we consider eastern Laurussia as a

whole, this number rises to 11 of the 17 described species.

They represent an impressive diversity considering the

relative scarcity of Devonian preovules globally. It places

eastern Laurussia as the region presenting the largest

diversity in seed plants at the end of the Devonian. This

strongly contrasts with the information given by other

contemporaneous floras.

The Belgian Evieux Flora, preovules excepted, is

much less diverse than coeval Western Gondwana floras.

This is particularly the case when comparing the number

of Archaeopteris species at both sides of the Acadian

mountains (Fairon-Demaret et al. 2001; Streel and

Marshall 2006). American deposits yielded at least four

Archaeopteris species (Archaeopteris macilenta, A.

halliana, Ahibernica hibernica and Asphenophyllifolia

sphenophyllifolia) when coeval Belgian and UK assem-

blages only contain a single species A. halliana (syn. A.

roemeriana). Such a difference has been tentatively

explained by the very contrasted climatic conditions

existing in the two regions (Thorez and Dreesen 1986).

At the end of Devonian times, east Avalonia was

located at the southeastern margin of Laurussia. Climate

was characterised by arid conditions as shown by the

occurrence of evaporites and pedogenic carbonates

interstratified within siliclastic sediments particularly

rich in feldspars (Thorez and Dreesen 1986). By contrast,

American deposits, from the western part of Laurusia, are

characteristics of more humid conditions (Scheckler 1986;

Thorez and Dreesen 1986). The difference between the

two palaeoregions has been explained by the rise of the

Variscan highs. The Variscan highs very likely intercepted

the humid trade winds, which lead to dryer conditions on

the Old Red Sandstone than in present-day North-Eastern

America where these mountains were absent (Streel and

Marshall 2006).

Archaeopteris is an endosporic heterosporous free-

sporing plant (Scheckler et al. 1997; Fairon-Demaret et al.

2001). Because of their flagellate antheroizoids, all extant

heterosporous free-sporing plants (Selaginellales, Isoe-

tales and Salviniales) need wet environments for the

fertilisation. Archaeopteris most probably did not have

exactly the same ecological requirements, but its

reproduction was presumably also more effective in

aquatic or amphibious environments and in some ways

may be considered potentially dysfunctional in dry

habitats (Bateman and DiMichele 1994). Even though

seasonal rains gave Archaeopteris, the opportunity to

reproduce (Thorez and Dreesen 1986; Thorez et al. 2006),

the dry conditions prevailing on the eastern part of the Old

Red Sandstone presumably prevented the development of

large and diverse Archaeopteris populations. This has been

suggested to explain why only one species (A. halliana)

was present in that palaeoregion (Fairon-Demaret et al.

2001) and that it had developed characters linked to xeric

conditions (Fairon-Demaret et al. 2001).

Contrary to Archaeopteris, seed plants are widely

accepted as being more adapted to dryer environments

(DiMichele et al. 1989; Haig and Westoby 1989; Rothwell

and Scheckler 1989; Bateman and DiMichele 1994). In

terms of reproduction, seed plants are endosporic

heterosporous plants, and, unlike Archaeopteris, the

megaspore is not released from the megasporangium.

The development of the integument provided increased

protection to the enclosed megagametophyte and likely

helped to reduce water loss and lessen the effects of

desiccation. The seed habit liberates these plants from the

obligate free water pollination of endosporic heterospor-

ous plants. This major advance gave seed plants a decisive

evolutionary advantage that helped them to colonise and

thrive in a range of ecological niches that were not readily

accessible to coeval free-sporing plants.

The very contrasted conditions prevailing between

actual North America and present north-eastern Europe

during the Famennian presumably drove an important

reduction of the wetlands in the latter. The consequences

of this on plant diversity were important. Whereas huge,

extensive and highly diversified Archaeopteris forests

flourished in western Laurussia, monospecific xeric A.

halliana forests are present on the eastern Laurussia

region. Other elements of the Late Devonian flora such as

lycophytes were also affected (Fairon-Demaret 1996a;

Cressler and Pfefferkorn 2005). Those contrasting climatic

conditions, however, likely favoured seed plants that had

probably an evolutionary advantage in dryer environ-

ments. Spermatophytes likely found in these dryer regions

8 C. Prestianni and P. Gerrienne

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occurred in less crowded niches where they could evolve

and diversify.

6. Conclusions

In this paper, we describe a new ovular structure from the

Late Devonian of Belgium. It conforms to the Moresnetia

morphotype. The cupule is composed of four quarters,

each containing one ovoid preovule with a glabrous,

basally entire integument, forming apically ca. 4–5 small

integumentary lobes. The specimens exhibit a range of

morphologies, which is interpreted as related to the

preovule maturity. This discovery increases the diversity

of Late Devonian spermatophytes in eastern Laurussia.

This region appears to be one of the hot spots of earliest

seed plant diversification. This diversity contrasts with the

relative uniformity of the remaining flora in the same

regions. For example, it is noticeable that Archaeopteris

forests are reduced and monospecific. The dry conditions

that prevailed in eastern Laurussia during the Famennian

very likely had a negative impact on the general diversity

but in parallel gave to spermatophytes an ecological boost

leading to an increased diversity in the area.

Acknowledgements

We thank M. Gaipl for having collected this material and givingit to the ULg. We also thank Dr M. Fairon-Demaret for her usefulcomments and support that would have justified her being co-author of this publication. We also thank Jonica Dos Remediosfor helping illustrating the paper.

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