New perspectives on the Mesozoic seed fern order Corystospermales based on attached organs from the Triassic of Antarctica

A new Triassic corystosperm is described from the Shackleton Glacier region of Antarctica. The compression fossils include cupulate organs (Umkomasia uniramia) and leaves (Dicroidium odontopteroides) attached to short shoot-bearing branches. The cupulate organs occur in groups near the apices of the short shoots, and each consists of a single axis with a pair of bracts and a subapical whorl of five to eight ovoid cupules. This unique architecture indicates that the cupules are individual megasporophylls rather than leaflets of a compound megasporophyll. A branch bearing an attached D. odontopteroides leaf provides the first unequivocal evidence that Umkomasia cupulate organs and Dicroidium leaves were produced by the same plants. Although this had previously been assumed based on organ associations, the new specimens are important in demonstrating that a single species of corystosperm produced the unique cupulate organs described here and the geographically and stratigraphically widespread and common D. odontopteroides leaf. Therefore, biostratigraphic, paleoecological, and phylogenetic studies that treat Dicroidium leaf morphospecies as proxies for biological species of entire plants should be reconsidered. Phylogenetic analysis suggests that the corystosperm cupule is an unlikely homologue for the angiosperm carpel or outer integument.     

The corystosperm pollen organ Pteruchus from the Triassic of antarctica

The permineralized corystosperm pollen organ Pteruchus is described from the early Middle Triassic of Antarctica. Pteruchus fremouwensis consists of an axis bearing numerous, helically arranged microsporophylls, each of which terminates in a distal flattened head. The axis is 1–2 mm in diameter and eustelic. Spherical to elliptical secretory cavities are present in the ground tissue of the axis, microsporophyll, and pollen sac wall. The basal stalk of the microsporophyll is vascularized by a C-shaped strand that gives rise to a midvein and numerous lateral veins in the distal head. At least 38 pollen sacs are borne on the abaxial surface of the microsporophyll head. These are arranged in pairs on either side of lateral veins. Each pollen sac is sessile, elongated, and uniloculate. The pollen sac wall is several cell layers thick early in ontogeny, but reduced to a single layer in thickness when mature. Dehiscence is longitudinal along the inner surface. Pollen is monosulcate and bisaccate, and of the Alisporites-type. The Triassic specimens are the first structurally preserved pollen organs of the Pteruchus-type and can be related to the associated corystosperm stem and leaf genera based on the presence of unique secretory cavities. The morphology of Pteruchus and the relationship of this pollen organ with other Mesozoic and Paleozoic pollen organs is discussed.     

Latisemenia longshania,gen. et sp. nov., a new Late Devonian seed plant from China

The earliest known ovules in the Late Devonian (Famennian) are borne terminally on fertile branches and are typically enclosed in a cupule. Among these ovules are some that have terete integumentary lobes with little or no fusion. Here, we report a new taxon, Latisemenia longshania, from the Famennian of South China, which bears cupulate ovules that are terminal as well as opposite on the fertile axis. Each ovule has four broad integumentary lobes, which are extensively fused to each other and also to the nucellus. The cupule is uniovulate, and the five flattened cupule segments of each terminal ovule are elongate cuneate and shorter than the ovule. Associated but not attached pinnules are laminate and Sphenopteris-like, with an entire or lobate margin. Latisemenia is the earliest known plant with ovules borne on the side of the fertile axis and may foreshadow the diverse ovule arrangements found among younger seed plant lineages that emerge in the Carboniferous. Following the telome theory, Latisemenia demonstrates derived features in both ovules and cupules, and the shape and fusion of integumentary lobes suggest effective pollination and protection to the nucellus. Along with other recent discoveries from China, Latisemenia extends the palaeogeographic range of the earliest seed plants.     

Studies on the cupulate seed genus Hydrasperma Long from Berwickshire and East Lothian in Scotland and County Kerry in Ireland

Twelve petrified cupules containing seeds of Hydrasperma tenuis Long from the Upper-Devonian/ Lower Carboniferous Transition Series from Ballyheigue, Kerry Head, Ireland have been studied. The cupules containing 2–6 seeds are borne in pairs. Each cupule is campanulate and composed of up to 24 oval to terete units. The axis of the cupule is forked, each resultant branch dividing in an alternate monopodial fashion producing six major axes. These divide repeatedly, each terminating in a narrow rounded tip. A reconstruction of a four-seeded cupule is presented. Hydrasperma longii sp. nov. is proposed for the first 'named' Scottish cupulate seeds named by Long as Hydrasperma cf. tenuis. Hydrasperma longii differs from H. tenuis: comparison of H. tenuis with other cupulate Lower Carboniferous seeds indicates that two major branching patterns of the cupule occur in early seed plants. Following an initial dichotomy, branching may either be entirely dichotomous, or alternately monopodial. The Aneurophytales appears to have the branching patterns consistent with its position as the ancestral group.     

Permineralized seed fern cupules from the Triassic of Antarctica: implications for cupule and carpel evolution

In this paper we describe the first anatomically preserved Mesozoic seed fern cupule–Petriellaea. The multiovulate cupules were produced singly at the end of a short dichotomizing axis. Cupules are bilateral with a dorsal groove and transverse narrow ventral opening. The vascular system of the cupule consists of a series of traces that extend up the dorsal surface of the cupule and down the ventral face. Ovules are orthotropus, sessile, and borne on the adaxial surface of the leaflike cupule either singly or in multiple rows. They are up to 1.5 mm long, triangular in transverse section, and characterized by a multilayered integument. Nucellus and integument are fused throughout their length, but no pollen chamber is present. In the chalaza is a small vascular disc of transfusion tracheids that represents the extent of the ovule vascular system. Ovules are interpreted as being fossilized at a prepollination stage, although a few possess some evidence of a cellularized megagametophyte. These permineralized cupules indicate that in at least one Mesozoic seed fern group, ovule enclosure resulted from the transverse folding (tip to petiole) of a megasporophyll bearing adaxial ovules. Cupule morphology and ovule enclosure in other Late Paleozoic and Mesozoic seed ferns is discussed.     

Pullaritheca longii gen. nov. and Kerryia mattenii gen. et sp. nov., lower carboniferous cupules with ovules of the Hydrasperma tenuis-type

New specimens of cupules assignable to Hydrasperma longii Matten et al. have been collected at Oxroad Bay in southern Scotland. Two of these bear large well-developed ovules that are tightly packed within the cupules and reveal the features of the relatively mature fructification. The attached ovules fall within the range of structural variation that encompasses both the isolated ovules that are the type specimens of H. tenuis and cupulate ovules described as H. tenuis from the uppermost Devonian of Ireland. However, the cupules from Scotland and Ireland have distinctly different morphologies. This demonstrates that H. tenuis ovules are produced by at least two different kinds of plants and reveals that ovule structure alone is not always a reliable taxonomic indicator among primitive gymnosperms. The genus Hydrasperma is restricted to isolated ovules because it can not be determined which, if either of the plants, represented by the currently-known cupulate morphologies, actually bore the type specimens of H. tenuis. Plants represented by the ovulate cupules from Scotland and Ireland are named Pullaritheca longii gen. nov. and Kerryia mattenii gen. et sp. nov. respectively.     

ON THE ORIGINS OF THE OVULE AND CUPULE IN LYGINOPTERID PTERIDOSPERMS

Camp, Wendell H., and Mary M. Hubbard. (U. Connecticut, Storrs.) On the origins of the ovule and cupule in Lyginopterid pteridosperms. Amer. Jour. Bot. 50(3): 235–243. Illus. 1963.—The recently described Eurystoma angulare of the Lower Carboniferous with its naked, dichotomously branched, ovule-bearing branch truss may be taken conceptually as a starting point in a series of evolutionary reductions and modifications involving other known forms which ultimately led to the cupule surrounding the solitary ovule of later lyginopterids. It is postulated that the integuments of these ovules also were derived from dichotomously branched lateral trusses which immediately subtended the primitive megasporangia, but of less complexity than that which produced the cupule. Eurystoma indicates that ovules evolved independently of leaves; therefore, ovules cannot be thought of as having been derived from leaf tissues. Evidence is presented indicating that, although these pteridosperms produced ovules of considerable complexity, they did not bear seeds but dropped the pollinated ovules before fertilization. The already specialized organization of the ovules of the Lower Carboníferous pteridosperms indicates that the group must have originated in the Devonian. The structure of the Lyginopterid ovule is reinterpreted, indicating a basic similarity to that of the angiospermous ovule.     

On the Current Classification System of Paleozoic Seeds

This paper summarizes the history of classifications of Paleozoic seeds and revaluates the previous classification systems of Paleozoic detached seeds. The current status of studies on Paleozoic. gymnosperms: has been deteched seeds and whole fossil gymnosperms indicates that Seward’s classification system for Paleozoic seeds inadequate since all the seeds of Cardiocarpales in his system are not cordaitean female reproductive organs as Seward’s suggested. It is shown from investigations of whole fossil plants that the members of Cardiocarpales were derived from at least three different major groups of Paleozoic gymnosperms. Moreover, Meyen’s suggestion that the gymnosperms be classified based on symmetry of seeds has been little supported since all the fossil gymnosperms have not shown structurally preserved seeds and organic attachment. In order to relate detached seeds to whole fossil gymnosperms, the present author suggests that five families: Lagenostomaceae, Pachytestaceae, Callospermariaceae, Cryptospermaceae and Cardiocarpaceae be established for Paleozoic seeds and the Order Trigonocarpales be renamed as Pachytestales since the genus Trigonocarpus does not now include structurally preserved seeds. Thus, the five families may be considered either as subdivisions of the three orders of detached seeds: Lagenostomales, Pachytestales and Cardiocarpales, or as female reproductive organs of whole fossil gymnosperms of the five Permo-Carboniferous major groups: Lyginopteridales, Medullosales, Callistophytales Gigantopteridales and Cordaitales. A Key to Paleozoic seeds is provided as follows: A. Seeds with a cupule; integument thin, simple, deeply lobed and less differentiated;nucellus united to integument up to the base of pollen chamber; pollen chamber complex ................................. Lagenostomales, Lagenostomaceae A. Seeds without a cupule; integument thick, complex, unlobed and differentiated into several layers; nucellus free within integument except at the base; pollon chamber simple ................................................................................................ B B. Seeds radially symmetrical in shape; integumentary bundles present; nucellus bundles typical ................................................... Pachytestales, Pachytestaceae B. Seeds bilaterally symmetrical in shape; integumentary bundles present or absent; nucellus bundles often untypical .................................... C (Cardiocarpales) C. Bundles absent in integument; main bundle C-shaped in transverse section with a sclerenchyma bundle ............................................ Cryptospermaceae C. Bundles present in integument; main bundle not C-shaped in transverse section without a sclerenchyma bundle ......................................................... D D. Seeds very small with secretory cavities in integument; nucellus bundles limited in nucellus platform .......................................... Callospermariaceae D. Seeds large without secretory cavities in integument; nucellus bundles limited in nucellus platform or not ....................................... Cardiocarpaceae     

THE CRETACEOUS PTERIDOSPERMS RUFLORINIA AND KTALENIA AND IMPLICATIONS ON CUPULE AND CARPEL EVOLUTION

The vegetative (Ruflorinia sierra) and fertile (Ktalenia circularis) organs of an Early Cretaceous pteridosperm collected from Santa Cruz Province in Argentina are described. The sterile leaf is at least tripinnate and bears decurrent secondary pinnae with obliquely attached, sharply pointed pinnules. The fertile member arises from the base of the vegetative rachis and bears two types of appendages, cupules and bracts. Bracts are attached to the main axis near cupules and are present in clusters of up to six. Cupules are sessile, spherical, and arranged in opposite or subopposite pairs along the axis. A small lip is present on one surface of the cupule. The number of seeds per cupule may be one or two, with each characterized by a distal nucellar beak and circular, chalazal scar. Cuticular anatomy, including the fine structure of the stomatal complex, is described for both vegetative and reproductive organs. The cupules of Ktalenia and other Mesozoic seed plants are compared, and a discussion presented regarding the possible function of the cupule.     

New evidence for laurasian corystosperms: Umkomasia from the Upper Triassic of Northern China

Recent finds of remarkable fossil plants from the Upper Triassic Yangcaogou Formation in Liaoning Province, PR China include branched, cupule-bearing structures referable to the corystosperm ovulate organ Umkomasia. This material is described and assigned to the proposed new species Umkomasia asiatica. The collection includes numerous isolated cupules and fragments of ultimate cupule-bearing axes. Two specimens consisting of portions of the main axis with attached, cupulate lateral axes have also been found. The main axis was at least 6.5 cm long, with each lateral axis bearing one to at least three pairs of stalked, ovoid cupules. The new Umkomasia is similar to U. franconica from the Jurassic of Germany, which is the only other known laurasian species, but the cupules are smaller and more elongated. It is also similar to many gondwanan forms, including the type species U. macleanii. Leaves associated with the Chinese Umkomasia species are tentatively referred to Thinnfeldia, and may have been produced by the same plant. Associated ovoid seeds with elongated, curved micropyles are similar to those of gondwanan species of Umkomasia. The fossils described here are, therefore, significant in representing the first report of corystosperm reproductive structures from Asia, and only the second report of Umkomasia from the entire northern hemisphere. The new Chinese fossils also support leaf-based evidence that the Corystospermales were present in Laurasia as early as the Late Triassic.     

Anatomically preserved Cycadeoidea (Cycadeoidaceae), with a reevaluation of systematic characters for the seed cones of Bennettitales

Four anatomically preserved ovulate cycadeoid cones have been recovered from three localities in Upper Cretaceous (Turonian/Coniacian-Late Campanian) sediments of Vancouver and Hornby Islands, British Columbia, Canada. All of the specimens are preserved by calcareous cellular permineralization and are quite similar to seed cones described as several species of Cycadeoidea and Bennettites. These cones, described as Cycadeoidea maccafferyi sp. nov., consist of tightly packed interseminal scales and ovulate sporophylls with terminal ovules. Two specimens also preserve remains of a small receptacle. Interseminal scales and ovulate sporophylls are oriented parallel to one another. Ovules are distinctly stellate at the base of the micropylar tube, and the sarcotesta consists of both longitudinally oriented tubular cells and large radially elongated cells attached to the sclerotesta. The vascular strand below each ovule is highly contorted in a pattern that is characteristic of contractile tissue in the roots of living plants. These specimens are the most recent anatomically preserved cycadeoid cones yet discovered, revealing details of the reproductive biology shortly before extinction of the clade. Superb preservation of the British Columbia cones confirms that Bennettitales lack a cupule, have radial seeds, and have a vascularized nucellus (but no integumentary tracheids), and that no pollen chamber is produced. Together with a new species of Williamsonia preserved at one of the same localities, these specimens reveal a clear set of contrasting systematic characters for differentiating between isolated seed cones of Williamsoniaceae and Cycadeoidaceae.     

Female flower and cupule structure in Balanopaceae,an enigmatic rosid family

The Balanopaceae, whose flowers were poorly known, have, in the past, been variously allocated to the Fagales, Euphorbiaceae, Salicales or other hamamelids and rosids (these groups being in Fagales, Malpighiales and Saxifragales, according to the Angiosperm Phylogeny Group). This paper attempts a clarification based on flower morphology. Female flowers and cupules were studied in Balanops vieillardii, young fruits in B. australiana. The cupules are simple involucres of bracts which are spirally arranged (according to a Fibonacci pattern) on the floral axis preceding the flower. They contrast with the complicated cupules of Fagaceae which consist of a condensed cymose ramification system of axes of several orders around the flower. Flowers appear later than most of the cupular bracts, in contrast to Fagaceae. In addition to a terminal flower there may be several smaller lateral flowers in the axil of cupular bracts, each surrounded by its own small cupule. The female flowers do not have a perianth. They consist of two to three large carpels. At anthesis, the ovary is completely septate; the syncarpous part (ovary and lower style) is completely symplicate. The carpels are free for most of their length, with the free parts once, twice or three times bifurcate, in contrast to simple in Fagales. The stigmatic surface covers the ventral side of each stigmatic branch and at the margins also spreads to the dorsal side. The stigma is wet and secretion appears holocrine. The two ovules per carpel are collateral and axile in early development. However, at anthesis they appear one above the other, because in one ovule the funicle greatly elongates. As the ovary elongates only above the placenta, the ovules appear basal at anthesis. The ovules are (weakly) crassinucellar, bitegmic (not unitegmic), anatropous, and intermediate between apotropous and epitropous (not apotropous). The ovules are mature at anthesis, in contrast to Fagales. In mature ovules the upper part of the nucellus disintegrates, and a weakly differentiated endothelium is present in the inner integument. The morphological results of this study support a position of Balanopaceae in Malpighiales, and not Fagales or other orders, and are thus in accordance with recent molecular results based on chloroplast rbcL sequences data. However, within Malpighiales, as opposed to molecular results, Balanopaceae agree more with Euphorbiaceae s.l. than with Dichapetalaceae/Trigoniaceae and Chrysobalanaceae/Euphroniaceae.     

Two early mississippian seeds from the price formation of Southwestern Virginia

Lagenospermum imparirameum Arnold, originally described from a few specimens of cupulate seeds borne on two or three times dichotomous branches, is now shown to be borne on more complex branching systems. Details of the cupule and seed morphology are added and an emended diagnosis of the taxon is given. A new species,Gnetopsis hispida, is described as the third occurrence of this genus and the first occurrence in beds of Lower Mississippian Age in North America. The classification, evolutionary implications, and dispersal biology are discussed for each of the seeds     

Unicellular pheromone glands of the pentatomid bug Nezara viridula (Heteroptera: Insecta): ultrastructure, classification, and proposed function

Male Nezara viridula produce sex pheromones from many independent single cells, each with a duct that opens onto the ventral abdominal surface. Despite the presence of a long duct and an associated end complex (in the form of a cupule and microvillus saccule), the structural organization of the cells that comprise the gland conform to Class 1 epidermal gland cell classification : a single cell surrounds the entire secretory complex. Each cuticular cupule contains a central bed of filaments and opens into a narrow tubular ductule that leads from the base of the cupule through the epidermis to the cuticle to open externally as a pore. The cuticle of the cupule is continuous with that of the ductule and has the appearance of three layers, although the inner (middle) layer may be a gap formed during construction of the complex. In young adult males, just molted, the ultrastructure of the cells and their inclusions indicate that they are not active. The region of the cell that is distal to the abdominal cuticle is reduced and the proximal region, surrounding the duct, is enlarged when compared with sexually mature (3-4 weeks old) adult males. At maturity the pheromone cells are enlarged distally around the cupule, but are reduced to a narrow sleeve proximally, around the ductule. Two characteristic cell profiles are evident, based on the shape of the cupule and the organelle content. Type A shows a broad opening to the cupule, an abundance of mitochondria, and few vesicular bodies. Type B has an elongated, narrow, vase-like opening to the cupule, few mitochondria, and numerous vesicular bodies. Type B cells are smaller and more abundant than Type A. Distribution within the epidermal layer also differs. It is likely that the different types represent cells producing different secretion profiles. However, the secretions retained by the standard fixation protocol within mature cells of both types look similar and appear to collect as crystalline bodies within the lumen. This may represent a common storage mechanism.     

Two new fossil flowers of magnoliid affinity from the Late Cretaceous of New Jersey

Two taxa of cupulate magnoliid fossil flowers, Cronquistiflora and Detrusandra, are described from the Late Cretaceous (Turonian, ∼90 million years before present [MYBP]) Raritan (or lower Magothy) Formation of New Jersey. The fossil taxa are represented by flowers at various stages of development, associated fragments of cup-shaped floral receptacles with attached anthers, and isolated anthers. Both taxa have laminar stamens with adaxial thecae and valvate dehiscence. Pollen is boat-shaped and foveolate in anthers associated with Cronquistiflora and spherical with reticulate ornamentation in Detrusandra. Cup-shaped receptacles are externally bracteose in both taxa. The receptacle of Cronquistiflora is broader than the campanulate one of Detrusandra. Cronquistiflora also has more carpels (∼50 in a spiral vs. ∼5 in a whorl or tight spiral). In Detrusandra the carpels are surrounded by dorsiventrally flattened structures (pistillodes?) that are remote from the attachment of the stamens near the distal rim of the receptacular cupule. Detrusandra stigmas are rounded and bilobed, while those of Cronquistiflora, although bilateral in symmetry, are somewhat peltate. The fossil taxa share prominent characters with extant cupulate magnoliids (e.g., Eupomatia, Calycanthus), but also share characters with other magnoliids including Winteraceae. These fossils represent taxa that are character mosaics relative to currently recognized families. Inclusion of these fossils in existing data matrices and ensuing phylogenetic analyses effect changes in tree topologies consistent with their mosaicism relative to modern taxa. But such analyses do not definitively demonstrate the affinities of the fossils other than illustrating that these fossils are generalized magnoliids. Additional analysis of modern and fossil magnoliids is necessary to fully appreciate the phylogenetic significance and positions of these fossil taxa. However, the results of the phylogenetic analyses do introduce the possibility that extinct taxa of Magnoliales with cupulate floral receptacles were transitional between basal angiosperms and those with tricolpate pollen. The fossils provide insights into the timing of evolution of character complexes now associated with coleopteran pollination.     

THE ANATOMY OF THE PISTILLATE INFLORESCENCE AND FLOWER OF CASUARINA VERTICILLATA LAMARCK (CASUARINACEAE)

The pistillate inflorescence of Casuarina verticillata is described as consisting of a primary axis bearing whorls of bracts with a cymule in the axil of each bract of the more central whorls. Each cymule consists of an atepallate, two-carpellate, syncarpous floret and two, lateral, once-lobed bracteoles. A “peripheral intercalary” meristem, in which divisions are primarily periclinal, forms a meshwork beneath the bracts from early development and moves the connate bracts centrifugally around the cymules and extends and binds the bracts, and to some extent the bracteoles, of the fertile part of the inflorescence together. Each bract receives a single trace; each cymule receives two traces. Each bundle extension of a cymule trace supplies: 1) a branch which joins its counterpart to become the anterior common carpellary bundle; 2) a second branch which joins its counterpart to become the posterior common carpellary bundle; and 3) a central branch which supplies a lateral bracteole. Within each floret, each common carpellary bundle provides a dorsal carpellary bundle, two ventral carpellary bundles (fertile anterior carpel) or one common ventral bundle (sterile posterior carpel). The ventral bundle-supplies join and form a single placental bundle which lies in the gynoecial septum, and which, in turn, supplies the two ovules in the anterior carpel. Whether the inflorescence is a simple racemose or a condensed cymose type cannot be determined from this species alone. The function of the sclerenchymatous, enclosing bracteoles and connate bracts is discussed.     

STRUCTURALLY PRESERVED FOSSIL PLANTS FROM ANTARCTICA. IV. TRIASSIC OVULES

Small, anatomically preserved ovules are described from specimens collected at Fremouw Peak in the central Transantarctic Mountains. The ovules occur within a silicified peat in the upper part of the Fremouw Formation, which is considered to be Early to Middle Triassic. Ovules are radially symmetrical and ovoid, with an integument that consists of a narrow endotesta and a complex sclerotesta. The bilayered nucellus has a characteristic scalloped appearance and is attached to the integument only at the base. Of 43 ovules examined, 50 to 60% contain cellular megagametophyte tissue. One specimen contains a possible archegonium with embryo. Although the ovules have not been found attached, their possible affinities are discussed in relation to the known flora from this locality and other, comparable-aged floras from Gondwana.     

Gill microcirculation of the air-breathing climbing perch, Anabas testudineus (Bloch):relationships with the accessory respiratory organs and systemic circulation

The general macrocirculation and branchial microcirculation of the air-breathing climbing perch, Anabas testudineus, was examined by light and scanning electron microscopy of vascular corrosion replicas. The ventral aorta arises from the heart as a short vessel that immediately bifurcates into a dorsal and a ventral branch. The ventral branch distributes blood to gill arches 1 and 2, the dorsal branch to arches 3 and 4. The vascular organization of arches 1 and 2 is similar to that described for aquatic breathing teleosts. The respiratory lamellae are well developed but lack a continuous inner marginal channel. The filaments contain an extensive nutritive and interlamellar network; the latter traverses the filament between, but in register with, the inner lamellar margins. Numerous small, tortuous vessels arise from the efferent filamental and branchial arteries and anastomose with each other to form the nutrient supply for the filament, adductor muscles, and arch supportive tissues. The efferent branchial arteries of arches 1 and 2 supply the accessory air-breathing organs. Arches 3 and 4 are modified to serve primarily as large-bore shunts between the dorsal branch of the ventral aorta and the dorsal aorta. In many filaments from arches 3 and 4, the respiratory lamellae are condensed and have only 1-3 large channels. In some instances in arch 4, shunt vessels arise from the afferent branchial artery and connect directly with the efferent filamental artery. The filamental nutrient and interlamellar systems are poorly developed or absent. The respiratory and systemic pathways in Anabas are arranged in parallel. Blood flows from the ventral branch of the ventral aorta, through gill arches 1 and 2, into the accessory respiratory organs, and then returns to the heart. Blood, after entering the dorsal branch of the ventral aorta, passes through gill arches 3 and 4 and proceeds to the systemic circulation. This arrangement optimizes oxygen delivery to the tissues and minimizes intravascular pressure in the branchial and air-breathing organs. The efficiency of this system is limited by the mixing of respiratory and systemic venous blood at the heart.     

Axial nature of the cupule‐bearing organ in Caytoniales

Abstract Caytoniales are an important group of seed plants, and the nature of their female reproductive organ may influence interpretations of the seed plant phylogeny and the origin of angiosperms. Although not convincingly demonstrated by clear evidence, cupules on previously described specimens were interpreted as being distichously arranged, implying that the cupule‐bearing organ in Caytoniales was a pinnate megasporophyll. Here a female reproductive organ of Paracaytonia hongtaoi gen. et sp. nov. (Caytoniales) is reported from Liaoning, China. The well preserved specimen clearly shows a spiral arrangement of cupules along the reproductive axis, suggesting that the cupule‐bearing organ in Caytoniales is not a megasporophyll but a branch. This new information on the axial nature of the cupule‐bearing organ in Caytoniales has significant implications on the placement of Caytoniales in the seed plant phylogeny and interpretation of the relationship between Caytoniales and angiosperms.