Paleobotany, in the broad sense, is the science that deals with the study of fossil plants. In the strict sense, it is a multidisciplinary science, where geology and botany play a preponderant role in the analyses and obtaining of information from the fossil plant record (where the study of fungi, algae, and bacteria fossils is also included) throughout the geological time.
The report of occurrence of fossil plants in the island of Madeira is relatively common when the pioneer works from the 19th century, dealing with the geology and flora of the island, are carefully read. The preservation of plant remains in volcanic islands owes to its geological history. Normally, volcanic islands are constructed through various eruptive processes over time, intercalated with periods of quiescence of variable duration, at a local or regional scale. These intervals constitute, from a biological point of view, an opportunity for the installation or recovery of the flora. Whether during the volcanic activity, whether in the quiescence period, there are various taphonomical processes (i.e. burial and preservation processes of remains of living organisms) that operate, important for the fossilization of floristic communities. Burial phenomena of plants can be of two types: phenomena directly associated to volcanism such as lava flows, pyroclastic fall, or pyroclastic flows; or phenomena indirectly associated to volcanism such as lahar (hyperconcentrated sedimentary flows), aggradation of waterlines due to landslides blockage or lava spills (many times forming temporary lakes), landslides and slope deposits and, finally, deposits of aeolic origin.
Since the 19th century, several deposits containing vegetal macrorests have been described all over the archipelago: rhizoliths from Piedade Dunes (Caniçal), São Jorge Deposit (Santana), Porto da Cruz Deposit and Deposit of Paul da Serra; and other mentioned deposits situated in Funchal (Pontinha and Formosa beach), Câmara de Lobos (sítio do Convento), and Porto Moniz (sítio da Lagoa). Of equal paleontological interest are the findings of tree trunks during tunnel excavations, which were never scientifically described. In Porto Santo, the presence of a lignite layer (ribeira do Calahau) is equally referred. Less common is the description of microrests, such as pollen and phytoliths, restricted to Piedade Dunes.
Historically, the first reference and illustration of plant macrofossils in the island of Madeira comes from this locality (Fig. 1) and emerges from the posthumous work of naturalist Thomas Edward Bowdich (1791?-1824), Excursions in Madeira and Porto Santo During the Autumn of 1823, published in 1825. Along the years, this locality was referred in many works of geological and biological nature, such as those from James Macaulay and Charles Lyell, due to its fossil content. Geologically, and according to Brum da Silveira, this sedimentary deposit consists of levels of aeolian sands of marine origin deposited in a subaereal environment and separated by paleosols, in which carbonated rhizo-concretions and terrestrial gastropods occur. This sedimentary sequence dates, according to Gleen Goodfriend, from 300 to 200 thousand years BP.
Several paleontological works exist about this place, mainly focusing on the fossil fauna (e.g. gastropods, birds, mammals, and reptiles). The work about stratigraphy, chronology, and paleoenvironment deposition was the one that addressed the most of the paleobotany of the place, whose micropaleontological analysis revealed the presence of pollen (vide Boraginaceae, Caryophyllaceae, Chenopodiaceae, Compositae [Cirsium-type, Centaurea; Linguliflorae and Tubuliflorae], Cruciferae, Ericaceae, Plantago, Succisa, Gramineae and Pinus) and phytolithes (dicotyledons and graminids). Plant macrofossils are represented by rhizo-concretions (i.e. mineral accumulation around a root forming a layer; later, the tubular cavity left by the root’s decomposition can be filled) (Fig. 1). By its nature, this type of fossils does not contain anatomical information, but evidence the successive colonization of the dunes by a shrub or tree-like vegetation.
São Jorge Deposit
According to what emerges in the texts of authors from the 19th century, São Jorge deposit was situated between ribeira do Marcos (later designated as ribeira do Marques or dos Arcos) and ribeira grande de São Jorge, in the then called ribeira do Meio. The name of this affluent derived from the name given to an interfluvial, situated in the left margin, known as Lombo do Meio. The two first references were published in two papers from 1837 written by Mouzinho de Albuquerque (1772-1846) and another by Vargas-Bedemar (1768-1847), describing the existence of a lignite layer in São Jorge. Worthy of mention is the existence of a chemical study conducted by James Smith that concludes that the lignite would have the same chemical constitution of turf. This locality is referred by several authors throughout the first half of the 19th century, whether in scientific works, whether in tourist guides about the island, for example those written by James Macaulay and Edward Harcourt.
The most important paleobotanical studies about this deposit started with the journey of Sir Charles Lyell (1797-1875) to the Madeira and Canarian archipelagos during the years 1853 and 1854. According to Leonard Wilson, Lyell’s trip intended to reevaluate the geological observations of the Island of Madeira and the Canaries published by Leopold von Buch (1774-1853), responsible for the scientific formulation of the catastrophic theory of “elevation craters”. This theory postulated that the lifting of lava layers, horizontally accumulated in the seafloor, would give origin to the formation of volcanic islands. Lyell, critic and unsatisfied with the then current theory, traveled to the sites so he can observe and document the geology of the islands himself. During his stay in Madeira, he met Georg Hartung (1821-1891), a naturalist that then lived in the island, which accompanied him during fieldwork. Lyell looked for outcrops of fluvial sediments intercalated with eruptive events in order to obtain proves of the gradual subaereal formation of Madeira Island. One of the key points was the visit to the lignite that surfaced in one of the affluents of ribeira grande de São Jorge, known then as ribeira do Meio, in January 18, 1854 (Fig. 2). The detailed inspection of the place by Lyell and Hartung led to the discovery of a deposit of fossil leaves, from which a collection of at least 150 specimens was collected. The fossil leaves of São Jorge were the first proof to support the uniformitarian ideas of the formation of volcanic islands. This locality was also target of a fossil collection between 1854 and 1855 by Hartung, from which a collection was sent to the Swiss paleobotanist Oswald Heer (1809-1883).
The collections from this deposit gave origin to two important publications, Ueber die Fossilen Pflanzen von St. Jorge in Madeira [About the fossil plants from St. Jorge in Madeira], from Oswald Heer, and On Some Vegetable Remains from Madeira, from Charles Bunbury. Heer’s paper, presented in a conference dated from November 5, 1855, and later published in 1857, continues to be the most important on the plant fossils from São Jorge. In this article, not only a general introduction of the geology of Madeira is given, but the main sedimentary outcrops and respective fossil contents are addressed as well. The current flora of the island is discussed, and the description of the stratigraphy and morphology is made, comparing the Madeiran specimens with similar European tertiary fossils from Switzerland and with the Madeiran flora. Three prints are attached to this publication. Two of them include 58 paleobotanical specimens (25 specimens in one print and 33 specimens in the second, being one of them a coleopterous), demonstrating the existence of 25 species of plants, including current and extinct species (Fig. 3). The third print illustrates the general stratigraphy of the island, together with an illustration from the São Jorge outcrops and the Piedade Dunes. In this work, Heer demonstrates not only the similitude of the fossil record with the flora of the island of Madeira at the time, but also shows the similarities between the extinct genera in the European continent and the current living genera, in an attempt to prove that the islands would have been connected to the continent, in a clear allusion to the Atlantis theory. The collection that gave origin to this publication was deposited in the Eidgenössische Technische Hochschule Zürich, in Switzerland.
|Oswald Heer (1857)||Charles Bunbury (1859)||Oswald Heer (1861) & George Hartung (1864)|
|Pteris aquiline||Pteris aquilina||Laurus canariensis|
|Trichomanes radicans||Woodwardia radicans||Myrica faya|
|Woodwardia radicans||Davalia canariensis||Oreodaphne foetens|
|Osmunda regalis||Aspidium lyelli||Osmunda regalis|
|Asplenium marinum||Aspidium? (Polystichum)||Phyllites ziegleri|
|Asplenium bunburyanum||Nephorodium??||Pteris aquilina|
|Aspidium lyelli||Nephorodium?||Pteris cretica (figurado em HARTUNG, 1864)|
|Salix lowei||Adiantum? psychodes||Woodwardia radicans|
|Corylus australis||Laurus canariensis|
|Ulmus suberosa||Oreodaphne foetens|
|Oreodaphne foetens||Corylus australis|
|Vaccinium maderense||Vaccinium maderense|
|Vinca major||Vaccinium myrtillus?|
|Myrtus communis||Erica arborea|
|Ilex hartungi||Ilex hartungii|
|Pistacia phaeacum||Phyllites hymenaeoides|
|Phyllites (Rhus?) ziegleri|
The next work regarding the plant fossils of São Jorge is presented by Bunbury to the Geological Society of London, in April 28, 1858, in a talk about Lyell’s collection. Later, it is published in the Quarterly Journal of the Geological Society under the title “On Some Vegetable Remains from Madeira”. This publication, shorter than Heer’s, compares both collections. Bunbury analyzes 140 specimens finding 11 species not present in Heer’s collection. Lyell’s collection was later reduced to less than 40 specimens and deposited in two British institutions: the Natural History Museum (London) and the Sedgwick Museum of Earth Sciences (Cambridge University). Another collection, mentioned in an enigmatic letter from Heer to Prof. H. G. Bronn (1800-1862), would belong to Hartung. In this letter dated from March 17, 1861, referring the devolution of 260 entomological and paleobotanical specimens to Bronn, Heer discusses the analysis of a fossil collection from São Jorge, which resulted in the identification of seven morphotypes (Fig. 3). The specimens identified in the previous letter are most likely specimens gathered by Hartung in his passages through the island between the years of 1854 and 1857. According to his biographical article, during the first part of the year of 1858 Hartung frequently visited Bronn to discuss the nature of the collected specimens during his trips through the Atlantic islands. Very likely, Heer’s letter from 1861 made reference to the analyses of these specimens. Heer also relates having received fossils collected in 1861 by James Yate Johnson. Supporting the idea of a private collection of fossil leaves from São Jorge exists the publication of the book Geologische Beschreibung der Inseln Madeira und Porto Santo [Geological description of the islands of Madeira and Porto Santo] from Georg Hartung and Karl von Mayer. This book has a section dedicated exclusively to the paleontology of the islands of Madeira and Porto Santo, where Hartung dedicates several pages to paleobotany, presenting fossils collected in São Jorge and in a new deposit in Porto da Cruz. The identifications were in charge of Oswald Heer. The inspection of the São Jorge Deposit is presented as a summary similar to Heer’s article, adding a new species for the deposit which is identified as new by Heer in the 1861 letter to Bronn.
After the collection and study period of the fossil leaves of S. Jorge, during the beginning of the second half of the 19th century, follows a period without news, during which recurrent citations of the main previous works exist. An exception is the article published by John Starkie Gardner in 1882, where the author describes his tentative visit to the São Jorge Deposit, reporting that this had been hidden by a collapse occurred in 1865. Even so, he visited the alleged site, making a stratigraphic description and comparing it with the one from Oswald Heer, yet not getting any fossil specimens. After this discovery period and study of the fossil remains of São Jorge and the beginning of the 21th century, there is only citation of the old works.
The exact place of the São Jorge Deposit is unknown, probably buried by a collapse, from which there are reports of occurrences in 1865, 1885, and 1901. Nevertheless, the Elucidário Madeirense reports that the collapse that covered the São Jorge outcrop in mid 1865 was cleaned, exposing the deposit again in 1917. The authors do not relate however, who would have proceeded to the cleaning of this site or what were the reasons. According to the descriptions provided by the various authors, the Deposit and the lignite strata would be at an altitude of 1014 feet (~300 m) above sea level, in the right margin of ribeira do Meio, in the confluence with ribeira grande de São Jorge. According to Hartung’s geological cut, the strata would have an East-West orientation and an inclination towards North. Above the outcrop, a vertical wall with more than 300 m altitude would exist, resulting from the watercourse. Because of its geological localization, the deposit is thought to be included in the Intermediate Volcanic Complex (5,57 to 1,8 Ma), according to the recent volcano-stratigraphy defined by Brum da Silveira in 2010. The presence of little evolved coals is probably related to a high concentration of organic matter, possibly coming from the forests that would have then covered the island, or to the in situ formation of an accumulation of bryophytes (e.g. Sphagnum sp.) in the small basin that would have given origin to the sedimentary deposit of São Jorge. The consequent recoating of these layers by sedimentary and volcanic sequences probably provided enough temperature and pressure for the formation of the lignite layers observed by the 19th century authors.
The analyses of the surviving fossils from São Jorge conducted at the beginning of the 21th century and reviewed by Góis Marques in 2003, point out that more than 2/3 of the determinations of the local fossil flora correspond to genera or species wrongly identified or undetermined. The revision of this flora revealed the presence of 19 different forms of plants, 14 of which are attributable to current genera: ferns (Osmunda regalis, Pteridium aquilinum, Arachniodes sp., Asplenium cf. onopteris, Asplenium sp., Asplenium aff. anceps, Woodwardia radicans, Davallia canariensis, Polystichum sp.) and dicotyledons (Ocotea foetens, Erica arborea, Myrtus communis, Vaccinium cf. padifolium sp., Rubus sp.). These results contrast with the 37 species originally described in the 19th century. This is due, above all, to the wrong preservation and incompleteness of the fossils, and to the scarce knowledge of the geology and botany in the time in which the original studies were developed. The fossil plants of São Jorge, mostly composed of Lauraceae (Til: Ocotea foetens) and ferns, present remarkable similarities with the floristic community of the Temperate Til Laurisilva. Taking into account the geological context in which this species association was present, it is possible that its origin is related with a fast transit (e.g. flash flood), evidenced by the fragmented state of the specimens and by their preservation in a breach of silty-clay matrix, of an old dead layer or soil, corresponding to the series of what is considered, at the beginning of the 21th century, Til Laurisilva.
Porto da Cruz Deposit
This deposit is located in the promontory of the Porto da Cruz village, Machico council (Fig. 3). According to George Hartung and Karl von Mayer, the discovery of fossil leaves in this locality was conducted by James Yate Johnson (1820-1900) in 1859, who, together with Georg Hartung, gathered such collection. The latter author published in his book from 1864 the description of the fossils as well as a print with detailed drawings from the analyzed specimens. The paleobotanical determinations were in charge of Oswald Heer. The total number of specimens is not known but a print is published with 11 specimens from Porto da Cruz (the 12º specimen included belongs to the São Jorge collection), where two species are recognized (Rubus and Carex). The whereabouts of this collection of plant fossils from Porto da Cruz is unknown.
In 1882, John Starkie Gardner visited Porto da Cruz Deposit, collecting dicotyledon and monocotyledon leaves but did not conduct any detailed study on the collected fossils. In 1928, Theodore Dru Alison Cockerell conducted a prospection in this locality, yet did not find any fossil. More recently, there is mention of this deposit in the work A field Guide to the Geology of Madeira, from Christopher Burton and Jim MacDonald. This guide briefly refers to the existence of fossil flora in Porto da Cruz, explaining the origin of the sedimentary deposit of this locality and describing the fossil occurrence.
There is knowledge of at least four more collections other than the missing one from Hartung. One of them, composed by only six specimens housed in the Sedgwick Museum of Earth Sciences (Cambridge University), gathered by George Walter Grabham (1882-1955), is not included in any bibliographical record. The second is the one from John Starkie Gardner, mentioned in his publication from 1882, which fossils were housed in the paleontological collections of the Natural History Museum (London). A third collection, collected in 2012 by the authors of this entry, is deposited in the University of Madeira, waiting for a more detailed study (Fig. 3). The fourth collection is the one housed in the Madeira Botanical Garden (Natural History Museum), which study has not been conducted.
According to the geological map of Madeira, Porto da Cruz Deposit is framed within the Higher Volcanic Complex (Funchal’s Unit), corresponding to “hyperconcentrated fluxes that give origin to tuffs and sandstones with interbedded conglomerates”. These are sealed by a mugearite spill dated 1,5 Ma, restricting the age of this sedimentary deposit to the interval between 1,8 Ma and 1,5 Ma. These deposits could be related to the impoundment of water lines by landslides or lava spills. The consequent siltation of a river valley upstream of a drainage blockage might have given origin to thick fluvial sedimentary deposits. The Porto da Cruz Deposit reveals the presence of scarce flora with only four morphotypes recognized and three genera identified, very likely due to the little exploitation that is was target. The macroflora reveals plants of a hydrophilic nature (e.g. Carex sp. and Equisetum sp.) and lianas (Rubus sp.). Very likely, this vegetation took advantage of a period following a flooding episode from the sedimentary basin, settling in the recently-created niche, or enough for a paleosol to be created. The consequent rise of the water level caused the plants to be buried in situ, which is supported by the large amount of Carex sp. leaves present in a single layer, by the fact that they are not fragmented, and because they are apparently oriented following an approximate S-N direction, as stated by Góis Marques.
According to the literature, or based on other materials (e.g. collection labels), other localities can be listed where plant fossils were observed or collected, which detailed study was never conducted.
Paul da Serra
Some authors refer the presence of lignite in Paul da Serra. It is carbonized plant material found in a paleosol underlying a pyroclastic deposit existing in this site. This material was recently used in order to date, using the radiocarbon method, the pyroclastic deposit issued by one of the most recent volcanic eruptions in the Upper Volcanic Complex (6 to 7 thousand years, according to Geldmacher and Brum da Silveira). Carlos Teixeira mentions the existence of teixo (yew; Taxus baccata) stems in this deposit.
Mouzinho de Albuquerque describes that west of Pontinha (Funchal) existed “fragments of plant roots petrified and converted in lime carbonate loaded in silica”. Still, this sedimentary deposit apparently did not survive, probably due to the expansion of the construction in Funchal area. Charles Lyell in turn, describes the discovery conducted by James Smith of Jordan Hill (1782-1867) in 1840, of carbonized roots and branches in Funchal. Likewise in Pontinha area, Oswald Heer relates having found in 1851 carbonized branches in volcanic tuffs, which he identified as being murta (myrtle; Myrtus sp.). The three references could correspond to one same deposit, given the similarities in the descriptions and geographic location.
Câmara de Lobos (sítio do Convento) and Porto Moniz (sítio da Lagoa)
The access to photographs dated from 2008 from the leaf fossil collection of the Madeira Botanical Garden (Natural History Museum) revealed labels with reference to the existence of tuffs with leaf impressions from the sítio do Convento, in Câmara de Lobos. The label is dated from August, 1931, having the specimens been collected by the canon Jaime de Gouveia Barreto. Another label present in the collection suggests the hypothesis of the existence of lignite (?) in sítio da Lagoa, in the council of Porto Moniz. These remains were also collected by canon Barreto in 1932, who writes in the label: “Very abundant in the excavations that were conducted to place a tap for irrigation water”.
Lietz and Schwarzbach as well as Goodfriend described the existence of a paleosol with lignite 100 m north of the mouth of ribeira do Calhau (eastern region of the island). The dating of this paleosol stipulated an age of 480±55 years BP, but a paleobotanical study attempting to identify possible plant remains was never performed.
The fossil collections and the deposits will continue to be important witnesses of the vegetation that once covered the island. Its study is fundamental for the comprehension of the origin and evolution of the Madeiran flora and the remaining Macaronesia islands. The fossil flora of São Jorge and Porto da Cruz, the best studied floras at the beginning of the 21st century, allowed the identification not only of species similar to the current ones, but also ecologies similar to the ones observed.
Bibliog.: printed: BOWDICH, Sara, Excursions in Madeira and Porto Santo During the Autumn of 1823, While on His Third Voyage to Africa, London, G. B. Whittaker, 1825; BRUM DA SILVEIRA, António et al., Notícia Explicativa da Carta Geológica da Ilha da Madeira na Escala 1:50.000: Folhas A e B, 1.ª ed., Funchal, Secretaria Regional do Ambiente e Recursos Naturais, 2010; BUNBURY, Charles J. F., «On Some Vegetable Remains from Madeira», Quarterly Journal of the Geological Society, n.º 15, 1859, pp. 50-59; BURTON, Christopher J. e MacDonald, Jim G., A Field Guide to the Geology of Madeira, Glasgow, Geological Society of Glasgow, 2008; FERNÁNDEZ-PALACIOS, José M., “Una Aproximación a la Historia de la Laurisilva Macaronésica”, Makaronesia, n.º 15, 2013, pp. 52-75; GELDMACHER, Jörg et al., “The 40Ar/39Ar Age Dating of the Madeira Archipelago and Hotspot Track (Eastern North Atlantic)”, Geochem. Geophys. Geosyst., n.º 1, 2000, pp. 1-26; GÓIS MARQUES, Carlos Alberto, Paleobotânica da Ilha da Madeira: Inventário e Revisão da Macroflora Fóssil de São Jorge e Porto da Cruz, Dissertação de Mestrado em Geologia (Estratigrafia, Sedimentologia e Paleontologia) apresentada à Universidade de Lisboa, Lisboa, texto policopiado, 2013; GOODFRIEND, Glenn A. et al., “The Quaternary Eolian Sequence of Madeira: Stratigraphy, Chronology, and Paleoenvironmental Interpretation”, Palaeogeography, Palaeoclimatology, Palaeoecology, n.º 120, 1996, pp. 195-234; HARCOURT, Edward Vernon, A Sketch of Madeira Containing Information for the Traveller, or Invalid Visitor, 1.ª ed., London, John Murray, 1851; HARTUNG, Georg e MAYER, Karl von, Geologische Beschreibung der Inseln Madeira und Porto Santo. Mit dem Systematischen Verzeichnisse der Fossilen Reste Dieser Inseln und der Azoren von Karl Mayer, Leipzig, W. Engelmann, 1864; HEER, Oswald, “Pflanzenreste von St. Jorge in Madeira (Lettre à M. Bronn)”, Neues Jahrbuch Für Mineralogie, Geognosie, Geologie und Petrefakten-Kunde, Jahrgang 1861, p. 315; Id., “Ueber die Fossilen Pflanzen von St. Jorge in Madeira”, Neue Denkschriften der Allgemeinen Schweizerischen Gesellschaft für die Gesamten Naturwissenschaften, n.º XV, 1857, pp. 1-40; LIETZ, Von J. e SCHWARZBACH, M. Köln, “Quartäre Sedimente auf der Atlantik-Insel Porto Santo (Madeira-Archipel) und Ihre Paläoklimatische Deutung”, E&G – Quaternary Science Journal, vol. 22, n.º 1, 1971, pp. 89-109; LYELL, Charles, A Manual of Elementary Geology, 5.ª ed., London, John Murray, 1855; LYELL, Charles, e LYELL, Katherine M., Life, Letters, and Journals of Sir Charles Lyell, Bart., London, John Murray, 1881; MACAULAY, James, “Notes on the Physical Geography, Geology and Climate of the island of Madeira”, Edinburgh New Philosophical Journal, n.º 29, 1840, pp. 336-375; MOUZINHO DE ALBUQUERQUE, Luiz da Silva, “Observações para Servirem para a História Geológica das Ilhas da Madeira, Porto Santo e Desertas”, Memorias da Academia Real das Sciencias de Lisboa, t. XII, pt. I, Lisboa, Typ. da Academia Real das Sciencias de Lisboa, 1837, pp. 1-28; PINTO, Manuel Serrano e BOUHEIRY, Annette, «The German Geologist Georg Hartung (1821–1891) and the Geology of the Azores and Madeira Islands», in JACKSON, Patrick W. (ed.), Four Centuries of Geological Travel: The Search for Knowledge on Foot, Bicycle, Sledge and Camel, London, Geological Society of London, 2007, pp. 229-238; SILVA, Fernando Augusto da e MENESES, Carlos Azevedo de, Elucidário Madeirense (3 vols.), 2.ª ed., Funchal, s.n., 1940; SMITH, James, «On the Geology of the Island of Madeira», Proceedings of the Geological Society of London, n.º 3, 1841, pp. 351-356; STARKIE GARDNER, John, «The Geology of Madeira», Quarterly Journal of the Geological Society, n.º 38, 1882, pp. 277-281; TEIXEIRA, Carlos, «Notas Sobre a Geologia das Ilhas Atlânticas», Anais da Faculdade de Sciências do Porto, vol. 33, 1948, pp. 193-233; VARGAS-BEDEMAR, Edouard Romeo, «Resumo das Observações Geológicas Feitas em uma Viagem às Ilhas da Madeira, Porto Santo e Açores, nos Anos de 1835 e 1836», Arquivo dos Açores, n.º 10, 1837, pp. 289-296; WILSON, Leonard G., «The Geological Travels of Sir Charles Lyell in Madeira and the Canary Islands, 1853–1854», in JACKSON, Patrick W. (ed.), Four Centuries of Geological Travel: The Search for Knowledge on Foot, Bicycle, Sledge and Camel, London, Geological Society of London, 2007, pp. 207-228; digital: LYELL, Charles e HARTUNG, Georg, On the Geological Structure of the Islands of Madeira & Porto Santo, 1856, pp. 1-130, ms. digitalized by the Edinburgh University Library for the Humboldt Project: http://humboldt.mpiwg-berlin.mpg.de/lyel_onthe_en_01_1855_LiSe/index_en.html (accessed on April, 27, 2015).
Carlos A. Góis Marques
Miguel Menezes de Sequeira