Rallus

Alcover, Josep Antoni, Pieper, Harald, Pereira, Fernando & Rando, Juan Carlos, 2015, Five new extinct species of rails (Aves: Gruiformes: Rallidae) from the Macaronesian Islands (North Atlantic Ocean), Zootaxa 4057 (2), pp. 151-190 : 172-178

Rallus sp. B

Terceira rail

[ Figures 18 , 19 C]

The material from Terceira consists of 13 associated skeletons (but most bones are very fragmentary). These bones probably also belong to an endemic species, convergent with the Pico rail, but the available material is very fragmentary and insufficient to establish clearly its specific distinction from known taxa, so we refer it as Rallus sp.

Material and collection information. Gruta do Natal: MCMa 1818.013, associated skeleton (LR tmt, R rad, L uln and R dis uln, 7 verts, LR cmc, fem frag, LR tbt frag, 6 ped phas); MCMa 1819.013, associated skeleton (hum frags, R uln frags, rad frags, syn, verts, tbt frags, LR tmt, ped pha); MCMa 1824.013, associated skeleton (man frags, R tbt frag, R tmt, ped pha)

Gruta de Santo Antonio: MCMa 1820.013, associated skeleton (sku, man frags, R cor, scap frag, R hum frag, syns frag, verts, quad, R fem frag, R tmt, R tbt frag, ped pha); MCMa 1825.013, associated skeleton (sku frags, R cor, syns frag, pel frags, L fem frag, LR tbt frags, tmt frags, verts); MCMa 1827.013, associated skeleton (sku frag, man frags, LR hum frags, uln frags, L cor and caudal part of R cor, sca frag, tbt frag, tmt frag, 10 vers, pha); MCMa 1830.013, associated skeleton (man frag, syn frag, LR fem frags, tbt frag, LR tmt frags, pha).

Gruta das Agulhas: MCMa 1821.013, associated skeleton (sku frag, LR hum, LR uln, R rad and L dis rad, LR cmc, LR cor, sca frag, fem frags, R tmt frags, LR tbt, 10 vers, 5 ped phas); MCMa 1822.013, associated skeleton (L hum frag, R uln, R cmc, L fem and R dis fem, tbt frag); MCMa 1828.013, associated skeleton (hum frag, tmt frag, LR tbt frag).

Furna de Santa Maria: MCMa 1823.013, associated skeleton (sku frag, man frags, R hum frag, LR sca frags, R tbt frag, R tmt and L dis tmt, vers, ped phas); MCMa 1826.013, associated skeleton (sku frags, L hum frag, LR uln frag, rad frag, R cmc and L prox cmc, L cor and R cranial end of cor, LR fem frags, LR tbt frags, LR tmt frags, vers, ped phas); MCMa 1829.013, associated skeleton (sku frag, tbt frag, vers).

All the material was collected by FP, JCR and JAA on 17–22 Sept 2012, with the collaboration of IA in Furna de Santa Maria.

Measurements: See Table 1.

A silicified specimen of Rallus sp. B. We report here the first avian body fossil described from ancient siliceous sinter (opal) deposits in the Azorean Islands, curated at the Colecção dos Montanheiros. The specimen comes from the Algar do Carvão, Terceira. It is a 3D preservation of feathers and soft body morphology ( Figure 18 ), derived from a silicification process that allows the preservation of features normally lost during fossilization. Silica (silicon oxide) is a mineral associated with exceptional preservation of fossils (e.g., Allison 1988; Channing et al. 2005; Manning et al. 2009). Some Azorean caves contain siliceous speleothems (stalactites, stalagmites and flowstone) made of opal. The opal is a hydrated amorphous form of silica that precipitates as hot water evaporates, becoming solid after drying. The silicification of a specimen results from its coating and encrustation by the dissolved opal and its subsequent drying. Exceptionally, the silification can affect carcasses of recently dead animals. Thesilica encrustation of the body of the dead specimen can occur within hours or days after death and before substantial soft tissue degradation takes place (i.e., before 2–3 days after death; Davis and Briggs 1998; Channing et al. 2005). It can preserve the feathers, nails, and the colonizing microbial communities.

Channing et al. (2005) described a silicified bird from Yellowstone Quaternary hot springs deposits. It consisted of an external mould of the body of a Fulica . According to the description of Nunes et al. (2004), Algar do Carvão is a volcanic cave located in the centre of Terceira Island, at 583 m above sea level. The entrance of the volcanic pit is 17 m x 27 m in size and connects with a vertical conduit, which drops ca. 45 m. After an uneven slope, the conduit [or passage] drops vertically once again and ends in a lake of clear water, at ca. 90 m below the mouth of the pit. The lake has a maximum depth of about 15 m and dries up almost completely in summer in years of low precipitation. The complex geological and biochemical phenomena which occur in the hydrogeological system of Algar do Carvão allowed the formation of stalactites and flowstone of amorphous silica (Si ≈ 77% to 82%). These are probably the most exuberant, rare and beautiful structures present in the volcanic caves of the Azores. These opal stalactites, milky-white coloured with reddish internal veins, cover a large area of the roof and walls of the pit and reach a length of about 1 m long and 40 cm to 50 cm in diameter. On the cave floor, opal flowstone coats several square meters. The presence of opal speleothems suggests the sporadic activity of a hot spring inside the cave.

Unlike the Fulica specimen described by Channing et al. (2005), which corresponds to an external cast that could have originated by the erosion of the body in a fully coated specimen or by the deposition of a carcass upon an opaline gum, the Rallus sp. B from the Algar do Carvão preserves its body. The specimen is a feathered carcass coated with a thin layer of opal. It suffered some damage during its extraction in the 1990s, and currently it is curated in the Museum of Os Montanheiros, at Angra do Heroísmo. The opal coat of the specimen was formed apparently as the result of a silica-enriched water flux upon a freshly dead specimen.

The specimen is 17.5 x 15.7 cm in size and apparently corresponds to an adult or to a full-grown specimen, preserving the body (abdomen and trunk), the two hindlimbs (the right is broken at the level of the proximal end of tarsometatarsus; the left is broken at the level of the toes), and the right wing. The tail is not visible and the specimen is broken at the level of the neck, lacking the entire head. A hole allows the right coracoid to be seen, fully formed, in its anatomical position. The tip of the wing is slightly broken, but the total length of the wing can be estimated at 9–10 cm. The length of the tarsosometatarsus is ca. 4 cm, and the specimen might have attained a height of c. 12 cm. It is possible to see the skin scales on the proximal right tarsometatarsus. The hindlimbs appear to wear fewer feathers than in R. aquaticus . The tibiotarsus seems to be naked until its upper third.

The plumage of the ventral part of the body has the appearance of a juvenile (i.e. loose feathers).

Age: Holocene-Recent. The silicified specimen of Rallus sp. B found in Algar do Carvão, (Terceira Island) a volcanic pit of a Late-Holocene age (Forjaz et al. 2004), indicates a very recent extinction. The radiometric dating analysis of charcoal found beneath the basaltic lava flow at Algar do Carvão rendered an age of 1730± 40 years BP for the lava flow (Forjaz et al. 2004), and the silicified specimen was apparently deposited at an indeterminate moment after this flow.

Figures

FIGURE 18. Photograph of a silicified specimen of Rallus sp. B from Algar do Carvão, Terceira. A. General view. B. Detail of the tarsometatarsus. The scales of the tarsometatarsus are visible in the section free of silica. Scale bar of A: 4 cm. Scale bar of B: 2 cm.

FIGURE 19. Bones of Rallus sp. A from Graciosa (B) and Rallus sp. B from Terceira (C) compared with the same bones of Rallus montivagorum n. sp. from Pico (A). From L to R: Humerus, caudal view: MCMa 1943.014; MCMa 1961.04 (L humerus, specular image), MCMa 1821.013. Ulna, dorsal view: MCMa 1867, MCMa 1821 (L ulna, specular image). Carpometacarpus, ventral view: MCMa 1856, MCMa 1821.013 (L carpometacarpus, specular image), MCMa 1964.014. Coracoid, dorsal view: MCMa 1918, MCMa 1969.014, MACa 1825. Femur, cranial view: MCMa 1880, MCMa 1965.014, MCMa 1822 (L femur, specular image). Tibiotarsus, cranial view: IMEDEA 104097, MCMa 1982.014, MCMa 1821.013. Tarsometatarsus, dorsal view: MCMa 1949, MCMa 1968.014 (L tarsometatarsus, specular image), MCMa 1820.013. Scale bar: 2 cm.