| 1. |
- Balthasar, Uwe
(författare)
-
An Early Cambrian organophosphatic brachiopod with calcitic granules
- 2007
-
Ingår i: Palaeontology. - : Wiley. - 0031-0239 .- 1475-4983. ; 50:6, s. 1319-1325
-
Tidskriftsartikel (refereegranskat)abstract
- The linguliform brachiopod Eoobolus from the Early Cambrian Mural Formation (Jasper National Park, Canadian Rocky Mountains) exhibits various calcitic features in its otherwise apatitic shell. It is argued here that the decomposition of the organic matter within the shell led to a microenvironment similar to those resulting in the phosphatization of soft tissues. This diagenetic regime encouraged the initial precipitation of apatite cements followed by calcite cements. By fully coating primary structures early apatite cements separate primary structures from the later precipitation of calcite cement. Round calcareous grains, about 3 µm in size, that occur in the centre of apatite botryoids must therefore represent original components of the shell. The equivalent pits of such calcareous granules are seen in the larval shells of many Palaeozoic linguliform brachiopods. This suggests that mixed organophosphatic-calcareous shells were relatively common at that time but that they have been overlooked owing to the obliteration of original calcareous structures by traditional acid preparation methods for the extraction of phosphatic fossils. The Eoobolus shell structure is intermediate between purely organophosphatic and calcitic shells. Although one such genus is not sufficient to reconstruct the ancestral composition of the brachiopod shell, it provides a means of recognizing other transitional forms that are needed to understand fully the shift in shell mineralogy.
|
|
| 2. |
|
|
| 3. |
- Balthasar, Uwe, et al.
(författare)
-
Early Cambrian "soft-shelled" brachiopods as possible stem-group phoronids
- 2009
-
Ingår i: Acta Palaeontologica Polonica. - 0567-7920 .- 1732-2421. ; 54:2, s. 307-314
-
Tidskriftsartikel (refereegranskat)abstract
- Brachiopods and phoronids are widely recognised as closely related lophophorate phyla. but the lack of morphological intermediates linking the bivalved bodyplan of brachiopods with tubular phoronids has frustrated precise phylogenetic placement. Here we describe Lingulosacculus nuda gen. et sp. nov., a new "soft-shelled" brachiopod from the Early Cambrian Mural Formation of western Alberta which provides a plausible candidate for a phoronid stem-group within (paraphyletic) Brachiopoda. In addition to its non-biomineralised shell, L. nuda had a ventral valve with an exceptionally long, pocket-like extension (pseudointerarea) that Would have allowed the transformation of criss-crossing brachiopod-type musculature to the longitudinal arrangement typical of phoronids. "Soft-shelled" linguliform brachiopods have previously been reported from both the Chengjiang and Burgess Shale Lagerstatten which, together with L. nuda. probably represent two independent losses of shell mineralisation in brachiopods.
|
|
| 4. |
|
|
| 5. |
- Balthasar, Uwe, et al.
(författare)
-
Homologous skeletal secretion in tommotiids and brachiopods
- 2009
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 37:12, s. 1143-1146
-
Tidskriftsartikel (refereegranskat)abstract
- Tommotiids are distinctive components of the early Cambrian small shelly fauna, almost invariably represented by isolated phosphatic sclerites derived from a multielement protective cover (scleritome). The unusual range of tommotiid sclerite morphologies and unknown construction of the scleritome have severely hampered our understanding of their phylogenetic affinities. However, recent description of rare, articulated scleritome material belonging to the tommotiid genera Eccentrotheca and Paterimitra support the hypothesis that some tommotiids fall within the stem group of the lophophorate phyla Phoronida and Brachiopoda and that at least some tommotiid sclerites are homologous precursors of the shells of organophosphatic brachiopods. Here we show that the shell microstructure of Eccentrotheca and Paterimitra share substantial similarities with paterinid brachiopods. While paterinids possess an overall brachiopod morphology, their microstructure appears more similar to Eccentrotheca and Paterimitra than to nonpaterinate lingulids. These findings strongly support the existence of a brachiopod total group that is solidly rooted within tommotiids, and identify the organophosphatic skeletal composition as plesiomorphic with calcareous shells as derived. The microstructural changes of the proposed tommotiid-brachiopod transition probably reflect an adaptation to fluctuating food and phosphorous intake that came with the switch to a sessile life style at the base of the tommotiid clade.
|
|
| 6. |
- Balthasar, Uwe
(författare)
-
Mummpikia gen. nov and the origin of calcitic-shelled brachiopods
- 2008
-
Ingår i: Palaeontology. - : Wiley. - 0031-0239 .- 1475-4983. ; 51:2, s. 263-279
-
Tidskriftsartikel (refereegranskat)abstract
- Phosphatised limestone-hosted and shale-hosted specimens of the obolellid Mummpikia nuda gen. et comb. nov. and two further unidentified obolellids from the Early Cambrian Mural Formation (jasper National Park, Canadian Rocky Mountains) provide novel insights into the shell microstructure of obolellids and the nature of their pedicle. The shell was penetrated by abundant canals of a sub-pm diameter and the anterior tip of the delthyrium forms a projection into the body cavity that is penetrated by a thin canal. It is argued that both shell microstructure and posterior margin are linguliformean characters and that obolellids hold a position basal in the rhynchonelliform stem-group.
|
|
| 7. |
- Balthasar, Uwe
(författare)
-
The brachiopod Eoobolus from the Early Cambrian Mural Formation (Canadian Rocky Mountains)
- 2009
-
Ingår i: Palaeontologische Zeitschrift. - : Springer Science and Business Media LLC. - 0031-0220 .- 1867-6812. ; 83:3, s. 407-418
-
Tidskriftsartikel (refereegranskat)abstract
- The Early Cambrian brachiopod, Eoobolus, is one of the first representatives of the superfamily, Linguloidea, the defining characteristics of which include the classical morphology of oval shells and a pedicle that emerges from between the two valves. The material described here from the Mural Formation (Jasper National Park, Canadian Rocky Mountains) provides well-preserved muscle scars and larval shells that allow a discussion of the muscle system and the larval morphology of Eoobolus. The dorsal larval shell exhibits a morphology similar to other Cambrian linguloids, but also to paterinids, Mickwitzia muralensis, and some rhynchonelliforms. This suggests that there was a lesser degree of disparity among brachiopod larvae in the Cambrian than there is today. The muscle system of Eoobolus is similar to other linguloids, but differs from that of Recent lingulids and discinids by having one or two more pairs of oblique muscles. New data on the distribution of features characteristic of the family Eoobolidae question the validity of this family.
|
|
| 8. |
- Butterfield, N.J., et al.
(författare)
-
Fossil diagenesis in the Burgess Shale
- 2007
-
Ingår i: Palaeontology. - : Wiley. - 0031-0239 .- 1475-4983. ; 50:3, s. 537-543
-
Tidskriftsartikel (refereegranskat)abstract
- Current models for the exceptional preservation of Burgess Shale fossils have focused on either the HF-extractable carbonaceous compressions or the mineral films identified by elemental mapping. BSEM, EDX and microprobe analysis of two-dimensionally preserved Marpolia, Wiwaxia and Burgessia identifies the presence of both carbonaceous and aluminosilicate films for most features, irrespective of original lability. In the light of the deep burial and greenschist facies metamorphism documented for the Burgess Shale, the aluminosilicate films are identified as products of late-stage volatilization and coincident mineralization of pre-existing compression fossils, whereas the three-dimensionally preserved gut-caecal system of Burgessia is interpreted as an aluminosilicate replacement of a pre-existing carbonate phase. The case for late diagenetic emplacement of aluminosilicate minerals is supported by the extensive aluminosilicification of trilobite shell and (originally) calcareous veinlets in the Burgess Shale, as well as documentation of other secondarily aluminosilicified compression fossils. By distinguishing late diagenetic alteration from the early diagenetic processes responsible for exceptional preservation, it is possible to reconcile the range of preservational modes currently expressed in the Burgess Shale.
|
|
| 9. |
- Grazhdankin, Dmitriy V., et al.
(författare)
-
Carbonate-hosted Avalon-type fossils in arctic Siberia
- 2008
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 36:10, s. 803-806
-
Tidskriftsartikel (refereegranskat)abstract
- Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted. assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleoecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionafly with taphonomic phantoms of Avalon-type taxa (Miaohetype taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues.
|
|
| 10. |
- Holmer, Lars E., et al.
(författare)
-
The enigmatic early cambrian salanygolina : a stem group of rhynchonelliform chileate brachiopods?
- 2009
-
Ingår i: Palaeontology. - : Wiley. - 0031-0239 .- 1475-4983. ; 52:1, s. 1-10
-
Tidskriftsartikel (refereegranskat)abstract
- New material of the enigmatic brachiopod Salanygolina obliqua Ushatinskaya from the Early Cambrian of Mongolia shows that it has a colleplax- a triangular plate - in the umbonal perforation, which is enlarged by resorption. This structure is otherwise only known from the equally enigmatic Palaeozoic orders Chileida and Dictyonellida (Rhynchonelliformea, Chileata). The colleplax in Salanygolina is here considered to be homologous with that of the chileates. Salanygolina is also provided with a ridge-like pseudodeltidium, which is another chileate feature. Other characters of Salanygolina, like the radial arrangement of adductor muscle scars and postero-medially placed internal oblique muscles are characteristic of chileates, but also found in the paterinates. In contrast, mixoperipheral dorsal valves with low rudimentary interareas are well known in paterinates, but not yet recorded from chileates. Thus, Salanygolina shows a mosaic combination of morphologic characters, known both from the paterinates and chileates, indicating that it may represent a stem group of the rhynchonelliform chileate brachiopods. The laminar phosphatic secondary shell of Salanygolina is composed of closely packed and nearly identical hexagonal prisms, oriented with their long axis normal to the laminae in a honeycomb pattern. The prism walls appear to have originally been composed of organic membranes and might represent precursors of the organic sheaths of calcite fibers that are typical of calcitic shells with a fibrous microstructure.
|
|
