14 results found
Brazeau MD, Friedman M, 2015, The origin and early phylogenetic history of jawed vertebrates, NATURE, Vol: 520, Pages: 490-497, ISSN: 0028-0836
Brazeau MD, de Winter V, 2015, The hyoid arch and braincase anatomy of Acanthodes support chondrichthyan affinity of 'acanthodians', PROCEEDINGS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES, Vol: 282, ISSN: 0962-8452
Giles S, Coates MI, Garwood RJ, et al., 2015, Endoskeletal structure in Cheirolepis (Osteichthyes, Actinopterygii), An early ray-finned fish, PALAEONTOLOGY, Vol: 58, Pages: 849-870, ISSN: 0031-0239
Giles S, Friedman M, Brazeau MD, 2015, Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome, NATURE, Vol: 520, Pages: 82-U175, ISSN: 0028-0836
Brazeau MD, Friedman M, 2014, The characters of Palaeozoic jawed vertebrates, ZOOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Vol: 170, Pages: 779-821, ISSN: 0024-4082
Friedman M, Brazeau MD, 2013, PALAEONTOLOGY A jaw-dropping fossil fish, NATURE, Vol: 502, Pages: 175-177, ISSN: 0028-0836
BRAZEAU MARTIND, 2012, A revision of the anatomy of the Early Devonian jawed vertebrate Ptomacanthus anglicus Miles, Palaeontology, Vol: 55, Pages: 355-367, ISSN: 0031-0239
Anderson PS, Friedman M, Brazeau MD, et al., 2011, Initial radiation of jaws demonstrated stability despite faunal and environmental change., Nature, Vol: 476, Pages: 206-209
More than 99 per cent of the roughly 58,000 living vertebrate species have jaws. This major clade, whose members are collectively known as gnathostomes ('jawed mouths'), made its earliest definitive appearance in the Silurian period, 444-416 million years (Myr) ago, with both the origin of the modern (crown-group) radiation and the presumptive invasion of land occurring by the end of the Devonian period (359 Myr ago). These events coincided with a major faunal shift that remains apparent today: the transition from Silurian ecosystems dominated by jawless fishes (agnathans) to younger assemblages composed almost exclusively of gnathostomes. This pattern has inspired several qualitative descriptions of the trophic radiation and ecological ascendance of the earliest jawed vertebrates. Here we present a quantitative analysis of functional variation in early gnathostome mandibular elements, placing constraints on our understanding of evolutionary patterns during this critical interval. We document an initial increase in functional disparity in the Silurian that stabilized by the first stage of the Devonian, before the occurrence of an Emsian (∼400 Myr ago) oxygenation event implicated in the trophic radiation of vertebrates. Subsequent taxonomic diversification during the Devonian did not result in increased functional variation; instead, new taxa revisited and elaborated on established mandibular designs. Devonian functional space is dominated by lobe-finned fishes and 'placoderms'; high disparity within the latter implies considerable trophic innovation among jaw-bearing stem gnathostomes. By contrast, the major groups of living vertebrates--ray-finned fishes and tetrapods--show surprisingly conservative mandibular morphologies with little indication of functional diversification or innovation. Devonian gnathostomes reached a point where they ceased to accrue further mandibular functional disparity before becoming taxonomic dominants relative to 'ostr
BRAZEAU MARTIND, 2011, Problematic character coding methods in morphology and their effects, Biological Journal of the Linnean Society, Vol: 104, Pages: 489-498, ISSN: 0024-4066
Friedman M, Brazeau MD, 2011, Sequences, stratigraphy and scenarios: what can we say about the fossil record of the earliest tetrapods?, Proc Biol Sci, Vol: 278, Pages: 432-439
Past research on the emergence of digit-bearing tetrapods has led to the widely accepted premise that this important evolutionary event occurred during the Late Devonian. The discovery of convincing digit-bearing tetrapod trackways of early Middle Devonian age in Poland has upset this orthodoxy, indicating that current scenarios which link the timing of the origin of digited tetrapods to specific events in Earth history are likely to be in error. Inspired by this find, we examine the fossil record of early digit-bearing tetrapods and their closest fish-like relatives from a statistical standpoint. We find that the Polish trackways force a substantial reconsideration of the nature of the early tetrapod record when only body fossils are considered. However, the effect is less drastic (and often not statistically significant) when other reliably dated trackways that were previously considered anachronistic are taken into account. Using two approaches, we find that 95 per cent credible and confidence intervals for the origin of digit-bearing tetrapods extend into the Early Devonian and beyond, spanning late Emsian to mid Ludlow. For biologically realistic diversity models, estimated genus-level preservation rates for Devonian digited tetrapods and their relatives range from 0.025 to 0.073 per lineage-million years, an order of magnitude lower than species-level rates for groups typically considered to have dense records. Available fossils of early digited tetrapods and their immediate relatives are adequate for documenting large-scale patterns of character acquisition associated with the origin of terrestriality, but low preservation rates coupled with clear geographical and stratigraphic sampling biases caution against building scenarios for the origin of digits and terrestrialization tied to the provenance of particular specimens or faunas.
Friedman M, Brazeau MD, 2010, A reappraisal of the origin and basal radiation of the Osteichthyes, Journal of Vertebrate Paleontology, Vol: 30, Pages: 36-56, ISSN: 0272-4634
Brazeau MD, 2009, The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins., Nature, Vol: 457, Pages: 305-308
Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic 'acanthodians' are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418-412 Myr bp) acanthodian, Ptomacanthus anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters.
Brazeau MD, Jeffery JE, 2008, The hyomandibulae of rhizodontids (Sarcopterygii, stem-tetrapoda)., J Morphol, Vol: 269, Pages: 654-665, ISSN: 0362-2525
Despite its important role in the study of the evolution of tetrapods, the hyomandibular bone (the homologue of the stapes in crown-group tetrapods) is known for only a few of the fish-like members of the tetrapod stem-group. The best-known example, that of the tristichopterid Eusthenopteron, has been used as an exemplar of fish-like stem-tetrapod hyomandibula morphology, but in truth the conditions at the base of the tetrapod radiation remain obscure. We report, here, four hyomandibulae, from three separate localities, which are referable to the Rhizodontida, the most basal clade of stem-tetrapods. These specimens share a number of characteristics, and are appreciably different from the small number of hyomandibulae reported for other fish-like stem-tetrapods. While it is unclear if these characteristics represent synapomorphies or symplesiomorphies, they highlight the morphological diversity of hyomandibulae within the early evolution of the tetrapod total-group. Well-preserved muscle scarring on some of these hyomandibulae permit more robust inferences of hyoid arch musculature in stem-tetrapods.
Brazeau MD, Ahlberg PE, 2006, Tetrapod-like middle ear architecture in a Devonian fish., Nature, Vol: 439, Pages: 318-321
Few fossils show the incipient stages of complex morphological transformations. For example, the earliest stages in the remodelling of the spiracular tract and suspensorium (jaw suspension) of osteolepiform fishes into the middle ear of tetrapods have remained elusive. The most primitive known tetrapods show a middle ear architecture that is very different from osteolepiforms such as Eusthenopteron, with little indication of how this transformation took place. Here we present an analysis of tetrapod middle ear origins that is based on a detailed study of Panderichthys, the immediate sister taxon of tetrapods. We show that the spiracular region is radically transformed from osteolepiforms and represents the earliest stages in the origin of the tetrapod middle ear architecture. The posterior palatoquadrate of Panderichthys is completely tetrapod-like and defines a similarly tetrapod-like spiracular tract. The hyomandibula has lost its distal portion, representing a previously unrecognized advance towards a stapes-like morphology. This spiracular specialization suggests that the middle ear of early tetrapods evolved initially as part of a spiracular breathing apparatus.
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