Dinosaur biostratigraphy

Dinosaur biostratigraphy studies the distribution of dinosaur taxa through rock layers. It can be useful for dating and correlating rock units and reconstructiong ancient ecosystems. Most dinosaur-bearing rock formations do not contain multiple distinct stratigraphically separated faunas.^[1] Typically dinosaur faunas are static throughout a formation or change piecemeal over time.^[1] Faunal turnover usually occurs between formations.^[1] The fossil record can give an appearance of faunal turnover due to multiple causes including evolution, migration, or changing preservational biases.^[2] Turnover events can have extremely minor causes like the migration of a taxon to a new area or extremely conspicuous ones like an ecosystem destroying catastrophe.^[2] Since the fossil record is incomplete assessing the nature and causes of faunal turnovers is fraught with difficulty, except in cases where the fossil record is "unusually complete."^[2]

Dinosaur Park Formation

The Dinosaur Park Formation can be divided into at least two distinct faunas. The lower part of the formation is characterized by the abundance of Corythosaurus and Centrosaurus. This group of species is replaced higher in the formation by a different ornithischian fauna characterized by the presence of Lambeosaurus and Styracosaurus. The appearance of several new, rare species of ornithischian at the very top of the formation may indicate that a third distinct fauna had replaced the second during the transition into younger, non-Dinosaur Park sediments, at the same time an inland sea transgresses onto land, but there are fewer remains here. An unnamed pachyrhinosaur, Chasmosaurus irvinensis, and Lambeosaurus magnicristatus may be more common in this third fauna.^[3][4]

The timeline below follows a synthesis presented by Arbour et al. 2009 with additional information from Evans et al. 2009

Horseshoe Canyon Formation

The timeline below follows a synthesis presented by Arbour et al. 2009.

Two Medicine Formation

In 2001, David Trexler published a review of evidence for alleged faunal turnover in the Two Medicine Formation of Montana.^[1] The fossil record can give an appearance of faunal turnover due to multiple causes including evolution, migration, or changing preservational biases.^[2] Turnover events can have extremely minor causes like the migration of a taxon to a new area or extremely conspicuous ones like an ecosystem destroying catastrophe.^[2] Since the fossil record is incomplete assessing the nature and causes of faunal turnovers is fraught with difficulty, except in cases where the fossil record is "unusually complete."^[2] However, fossils recovered from the lower and middle part of the Two Medicine Formation tend to be scrappiy and harder to identify than those from its higher strata.^[2] In fact, identifiable skeletons from the lowest 100m of the formation are almost completely unknown and rare from the middle of the formation.^[2] Despite the low quality of the remains, however, the fossils present an extremely diverse ecosystem.^[2] Hypacrosaurus coexisted with Maiasaura for some time, as Hypacrosaurus remains have been found lower in the formation than was earlier known.^[5] The discovery of Gryposaurus latidens in Maiasaura's range has shown that the border between hypothesized distinct faunas in the upper and middle is less distinct than once thought.^[5] Faunal divisions between the lower and middle parts of the formation are very speculative due to the relative absence of fossils in the lowest strata.^[5] Further, there are tooth remains that seem to show the presence of certain taxa are unbroken through-out the whole of the formation.^[5] However, there seems to have been a major diversification in ornithischian taxa after the appearance of Maiasaura.^[5] Thorough examination of strata found along the Two Medicine River (which exposes the entire upper half of the Two Medicine Formation) indicates that the apparent diversification was a real event rater than a result of preservational biases.^[5] Erosional loss of chronologically equivalent Montanan strata makes comparing the Two Medicine formation to the Judith River Group difficult.^[5]

Achelousaurus.

Earlier 1992 work by Jack Horner speculated that transitional species evolved in the uppermost part of the Two Medicine Formation during the Bearpaw Transgression for a half-million year span as the transgression inundated the Judith River Formation and later the Two Medicine area, gradually destroying the dinosaurs' preferred habitats.^[6] Horner cited certain ceratopsid and pachycephalosaurid species as possible evidence for his hypothesis.^[6] Trexler accepted that the proposal might be true for some taxa, but notes that the effected hadrosaurid and tyrannosaurid taxa had actually evolved before the Bearpaw Transgression began.^[6] The most likely cause in the difference between dinosaur faunas in the Two Medicine Formation and Judith River Group is the latter being adapted to moist coastal lowlands and the former being a higher elevation somewhat arid region.^[7] Ecological separation between the different areas is the most likely explanation for faunal differences if there was no natural barrier.^[7] However there are many observations about the regions that this explanation fails to account for.^[7] Post mortem carcass transport or migration should result in shared taxa between formations but none are known.^[7] No evidence of gradual migration of Judith River taxa to the Two Medicine Formation exists even though the Two Medicine Formation should have gradually become coastal lowlands as the Judith River region was inundated by the Bearpaw Transgression. The absence of evidence for migration is conspicuous given the discovery of hundreds of dinosaur specimens in the upper Two Medicine.^[7] The Judith river formations and Two Medicine Formation shared the same northeasterly paleoflow direction in their streams, meaning they should have the same erosional source material, yet there is no evidence of this reflected in their respective facies.^[8] The Two Medicine and Judith River regions being arid and moist respectively despite exist within 100 km of each other and within the same rainshadow cast by the high mountains to the west.^[9] Trexler concludes that previously hypothesized faunal turnover boundaries may actually be due to preservational biases.^[9]

Footnotes

1. ^ ^{a b c d} "Abstract," Trexler (2001); page 298.
2. ^ ^{a b c d e f g h i} "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 304.
3. ^ Ryan and Evans (2005).
4. ^ Evans, D.C., Bavington, R. and Campione, N.E. (2009). "An unusual hadrosaurid braincase from the Dinosaur Park Formation and the biostratigraphy of Parasaurolophus (Ornithischia: Lambeosaurinae) from southern Alberta." Canadian Journal of Earth Sciences, 46(11): 791-800. doi:10.1139/E09-050
5. ^ ^{a b c d e f g} "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 306.
6. ^ ^{a b c} "Faunal Turnover, Migration, and Evolution," Trexler (2001); page 307.
7. ^ ^{a b c d e} "Conclusions," Trexler (2001); page 307.
8. ^ "Conclusions," Trexler (2001); pages 307-308.
9. ^ ^{a b} "Conclusions," Trexler (2001); page 308.

References

	Dinosaurs portal
	Earth sciences portal
	Paleontology portal

• Ryan, M.J., and Evans, D.C. 2005. Ornithischian dinosaurs. In: Currie, P.J., and Koppelhus, E.B. (eds), Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed. Indiana University Press: Bloomington and Indianapolis, 312-348.
• Trexler, D., 2001, Two Medicine Formation, Montana: geology and fauna: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 298–309.