[dinosaur] Archosauriform tooth assemblage from Middle Triassic of Tanzania

Ben Creisler
[email protected]

A new paper in open access:

Devin K. Hoffman, Hunter R. Edwards, Paul M. Barrett & Sterling J. Nesbitt
(2019)
Reconstructing the archosaur radiation using a Middle Triassic
archosauriform tooth assemblage from Tanzania.
PeerJ 7:e7970
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Following the Permo–Triassic mass extinction, Archosauriformes--the clade
that includes crocodylians, birds, and their extinct relatives outside
crown Archosauria--rapidly diversified into many distinct lineages, became
distributed globally, and, by the Late Triassic, filled a wide array of
resource zones. Current scenarios of archosauriform evolution are ambiguous
with respect to whether their taxonomic diversification in the Early–Middle
Triassic coincided with the initial evolution of dietary specializations
that were present by the Late Triassic or if their ecological disparity
arose sometime after lineage diversification. Late Triassic archosauriform
dietary specialization is recorded by morphological divergence from the
plesiomorphic archosauriform tooth condition (laterally-compressed crowns
with serrated carinae and a generally triangular lateral profile).
Unfortunately, the roots of this diversification are poorly documented,
with few known Early­-Middle Triassic tooth assemblages, limiting
characterizations of morphological diversity during this critical, early
period in archosaur evolution. Recent fieldwork (2007–2017) in the Middle
Triassic Manda Beds of the Ruhuhu Basin, Tanzania, recovered a tooth
assemblage that provides a window into this poorly sampled interval. To
investigate the taxonomic composition of that collection, we built a
dataset of continuous quantitative and discrete morphological characters
based on in situ teeth of known taxonomic status (e.g., Nundasuchus,
Parringtonia: N = 65) and a sample of isolated teeth (N = 31). Using crown
heights from known taxa to predict tooth base ratio (= base length/width),
we created a quantitative morphospace for the tooth assemblage. The
majority of isolated, unassigned teeth fall within a region of morphospace
shared by several taxa from the Manda Beds (e.g., Nundasuchus,
Parringtonia); two isolated teeth fall exclusively within a "Pallisteria"
morphospace. A non-metric multidimensional scaling ordination (N = 67) of
11 binary characters reduced overlap between species. The majority of the
isolated teeth from the Manda assemblage fall within the Nundasuchus
morphospace. This indicates these teeth are plesiomorphic for
archosauriforms as Nundasuchus exhibits the predicted plesiomorphic
condition of archosauriform teeth. Our model shows that the conservative
tooth morphologies of archosauriforms can be differentiated and assigned to
species and/or genus, rendering the model useful for identifying isolated
teeth. The large overlap in tooth shape among the species present and their
overall similarity indicates that dietary specialization lagged behind
species diversification in archosauriforms from the Manda Beds, a pattern
predicted by Simpson’s "adaptive zones" model. Although applied to a single
geographic region, our methods offer a promising means to reconstruct
ecological radiations and are readily transferable across a broad range of
vertebrate taxa throughout Earth history.


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