Review of avialans from the Maastricht Formation (3 Viewers)

[albertonykus]

Field, D.J., J. Benito, S. Werning, A. Chen, P.-C. Kuo, A. Crane, K.E. Widrig, D.T. Ksepka, and J.W.M. Jagt (2024)
Remarkable insights into modern bird origins from the Maastrichtian type area (north-east Belgium, south-east Netherlands)
Netherlands Journal of Geosciences 103: e15
doi: 10.1017/njg.2024.11

For centuries, fossils from the Maastrichtian type locality and adjacent quarries have provided key evidence of vertebrate diversity during the latest Cretaceous, yet until recently the Maastrichtian type area had revealed no important insights into the evolutionary history of birds, one of the world’s most conspicuous groups of extant tetrapods. With the benefit of high-resolution micro-CT scanning, two important avian fossils from the Maastrichtian type area have now been examined in detail, offering profound, complementary insights into the evolutionary history of birds. The holotype specimens of these new taxa, Janavis finalidens Benito, Kuo, Widrig, Jagt and Field, 2022, and Asteriornis maastrichtensis Field, Benito, Chen, Jagt and Ksepka, 2020, were originally collected in the late 1990s, but were only investigated in detail more than twenty years later. Collectively, Janavis and Asteriornis provide some of the best evidence worldwide regarding the factors that influenced stem bird extinction and crown bird survivorship through the Cretaceous-Palaeogene transition, as well as insights into the origins of key anatomical features of birds such as an extensively pneumatised postcranial skeleton, a kinetic palate, and a toothless beak. Asteriornis also provides scarce evidence of a Cretaceous-aged divergence time calibration within the avian crown group, while together, Janavis and Asteriornis constitute the only documented co-occurrence of crown birds and non-neornithine avialans. Here, we review key insights into avian evolutionary history provided by these discoveries from the Maastrichtian stratotype, document undescribed and newly discovered Maastrichtian fossils potentially attributable to Avialae and provide the first histological data for the holotype of Asteriornis, illustrating its skeletal maturity at the time of its death.

 

[Fred Ruhe]

Figure 1. Main matrix blocks comprising the holotype specimens of Asteriornis maastrichtensis (NHMM 2013 008; upper right) and Janavis finalidens (NHMM RD 271; lower left).


Figure 2. The four matrix blocks comprising the holotype of Asteriornis maastrichtensis (NHMM 2013 008); the one containing the skull is in the upper left. An ovoid depression just above the femur shaft on that block illustrates the location of the femur shaft sample for palaeohistology. Scale bar equals 10 mm.


Figure 3. Position of the skeletal elements of Asteriornis maastrichtensis within the four matrix blocks comprising the holotype specimen (NHMM 2013 008), with the matrix rendered (left) and digitally removed (right). Scale bar equals 10mm (modified from Field et al., 2020b).


Figure 4. Skeletal reconstructions of Asteriornis and Janavis. Hindlimbs of Asteriornis are longer than previously reconstructed (Field et al., 2020b), bolstering the interpretation that Asteriornis was a long-legged bird with a possible wading ecology.


Figure 5. 3D-printed physical model of the holotype skull of Asteriornis maastrichtensis as preserved (above), and highresolution CT scan rendering (below). Scale bar equals 10 cm.


Figure 6. Artist’s reconstruction of Asteriornis and Janavis in their natural habitat, along the shore of a shallow sea in the Maastrichtian of northern Europe. Illustrations courtesy of Phillip Krzeminski. Used with permission.


Figure 7. A. NHMM K 4925, distal end of left tarsometatarsus in oblique plantar view; B. NHMM JJ 11919, unidentified avialan-like fragment in three alternative views (standard anatomical orientations not possible given the uncertain nature of the fragment), both from the Maastrichtian type locality (see text for details). Scale bars equal 1 mm.


Figure 8. Newly discovered avialan-like pedal phalanx from the Type Maastrichtian (NHMM 2015 172x) in (A) right lateral, (B) left lateral, (C) ventral, (D) dorsal, (E) proximal, and (F) distal views. The specimen represents possible gut contents from a mosasaur. Scale bar equals 1 mm.


Figure 9. Histological section through the holotype femur of Asteriornis maastrichtensis (NMMH 2013 008), section DF-2. ICL: inner circumferential lamellae; OCL: outer circumferential lamellae; PO: primary osteons within parallel-fibred bone; Trans: transition to more organised, parallel-fibered bone. Scale bar equals 1 mm. Inset shows the same section imaged under a single plane polariser plus full wave retarder (λ = 530 nm) at 90°.


Figure 10. Mandible of the holotype of Asteriornis maastrichtensis in dorsal and ventral views. LMP: left medial process; RMP: right medial process. The right medial process was originally identified as the right retroarticular process by Field et al. (2020b). Subsequently, this structure was reidentified as the twisted and displaced medial process of the right mandible. Scale bar equals 10 mm. Figure modified from Crane et al. (2023).


Figure 11. Detailed morphology of the quadrate of the holotype of Asteriornis maastrichtensis (NHMM 2013 008), modified from Kuo et al. (2023).


Figure 12. Hindlimb elements from the holotype of Asteriornis maastrichtensis (NHMM 2013 008). Scale bar equals 10 mm.
Modified from Field et al. (2020b).


Figure 13. Phylogenetic interrelationships of Pygostylia, illustrating the inferred phylogenetic position of Janavis finalidens (NHMM/RD 271) as the sister taxon of Ichthyornis. Collectively, Janavis and Ichthyornis comprise the only valid representatives of Ichthyornithes, inferred to represent the sister group to the rest of Ornithurae. Illustration for Neornithes is a life reconstruction of Asteriornis maastrichtensis. Illustrations courtesy of Roc Olivé, used with permission. Topology simplified from Benito et al. (2022b).


Fred