Scandiavis mikkelseni, Avian Phylogenetic Relationships and Brain Evolution (1 Viewer)

[Fred Ruhe]

Miriam Heingård, Grace Musser, Stephen A. Hal & Julia A. Clarke, 2021

New Remains of Scandiavis mikkelseni Inform Avian Phylogenetic Relationships and Brain Evolution

Diversity. 13 (11): Article 651.
doi:10.3390/d13120651

Abstract and free pdf: New Remains of Scandiavis mikkelseni Inform Avian Phylogenetic Relationships and Brain Evolution

Although an increasing number of studies are combining skeletal and neural morphology data in a phylogenetic context, most studies do not include extinct taxa due to the rarity of preserved endocasts. The early Eocene avifauna of the Fur Formation of Denmark presents an excellent opportunity for further study of extinct osteological and endocranial morphology as fossils are often exceptionally preserved in three dimensions. Here, we use X-ray computed tomography to present additional material of the previously described taxon Scandiavis mikkelseni and reassess its phylogenetic placement using a previously published dataset. The new specimen provides novel insights into the osteological morphology and brain anatomy of Scandiavis. The virtual endocast exhibits a morphology comparable to that of modern avian species. Endocranial evaluation shows that it was remarkably similar to that of certain extant Charadriiformes, yet also possessed a novel combination of traits. This may mean that traits previously proposed to be the result of shifts in ecology later in the evolutionary history of Charadriiformes may instead show a more complex distribution in stem Charadriiformes and/or Gruiformes depending on the interrelationships of these important clades. Evaluation of skeletal and endocranial character state changes within a previously published phylogeny confirms both S. mikkelseni and a putative extinct charadriiform, Nahmavis grandei, as charadriiform. Results bolster the likelihood that both taxa are critical fossils for divergence dating and highlight a biogeographic pattern similar to that of Gruiformes

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Fred

 

[Fred Ruhe]

Systematic Paleontology

Aves Linnaeus [67].
Neognathae Pycraft [68].
Pan-Charadriiformes Hood, Torres, Norell et Clarke, 2019
Scandiavis mikkelseni Bertelli, Lindow, Dyke and Mayr, 2013

Holotype Specimen

FU171x, a partial articulated skeleton of an adult individual preserved on a slab. The holotype is missing the zygomatic region of the skull, the left quadrate, the caudal portion of the mandible, the forelimbs and several ribs and pedal phalanges.

Referred Specimen

NHMD 625345 consists of a bird skull preserved within a calcium carbonate concretion (Figure 1). The skull is remarkably complete, preserved in three dimensions and shows no signs of deformation. The fossil has been split longitudinally along the medial plane,forming sub-samples that represent the right and left side of the skull, respectively. The cranial cavity has not been filled with secondary deposits but remains as an empty cavity.

Locality

Skarrehage Quarry, Silstrup Member, earliest Eocene, Fur Formation (early Ypresian), Island of Mors, Denmark, 5656041.5” N 852023.6” E (WGS84). The specimen derives from the stratigraphic level containing carbonate concretions around ash layer +15 of the formation. This layer sits immediately below ash layer +19, which is currently radiometrically dated to ~55.4 Ma

Referral and Emended Diagnosis

Prior to phylogenetic analysis, we assigned NHMD 625345 to Scandiavis based on the following diagnostic characters from Bertelli et al. [17]: (1) skull with long narial openings, (2) pars symphysialis of mandible with a flat ventral surface, and (3) presence of a dorsally
recurved processus retroarticularis of the mandible (character 238: state 1 and 239:1 of Musser and Clarke [18]. We additionally assigned NHMD 625345 to Scandiavis based on the following combination of characters from Musser and Clarke [18]: a rostrum that is slightly curved ventrally at the apex (2:1), nares that are both rostrally (13:1) and caudally (10:2) rounded, nares that are over half the length of that of the rostrum (11:3), nares that are holorhinal or rostral to the zona flexoria craniofacialis (15:1), presence of furrowing along the midline of the interorbital area (32:2), presence of a supraorbital crest (49:1) that
is dorsally projected (50:1), presence of a craniocaudally extensive fonticulus interorbitalis (90:2) that is not confluent with the fonticulus orbitocranialis (93:1), an occipital that is subequal in rostrocaudal position to that of the nuchal crest (158:2), a nuchal crest that
is ventral to the dorsal base of the postorbital process (159:1), and a symphysis of the mandible that is less than 1/5 the length of the mandible (216:1). Phylogenetic analysis of NHMD 625345 were additionally performed with NHMD 625345 as a separate taxon.
Results placed NHMD 625345 as the sister-taxon of Scandiavis.

Differential Diagnosis from Nahmavis grandei

Scandiavis mikkelseni differs from Nahmavis grandei in the following combination of character states: (1) nares that are more rostrocaudally and dorsoventrally extensive, (2) a more obtuse antorbital angle that is approximately 90 degrees, (3) a more rostrocaudally truncate cranium, (4) a rostroventrally oriented postorbital process [18], (5) a dorsoventrally wider rostral fenestra of the mandible, (6) a more prominent dorsal mandibular angle, (7) an articular of the mandible that is located markedly ventral to the ramus, (8) a more cranially projected crista cnemialis cranialis of the tibiotarsus [18], (9) a crista cnemialis cranialis with a more rounded distal apex than that of N. grandei, (10) presence of a notch along the distal rim of the medial condyle of the tibiotarsus, (11) a more shallow fossa parahypotarsalis lateralis in the tarsometatarsus, and (12) a shorter femur and tibiotarsus.

Fred


Figure 1. Photograph (A) and line drawing (B) of the right and left side of NHMD 625345 (Scandiavis mikkelseni). Bone is unfilled, the matrix is grey, and large voids in the bone are delimited in black. Anatomical abbreviations: fr, frontal bone; fm, foramen magnum; mnd, mandible; na, nares; nc, neural cavity; or, orbit.

Figure 2. CT visualization of Scandiavis mikkelseni (NHMD 625345) in (A) lateral, (B) dorsal, (C) rostrolateral, and (D) caudal aspect. (C) shows the right quadrate in lateral aspect. Anatomical abbreviations: ec, ectethmoid; fm, foramen magnum; fr, frontal bone, jug, jugal; nd, mandible; na, nares; oc, occipital condyle; iof, interorbital fonticulus; porb, postorbital process; prx, premaxilla; qdr, quadrate.

Figure 3. Strict consensus tree of 5335 steps recovered from analyses using the most recent Kimball et al. [66] tree as a molecular backbone constraint (CI = 0.168, RI = 0.472, RC = 0.079, HI = 0.832). Charadriiform, ralloid, and gruoid silhouettes are placed at the crowns of their respective clades. All extinct taxa are denoted with daggers. Gruiformes are highlighted in red, Charadriiformes are highlighted in blue, and Scandiavis mikkelseni and Nahmavis grandei are highlighted in dark blue. Bootstrap support values greater than 50% are denoted above branches.