Lagerpetidae

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Lagerpetidae
Temporal range:
Ma
Speculative life restoration of
Dromomeron gregorii
 in a bipedal stance
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Avemetatarsalia
Clade: Ornithodira
Clade: †Pterosauromorpha
Family: †Lagerpetidae
Arcucci, 1986
Genera

Lagerpetidae (

Dromomeron gigas and a specimen from the Santa Rosa Formation attributed to Dromomeron sp., were able to get quite large (femoral length 150–220 mm (5.9–8.7 in)).[8][9] Lagerpetid fossils are rare; the most common finds are bones of the hindlimbs, which possessed a number of unique features.[10]

Description

As with most early avemetatarsalians, the most characteristic adaptations of lagerpetids occurred in their hip, leg and ankle bones, likely as a result of these being the bones most commonly preserved. Hip material is only known in Lagerpeton and Ixalerpeton, which share three adaptations of the ilium (upper blade of the hip). The supraacetabular crest, a ridge of bone which lies above the acetabulum (hip socket), is thickest above the middle portion of the acetabulum, rather than the front of it. However, it also extends further forwards than in most dinosauromorphs, snaking along the length of the pubic peduncle (the area of the ilium which connects to the pubis). The ilium's facet for the pubis opens downwards, a trait also acquired by ornithischian dinosaurs.[6] The hip in general was wide, had a closed acetabulum (i.e. one with a bony inner wall), and had two sacral vertebrae, lacking many specializations of later dinosauromorphs, like dinosaurs.[4]

Skeletal diagram of Ixalerpeton polesinensis, restored in a quadrupedal stance. Known elements in white and unknown in gray.

Like other early

archosaurs (and archosaur relatives such as Euparkeria), the femur (thigh bone) was slender and S-shaped. The femoral head was thin when seen from above, and its apex projected about 45 degrees between medially (inwards) and anteriorly (forwards). Most archosaurs had three tubera (bumps) on their flattened femoral head, one at the middle of the anterolateral (forwards/outwards) surface, another at the middle of the posteromedial (backwards/inwards) surface, and a small third one which was near the apex of the femoral head. However, lagerpetids lack the anterolateral tuber, instead having an emargination in the head just below where the tuber would normally be expected. The femoral head itself was notably hook-shaped when seen from the side. The distal portion of the femur (i.e. the portion near the knee) had a pair of condyles (knobs) on either side of the rear surface, as well as a third knob-like structure known as a crista tibiofibularis, which was present just above the lateral condyle. The crista tibiofibularis was uniquely enlarged in lagerpetids, and undergoes further evolution in Ixalerpeton and particularly Dromomeron.[10]

The

pterosaurs and some early dinosaurs (coelophysoids, for example). A pair of small, pyramid-shaped structures rise up out of the astragalus, one in front of the facet for the tibia, and the other behind it. The one in front is similar to a structure found in dinosauriform ankles known as the anterior ascending process, and it may be homologous with it. However, the posterior ascending process (the one behind the tibial facet) is entirely unique to lagerpetids. The rear of the astragalus lacks a horizontal groove, similar to Tropidosuchus, theropods, and ornithischians, but unlike most other archosauriforms. Like pterosaurs and dinosaurs (but unlike Marasuchus and most other archosaurs), the facet on the calcaneum which receives the fibula is concave and there is no evidence of a pronounced rearward bump known as a calcaneal tuber.[10][6]

Classification

The lagerpetids were typically considered relatives of the

morphotype) to the data matrices used in the study by Muller et al. (2018).[7]

Cladogram simplified after Cabreira et al., 2016:[6]

Eucrocopoda

Euparkeria

Lagerpetidae
Dinosauriformes

By contrast, Kammerer et al. (2020),

pterosaurs, based on newly-described fossils of the jaw, forelimbs, and braincase.[3] Baron (2021) also recovered a similar result.[13]

References

  1. doi:10.1038/s41586-023-06359-z
    .
  2. ^
    PMID 32631980
    .
  3. ^
    S2CID 228077525
    .
  4. ^ a b c Arcucci, Andrea (1986). "New materials and reinterpretation of Lagerpeton chanarensis Romer (Thecodontia, Lagerpetonidae nov.) from the Middle Triassic of La Rioja, Argentina" (PDF). Ameghiniana. 23 (3–4): 3. Archived from the original (PDF) on July 19, 2019.
  5. ISBN 9781862393615
    . Retrieved 29 March 2016.
  6. ^
    PMID 27839975
    .
  7. ^
    S2CID 133873065
    .
  8. S2CID 131613066
    .
  9. S2CID 221751762
    .
  10. ^
    S2CID 83493714
    .
  11. S2CID 205254710. Archived from the original
    (PDF) on November 24, 2021.
  12. PMID 29690420
    .
  13. ISSN 0012-8252
    .
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