Hesperornithoides is one of those taxa that's been known publically for some time, largely due to Hartman et al.'s (2005) SVP abstract and poster presentation. At the time it was most notable for being an apparent troodontid from the Jurassic, although since 2008 we've known of anchiornithines which may also be examples of such. Unlike anchiornithines or the similarly Jurassic Archaeopteryx and scansoriopterygids though, Hesperornithoides isn't a slab specimen, but is preserved three dimensionally with little distortion. Here's Scott's excellent skeletal reconstruction-
While in many ways a generic basal paravian, there are a few weird things about it. The jugal has an extremely deep pneumatic recess anteriorly with a large funnel-shaped opening unlike any other maniraptoromorph (fig. 2). Velociraptor has a horizontal slit on its jugal under the anterior part of the orbit (Barsbold and Osmolska, 1999: Fig. 1B) and Zanabazar has "a longitudinally elongate dorsal slit on the medial surface of the bone" (Norell et al., 2009:34), so are both quite different. The newly described Archaeopteryx albersdoerferi has a feature in a similar position, but this seems to be only a diamond-shaped lateral fossa (Kundrat et al., 2018: Fig. 18A-B).
Another odd feature is the paraquadrate foramen is partially enclosed by the quadrate (fig. 3) unlike any other maniraptoriforms except Falcarius (Zanno, 2010: Fig. 1G) and Deinocheirus (Lee et al., 2014: scoring for character 118).
|Figure 3. Left quadrate and partial quadratojugal of Hesperornithoides miessleri (WYDICE-DML-001) in posterior view showing the quadrate (Qd) partially enclosed the paraquadrate foramen (Pqf) laterally. After Hartman et al. (2019).|
Given these characters and a few other resemblences to Morrison basal coelurosaurs Ornitholestes and Coelurus, peer reviewer Rauhut was curious about the possibility Lori was just a juvenile of one of those (it's about a third of their size- "the humerus of WYDICE-DML-001 is 29% the length of the Coelurus holotype, 17% the length of the Tanycolagreus holotype and 28% the length of the Ornitholestes holotype"). "In order to quantify the likelihood of it being a juvenile Ornitholestes, Coelurus or Tanycolagreus, we constrained trees pairing Hesperornithoides with each Morrison OTU. These were 11, 15 and 16 steps longer respectively than the most parsimonious trees, corroborating the abundant character evidence described above that Hesperornithoides is not referable to a Morrison non-maniraptoriform." Such characters include the mesiodistally constricted tooth roots, longitudinal sulcus on the neurocentral suture of mid and distal caudals, distal chevrons that are bifid both anteriorly and posteriorly, large U-shaped furcula, distodorsal radius flange, well developed semilunate carpal, extensor flange on metacarpal I, intermetacarpal scar on metacarpal II, tapered postacetabular process, shallow brevis fossa, posteroventral lobe on postacetabular process and deep anterior intercondylar groove on tibiotarsus. Still, its interesting we have some taxa that seem similar to both coelurid-grade coelurosaurs and basal paravians, like Caihong and Fukuivenator.
What about previously known Morrison paravians? Could Hesperornithoides be the same taxon? In the case of Koparion, definitely not. The teeth are extremely different (fig. 4, top). As we write, Koparion "differs from Hesperornithoides teeth in being more recurved, labiolingually wide (Basal Width / FABL ~.72 compared to ~.45), possessing large serrations as in derived troodontids, exhibiting mesial serrations that extend to within two serration lengths of the crown base, and possessing blood pits..." While I didn't include tooth-based taxa in my analysis, I wouldn't be surprised if Koparion was something that evolved large serrations convergently with troodontines since serration development is highly homoplasic in my paravian tree. "Paleopteryx" is a distal radius (BYU 2022) that I thought was similar to microraptorians back in 2007 (fig. 4, bottom left), which could certainly use a reexamination given all the new paravians described since then. While its not mentioned in the paper, Scott informed me the distal radius "is a bit squished and under-prepped" in Hesperornithoides, so we couldn't compare the two usefully although both have a pennaraptoran-like dorsal flange. Finally, there's proximal femur BYU 2023 that was first referred to Archaeopteryx by Jensen (1981) and Maniraptora indet. by Jensen and Padian (1989) (fig. 4, bottom right). I actually included this as an OTU but it "only overlaps with WYDICE-DML-001 at midshaft where it also lacks a fourth trochanter. Our analysis recovered BYU 2023 as a deinonychosaur that could belong to a troodontid or dromaeosaurid Hesperornithoides without an increase in tree length (see Positions of taxa pruned a posteriori in the supplementary information), but further comparison is limited." So a referral to Hesperornithoides is a distinct possibility that future discoveries might corroborate.
|Maniraptoromorph section of my Lori analysis after a posteriori deletion of taxa causing excess polytomies (see supp info for details). Hesperornithoides highlighted. Modified after Hartman et al. (2019).|
Finally, what is Hesperornithoides? Definitely a member of the deinonychosaur-avialan clade, as even placing it on the paravian stem takes 15 more steps. Beyond that, it's pretty uncertain. My final analysis recovered it as a troodontid, as in Hartman et al. (2005). Specifically in an intermediate clade closer to troodontines than sinovenatorines and 'jinfengopterygines' but not as close as Sinornithoides, Byronosaurus or Gobivenator. Other members of this clade include Daliansaurus, Xixiasaurus and Sinusonasus. But the analysis used in our first submission and 2018 SVP presentation found it to be a basal dromaeosaurid, along with Caihong, Tianyuraptor and Zhenyuanlong. Characters like the dorsally displaced maxillary fenestra, posteroventral maxillary fossa (like Zhenyuanlong and microraptorians), mesial serrations and large lateral teeth could support this, and it's only two steps longer in the final analysis too. But it could also be the first branching avialan, or an archaeopterygid (due to its similarity to Caihong which now falls in that family), or its troodontid clade can move to be the most stemward troodontids, all in two steps each. I've said before that I view phylogenetic results more as a set of probabilities than a final tree, and I like to think both drafts of the paper reflect that. I didn't go into this analysis with an agenda to place Lori in any particular position, and I found the other Jurassic paravians to have similarly uncertain relationships so I'm not surprised.
And with that, welcome to Lori Week! We'll be discussing my phylogenetic analysis and results, what I thought of peer review, how to add your own taxon to the Lori matrix and how to check constraint trees of alternative topologies, what ideas I snuck in the paper, and so much more. Hold on tight, I'm in the peer-reviewed literature...
References- Jensen, 1981. Another look at Archaeopteryx as the worlds oldest bird. Encyclia, The Journal of the Utah Academy of Sciences, Arts, and Letters. 58, 109-128.
Jensen and Padian, 1989. Small pterosaurs and dinosaurs from the Uncomphagre fauna (Brushy Basin Member, Morrison Formation: ?Tithonian), Late Jurassic, western Colorado. Journal of Paleontology. 63(3), 364-373. DOI: 10.1017/S0022336000019533
Barsbold and Osmólska, 1999. The skull of Velociraptor (Theropoda) from the Late Cretaceous of Mongolia. Acta Palaeontologica Polonica. 44(2), 189-219.
Hartman, Lovelace and Wahl, 2005. Phylogenetic assessment of a maniraptoran from the Morrison Formation. Journal of Vertebrate Paleontology. 25(3), 67A-68A. DOI: 10.1080/02724634.2005.10009942
Norell, Makovicky, Bever, Balanoff, Clark, Barsbold and Rowe, 2009. A review of the Mongolian Cretaceous dinosaur Saurornithoides (Troodontidae: Theropoda). American Museum Novitates. 3654, 63 pp. DOI: 10.1206/648.1
Zanno, 2010. Osteology of Falcarius utahensis: Characterizing the anatomy of basal therizinosaurs. Zoological Journal of the Linnaean Society. 158, 196-230. DOI: 10.1371/journal.pone.0198155
Lee, Barsbold, Currie, Kobayashi, Lee, Godefroit, Escuillie and Tsogtbaatar, 2014. Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature. 515, 257-260. DOI: 10.1038/nature13874
Kundrat, Nudds, Kear, Lü and Ahlberg, 2018. The first specimen of Archaeopteryx from the Upper Jurassic Mörnsheim Formation of Germany. Historical Biology. 31(1), 3-63. DOI: 10.1080/08912963.2018.1518443
Hartman, Mortimer, Wahl, Lomax, Lippincott and Lovelace, 2019. A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight. PeerJ. 7:e7247. DOI: 10.7717/peerj.7247