Guiyu and Psarolepis enter the LRT together

Today’s study confirms
the phylogenetic analyses of prior workers, like Zhu et al. 2012 and others cited therein. They considered Guiyu (Silurian, 419 mya) the earliest articulated bony fish. Zhu and Zhau 2009 described it as a basal lobe-finned fish with some ray-finned traits.

Figure 1. Guinyu in situ, as originally restored and as restored here based on the in situ data. Psarolepis shares the median spike seen here.

Figure 1. Guinyu in situ, as originally restored and as restored here based on the in situ data. Psarolepis shares the median spike seen here.

Today’s study offers a new reconstruction
for Guiyu: less like a sarcopterygian (Fig. 1) and more like the placoderm, Stensioella, from which it arises in the large reptile tree (LRT, 1480 taxa). So Guiyu documents the transition from placoderms to most bony fish (except catfish and sturgeons). Zhu et al. 2012 did not report the placoderm connection, perhaps because their reconstruction did not look like a placoderm. They did not notice the armored pectoral and pelvic fins were separate from the torso and tail.

Figure 2. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved.

Figure 2. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved. This rather complete taxon provides phylogenetic bracketing clues to the lateral skull and post-crania missing in a sister taxon, Psarolepis (Fig. 3). This taxon documents the transition from placoderms to bony fish.

Zhu et al. 2012 reported,
“Guiyu and Psarolepis have been placed as stem sarcopterygians in earlier studies, even though they manifested combinations of features found in both sarcopterygians and actinopterygians (e.g. pectoral girdle structures, the cheek and operculo-gular
bone pattern, and scale articulation). When Guiyu was first described based on an exceptionally well-preserved holotype specimen, it also revealed a combination of osteichthyan and nonosteichthyan features, including spine-bearing pectoral girdles and
spine-bearing median dorsal plates found in non-osteichthyan gnathostomes as well as cranial morphology and derived macromeric squamation found in crown osteichthyans. In
addition, Guiyu provided strong corroboration for the attempted restoration of Psarolepis romeri based on disarticulated cranial, cheek plate, shoulder girdle and scale materials.”

FIgure 2. Psarolepis skull restored from published data.

FIgure 2. Psarolepis skull restored from published data. Hypothetical opercula are not present, based on Guiyu.

Most of the cranial bones of Psarolepis are fused to one another.
Unfortunately that provides few clues to figure out bone outlines. Here (Fig. 2) Psarolepis is restored based on patterns found throughout the clade. The colors applied to each bone makes this restoration challenge a bit easier and certainly easier to convey to readers.

FIgure 3. Guiyu skull reconstruction in closer view. Mandibles in dorsal view.

FIgure 3. Guiyu skull reconstruction in closer view. Mandibles in dorsal view.

Zhu et al. 2014 followed tradition in their abstract:
“Living gnathostomes (jawed vertebrates) include chondrichthyans (sharks, rays and chimaeras) and osteichthyans or bony fishes. Living osteichthyans are divided into two lineages, namely actinopterygians (bichirs, sturgeons, gars, bowfins and teleosts) and sarcopterygians (coelacanths, lungfishes and tetrapods). [1] It remains unclear how the two osteichthyan lineages acquired their respective characters and how their common osteichthyan ancestor arose from non-osteichthyan gnathostome groups. [2] Here we present the first tentative reconstruction of a 400-million-year-old fossil fish (Psarolepis) from China; this fossil fish combines features of sarcopterygians and actinopterygians and
yet possesses large, paired fin spines previously found only in two extinct gnathostome groups (placoderms and acanthodians). [3] This early bony fish provides amorphological link between osteichthyans and non-osteichthyan groups. It changes the polarity of many characters used at present in reconstructing osteichthyan interrelationships and offers new insights into the origin and evolution of osteichthyans.” [4]

The following notes answer issues raised above:

  1. The LRT separates actinopterygians into several ray-fin clades. Sturgeons, bichirs, and catfish nest apart from bowfins, gars and teleosts.
  2. That lack of clarity is resolved in the LRT.
  3. They are forgetting bichirs, sturgeons, sticklebacks, and Guiyu.
  4. All the more so if they had only included Entelognathus and Guiyu.
  5. Adding Tinirau and LRT taxa helps separate Eusthenopteron and kin from their traditional, and now offshoot link to Tetrapoda.

Zhu et al. 2014 also reported, 
“Psarolepis was first placed within sarcopterygians, as a basal member of Dipnormorpha or among the basal members of Crossopterygii. The new features revealed by the shoulder girdle and cheek materials reported here indicate that Psarolepis may occupy a more basal position in osteichthyan phylogeny.” The LRT resolved this historical issue by including pertinent and key taxa.

Zhu et al. 2014 produced two cladograms
when they introduced Psarolepis, neither of which included Guiyu. In cladogram A Zhu et al. nested Psarolepis between the spiny sharks (acanthodians) and ray-fin fish beginning with the bichir, Polypterus + lungfish. In cladogram B Zhu et al. nested Psarolepis between Polypterus and lobe-fin fish beginning with coelacanths. The placoderms, Entelognathus (Zhu et al. 2013) and Stensioella (Broilli 1933) were not mentioned or included. Both are ougroups to Guiyu and Psarolepis in the LRT. Acanthodians are not primitive or basal to bony fish. They are derived bony fish in the LRT as we learned earlier here. Contra traditions, the most interesting taxa that transition to tetrapods are all slow-moving bottom feeders, not swift open water predators.

Figure 4. Subset of the LRT updated with new basal vertebrates.

Figure 4. Subset of the LRT updated with new basal vertebrates.

Dr. Zhu Min
is the lead author on several of these new discoveries and publications shedding light on key taxa at the origin of bony fish, tetrapods and ultimately humans.


References
Broili F 1933. Weitere Fischreste aus den Hunsrickschiefern. Situngsbirechte der bayerischen Akademie der Wissenschaften, Mathematisch-Naturewissenschaftliche Klasse 2: 269–313.
Zhu M and Zhau W-J 2009. The Xiaoxiang Fauna (Ludlow, Silurian) – a window to explore the early diversification of jawed vertebrates. Abstract from: Rendiconti della Società Paleontologica Italiana. 3 (3): 357–358.
Zhu M, Yu X, Choo B, Qu Q, Jia L, et al. 2012.  Fossil Fishes from China Provide First Evidence of Dermal Pelvic Girdles in Osteichthyans. PLoS ONE 7(4): e35103. doi:10.1371/journal.pone.0035103
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.
Zhu M, Yu X-B and Janvier P 2014. A primitive fossil fish sheds light on the origin of bony fishes. Nature 287:607–610.

wiki/Psarolepis
wki/Stensioella

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